Interfraction and intrafraction changes in amplitude of breathing motion in stereotactic liver radiotherapy

Novel utilization and quantification of Xsight diaphragm tracking for respiratory motion compensation in Cyberknife Synchrony treatment of liver tumors

PURPOSE

The Xsight lung tracking system (XLTS) utilizes an advanced image processing algorithm to precisely identify the position of a tumor and determine its location in orthogonal x-ray images, instead of finding fiducials, thereby minimizing the risk of fiducial insertion-related side effects. To assess and gauge the effectiveness of CyberKnife Synchrony in treating liver tumors located in close proximity to or within the diaphragm, we employed the Xsight diaphragm tracking system (XDTS), which was based on the XLTS.

METHODS

We looked back at the treatment logs of 11 patients (8/11 [XDTS], 3/11 [Fiducial-based Target Tracking System-FTTS]) who had liver tumors in close proximity to or within the diaphragm. And the results are compared with the patients who undergo the treatment of FTTS. The breathing data information was calculated as a rolling average to reduce the effect of irregular breathing. We tested the tracking accuracy with a dynamic phantom (18023-A) on the basis of patient-specific respiratory curve.

RESULTS

The average values for the XDTS and FTTS correlation errors were 1.38 ± 0.65  versus 1.50 ± 0.26 mm (superior-inferior), 1.28 ± 0.48  versus 0.40 ± 0.09 mm (left-right), and 0.96 ± 0.32  versus 0.47 ± 0.10 mm(anterior-posterior), respectively. The prediction errors for two methods of 0.65 ± 0.16  versus 5.48 ± 3.33 mm in the S-I direction, 0.34 ± 0.10  versus 1.41 ± 0.76 mm in the A-P direction, and 0.22 ± 0.072  versus 1.22 ± 0.48 mm in the L-R direction. The coverage rate of FTTS slightly less than that of XDTS, such as 96.53 ± 8.19% (FTTS) versus 98.03 ± 1.54 (XDTS). The prediction error, the motion amplitude, and the variation of the respiratory center phase were strongly related to each other. Especially, the higher the amplitude and the variation, the higher the prediction error.

CONCLUSION

The diaphragm has the potential to serve as an alternative to gold fiducial markers for detecting liver tumors in close proximity or within it. We also found that we needed to reduce the motion amplitude and train the respiration of the patients during liver radiotherapy, as well as control and evaluate their breathing.

Accuracy of robotic radiosurgery in renal cell carcinoma

PURPOSE

Although emerging clinical evidence supports robotic radiosurgery as a highly effective treatment option for renal cell carcinoma (RCC) less than 4 cm in diameter, delivery uncertainties and associated target volume margins have not been studied in detail. We assess intrafraction tumor motion patterns and accuracy of robotic radiosurgery in renal tumors with real-time respiratory tracking to optimize treatment margins.

METHODS

Delivery log files from 165 consecutive treatments of RCC were retrospectively analyzed. Five components were considered for planning target volume (PTV) margin estimation: (a) The model error from the correlation model between patient breath and tumor motion, (b) the prediction error from an algorithm predicting the patient breathing pattern, (c) the targeting error from the treatment robot, (d) the inherent total accuracy of the system for respiratory motion tracking, and (e) the margin required to cover potential target rotation, simulated with PTV rotations up to 10°.

RESULTS

The median tumor motion was 10.5 mm, 2.4 mm and 4.4 mm in the superior-inferior, left-right, and anterior-posterior directions, respectively. The root of the sum of squares of all contributions to the system's inaccuracy results in a minimum PTV margin of 4.3 mm, 2.6 mm and 3.0 mm in the superior-inferior, left-right and anterior-posterior directions, respectively, assuming optimal fiducial position and neglecting target deformation.

CONCLUSIONS

We have assessed kidney motion and derived PTV margins for the treatment of RCC with robotic radiosurgery, which helps to deliver renal treatments in a more consistent manner and potentially further improve outcomes.

Technical note: The migration distance - a unidirectional distance metric for region-of-interest comparisons

BACKGROUND

The radiotherapy process relies on several metrics in determining a notion of "distance" from one three-dimensional region-of-interest (ROI) to another. The majority are symmetric (or commutative) and do not contain information pertaining to directionality. Growth versus regression, for example, is not inherently distinguished by these metrics.

PURPOSE

The purpose of this work was to formalize a unidirectional distance metric, motivated by radiotherapy margin concepts, which we term the migration distance. Informally, the migration distance from ROI X $X$ to Y $Y$ is the minimum isotropic expansion of X $X$ such that Y $Y$ is completely encompassed by the expansion. If Y $Y$ is contained within X $X$ , the migration distance is negative with magnitude equal to the maximum isotropic contraction of X $X$ such that Y $Y$ remains contained within contraction. The metric is demonstrated by quantifying glioblastoma interfraction target changes.

METHODS

An explicit mathematical formulation of the migration distance is presented and contrasted with the related Hausdorff distance. The results are demonstrated for the gross tumor volume (GTV) dynamics of a glioblastoma cohort consisting of 111 patients that underwent standard chemoradiotherapy with offline MR imaging at planning, fraction 10, fraction 20, and 1-month post radiotherapy.

RESULTS

The mean ± SD of the GTV migration distance relative to planning was 5.9 ± 3.9 mm at fraction 10, 6.2 ± 4.4 mm at fraction 20, and 7.9 ± 7.1 mm at 1-month post radiotherapy. The maximum GTV migration distance across all patients at the same timepoints was 20.4, 20.7, and 45.5 mm, respectively.

CONCLUSIONS

We have proposed and demonstrated a unidirectional distance metric. The migration distance may have applications in the quantification of anatomical changes, planning target volume designs, and dosimetric radiotherapy plan assessment.

Dosimetric Effect of Target Position Accuracy on Single-Isocenter Multiple Liver Metastases SBRT

Purpose: To evaluate the dosimetric effects of intrafraction baseline shifts combined with rotational errors on Four-dimensional computed tomography-guided stereotactic body radiotherapy for multiple liver metastases (MLMs). Methods: A total of 10 patients with MLM (2 or 3 lesions) were selected for this retrospective study. Baseline shift errors of 0.5, 1.0, and 2.0 mm; and rotational errors of 0.5°, 1°, and 1.5°, were simulated about all axes. All of the baseline shifts and rotation errors were simulated around the planned isocenter using a matrix transformation of 6° of freedom. The coverage degradation of baseline shifts and rotational errors were analyzed according to the dose to 95% of the planning target volume (D95) and the volume covered by 95% of the prescribed dose (V95), and related changes in gross tumor volume were also analyzed. Results: At the rotation error of 0.5° and the baseline offset of less than 0.5 mm, the D95 and V95 values of all targets were >95%. For rotational errors of 1.0° (combined with all baseline shift errors), 36.3% of targets had D95 and V95 values of <95%. Coverage worsened substantially when the baseline shift errors were increased to 1.0 mm. D95 and V95 values were >95% for about 77.3% of the targets. Only 11.4% of the D95 and V95 values were >95% when the baseline shift errors were increased to 2.0 mm. When the rotational error was increased to 1.5° and baseline shift errors increased to 1.0 mm, the D95 and V95 values were >95% in only 3 cases. Conclusions: The multivariate regression model analysis in this study showed that the coverage of the target decreased further with reduced target volume, increasing the baseline drift, the rotation error, and the distance to the target.

Multi-parametric MRI for radiotherapy simulation

Magnetic resonance imaging (MRI) has become an important imaging modality in the field of radiotherapy (RT) in the past decade, especially with the development of various novel MRI and image-guidance techniques. In this review article, we will describe recent developments and discuss the applications of multi-parametric MRI (mpMRI) in RT simulation. In this review, mpMRI refers to a general and loose definition which includes various multi-contrast MRI techniques. Specifically, we will focus on the implementation, challenges, and future directions of mpMRI techniques for RT simulation.

A novel external/internal tumor tracking approach to compensate for respiratory motion baseline drifts

Objective.Real-time respiratory tumor tracking as implemented in a robotic treatment unit is based on continuous optical measurement of the position of external markers and a correlation model between them and internal target positions, which are established with X-ray imaging of the tumor, or fiducials placed in or around the tumor. Correlation models are created with fifteen simultaneously measured external/internal marker position pairs divided over the respiratory cycle. Every 45-150 s, the correlation model is updated by replacing the three first acquired data pairs with three new pairs. Tracking simulations for >120.000 computer-generated respiratory tracks demonstrated that this tracking approach resulted in relevant inaccuracies in internal target position predictions, especially in case of presence of respiratory motion baseline drifts.Approach.To better cope with drifts, we introduced a novel correlation model with an explicit time dependence, and we proposed to replace the currently applied linear-motion tracking (LMT) by mixed-model tracking (MMT). In MMT, the linear correlation model is extended with an explicit time dependence in case of a detected baseline drift. MMT prediction accuracies were then established for the same >120.000 computer-generated patients as used for LMT.Main results.For 150 s update intervals, MMT outperformed LMT in internal target position prediction accuracy for 93.7 ∣ 97.2% of patients with 0.25 ∣ 0.5 mm min^-1linear respiratory motion baseline drifts with similar numbers of X-ray images and similar treatment times. For the upper 25% of patients, mean 3D internal target position prediction errors reduced by 0.7 ∣ 1.8 mm, while near maximum reductions (upper 10% of patients) were 0.9 ∣ 2.0 mm.Significance.For equal numbers of acquired X-ray images, MMT greatly improved tracking accuracy compared to LMT, especially in the presence of baseline drifts. Even with almost 50% less acquired X-ray images, MMT still outperformed LMT in internal target position prediction accuracy.

Quantifying 6D tumor motion and calculating PTV margins during liver stereotactic radiotherapy with fiducial tracking

Objective

Our study aims to estimate intra-fraction six-dimensional (6D) tumor motion with rotational correction and the related correlations between motions of different degrees of freedom (DoF), as well as quantify sufficient anisotropic clinical target volume (CTV) to planning target volume (PTV) margins during stereotactic body radiotherapy (SBRT) of liver cancer with fiducial tracking technique.

Methods

A cohort of 12 patients who were implanted with 3 or 4 golden markers were included in this study, and 495 orthogonal kilovoltage (kV) pairs of images acquired during the first fraction were used to extract the spacial position of each golden marker. Translational and rotational motions of tumor were calculated based on the marker coordinates by using an iterative closest point (ICP) algorithm. Moreover, the Pearson product-moment correlation coefficients (r) were applied to quantify the correlations between motions with different degrees of freedom (DoFs). The population mean displacement (MP¯), systematic error (Σ) and random error (σ) were obtained to calculate PTV margins based on published recipes.

Results

The mean translational variability of tumors were 0.56, 1.24 and 3.38 mm in the left-right (LR, X), anterior-posterior (AP, Y), and superior-inferior (SI, Z) directions, respectively. The average rotational angles θX , θY and θZ around the three coordinate axes were 0.88, 1.24 and 1.12, respectively. (|r|&gt;0.4) was obtainted between Y -Z , Y - θZ , Z -θZ and θX - θY . The PTV margins calculated based on 13 published recipes in X, Y, and Z directions were 1.08, 2.26 and 5.42 mm, and the 95% confidence interval (CI) of them were (0.88,1.28), (1.99,2.53) and (4.78,6.05), respectively.

Conclusions

The maximum translational motion was in SI direction, and the largest correlation coefficient of Y-Z was obtained. We recommend margins of 2, 3 and 7 mm in LR, AP and SI directions, respectively.

Radiotherapy respiratory motion management in hepatobiliary and pancreatic malignancies: a systematic review of patient factors influencing effectiveness of motion reduction with abdominal compression

BACKGROUND

The effectiveness of abdominal compression for motion management in hepatobiliary-pancreatic (HPB) radiotherapy has not been systematically evaluated.

METHODS & MATERIALS

A systematic review was carried out using PubMed/Medline, Cochrane Library, Web of Science, and CINAHL databases up to 1 July 2021. No date restrictions were applied. Additional searches were carried out using the University of Manchester digital library, Google Scholar and of retrieved papers' reference lists. Studies conducted evaluating respiratory motion utilising imaging with and without abdominal compression in the same patients available in English were included. Studies conducted in healthy volunteers or majority non-HPB sites, not providing descriptive motion statistics or patient characteristics before and after compression in the same patients or published without peer-review were excluded. A narrative synthesis was employed by tabulating retrieved studies and organising chronologically by abdominal compression device type to help identify patterns in the evidence.

RESULTS

The inclusion criteria were met by 6 studies with a total of 152 patients. Designs were a mix of retrospective and prospective quantitative designs with chronological, non-randomised recruitment. Abdominal compression reduced craniocaudal respiratory motion in the majority of patients, although in four studies there were increases seen in at least one direction. The influence of patient comorbidities on effectiveness of compression, and/or comfort with compression was not evaluated in any study.

CONCLUSION

Abdominal compression may not be appropriate for all patients, and benefit should be weighed with potential increase in motion or discomfort in patients with small initial motion (<5 mm). Patient factors including male sex, and high body mass index (BMI) were found to impact the effectiveness of compression, however with limited evidence. High-quality studies are warranted to fully assess the clinical impact of abdominal compression on treatment outcomes and toxicity prospective in comparison to other motion management strategies.

Analysis of the amplitude changes and baseline shifts of respiratory motion using intra-fractional CBCT in liver stereotactic body radiation therapy

PURPOSE

Using intra-fractional cone-beam CT (CBCT) to evaluate the amplitude changes and baseline shifts of respiratory motion in liver stereotactic body radiation therapy (SBRT).

METHODS

The amplitude changes and baseline shifts of respiratory motion for 24 liver patients were evaluated by the four-dimensional (4D) CT, inter- and intra-fractional CBCT. The difference of the average liver position errors among all treatment fractions and the 4D CT representthe baseline shifts. According to the baseline shifts, the ITV to PTV margin was recalculated and the plan was re-designed to compare the dosimetric variation.

RESULTS

The systematic and random errors of the baseline shifts for intra-fractional CBCT in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions were 0.99/1.60 mm, 2.03/2.46 mm, and 1.02/2.07 mm, respectively. The new ITV to PTV margins should be 4.0 mm, 7.0 mm, and 4.0 mm, respectively. The amplitude change of motion between the 4D CT and the intra-fractional CBCT was 1.03 ± 4.35 mm, with 31% of fractions exceeding 5 mm. To achieve the same dose coverage of the new PTV, the D_mean, V50, V40, V30, V25 of normal liver and maximum dose of the duodenum were significantly different.

CONCLUSIONS

Significant amplitude changes and baseline shifts of motion occurred during dose delivery compared with those in 4D CT. Using the ITV to PTV margin of 4.0 mm (LR), 7.0 mm (SI), and 4.0 mm (AP) can ensure the target dose coverage and keep the dose constrain of normal tissues at an acceptable level.

A systematic review and meta-analysis of liver tumor position variability during SBRT using various motion management and IGRT strategies

PURPOSE

To suggest PTV margins for liver SBRT with different motion management strategies based on a systematic review and meta-analysis.

METHODS

In accordance with Preferred-Reporting-Items-for-Systematic-Reviews-and-Meta-Analyses (PRISMA), a systematic review in PubMed, Embase and Medline databases was performed for liver tumor position variability. From an initial 533 studies published before October 2020, 36 studies were categorized as 18 free-breathing (FB; n_patients = 401), 9 abdominal compression (AC; n_patients = 145) and 9 breath-hold (BH; n_patients = 126). A meta-analysis was performed on inter- and intra-fraction position variability to report weighted-mean with 95% confidence interval (CI₉₅) in superior-inferior (SI), left-right (LR) and anterior-posterior (AP) directions. Furthermore, weighted-mean ITV margins were computed for FB (n_studies = 15, n_patients = 373) and AC (n_studies = 6, n_patients = 97) and PTV margins were computed for FB (n_studies = 6, n_patients = 95), AC (n_studies = 7, n_patients = 106) and BH (n_studies = 8, n_patients = 133).

RESULTS

The FB weighted-mean intra-fraction variability, ITV margins and weighted-standard-deviation in mm were SI-9.7, CI₉₅ = 9.3-10.1, 13.5 ± 4.9; LR-5.4, CI₉₅ = 5.3-5.6, 7.3 ± 7.9; and AP-4.2, CI₉₅ = 4.0-4.4, 6.3 ± 7.6. The inter-fraction-based results were SI-4.7, CI₉₅ = 4.3-5.1, 5.7 ± 1.7; LR-1.4, CI₉₅ = 1.1-1.6, 3.6 ± 2.7; and AP-2.8, CI₉₅ = 2.5-3.1, 4.8 ± 2.1. For AC intra-fraction results in mm were SI-1.8, CI₉₅ = 1.6-2.0, 2.6 ± 1.2; LR-0.7, CI₉₅ = 0.6-0.8, 1.7 ± 1.5; and AP-0.9, CI₉₅ = 0.8-1.0, 1.9 ± 1.7. The inter-fraction results were SI-2.6, CI₉₅ = 2.3-3.0, 5.2 ± 2.9; LR-1.9, CI₉₅ = 1.7-2.1, 4.0 ± 2.2; and AP-2.9, CI₉₅ = 2.5-3.2, 5.8 ± 2.7. For BH the inter-fraction variability, and the weighted-mean PTV margins and weighted-standard-deviation in mm were SI-2.4, CI₉₅ = 2.1-2.7, 5.6 ± 2.9; LR-1.8, CI₉₅ = 1.3-2.2, 5.5 ± 1.7; and AP-1.4; CI₉₅ = 1.2-1.7, 6.1 ± 2.1.

CONCLUSION

Our meta-analysis suggests a symmetric weighted-mean PTV margin of 6 mm might be appropriate for BH. For AC and FB, asymmetric PTV margins (weighted-mean margin of 4 mm (AP), 6 mm (SI/LR)) might be appropriate. For FB, if larger (>ITV margin) intra-fraction variability observed, the additional intra- and inter-fraction variability should be accounted in the PTV margin.

Transmission study of the Abdominal Compression plate (BodyFIX Diaphragm Control) for abdominal and stereotactic body radiotherapy

Purpose

Abdominal Compression is one of the methods available to minimize breathing motion during stereotactic body radiotherapy (SBRT), particularly for abdominal malignancies. It might be necessary to treat some tumors with radiation entering through the compression device. One clinically available compression plate device (Elekta BodyFIX Diaphragm Control) was evaluated to understand its impact on dosimetry during clinical treatments.

Methods

The BodyFIX compression device was CT scanned following departmental stereo scanning protocols. Treatment planning system (TPS) calculations were used to determine attenuation ratios through each section of the compression device: the outer frame, compression plate, and higher density couch fixation points and compression screw. TPS calculated skin doses where the compression plate will come in contact with the skin were recorded. All attenuation ratio fields were measured on an Elekta Versa HD linear accelerator. Where differences in attenuation were observed, TPS density overrides were found to bring calculated doses into agreement with measurement.

Results

The compression plate and frame showed low dose attenuation (3%–4%). Only minor density overrides for the frame were required due to artefacts from the limited CT field‐of‐view. The high‐density materials in the couch fixation points resulted in higher attenuation (14%–20%). Similarly, the compression screw recorded very high attenuation (44%–65%), depending on the length of screw used. Skin doses assessed from the TPS calculations showed dose build‐up under the compression plate that would result in skin receiving the maximum dose.

Conclusion

Compression devices can cause significant dose attenuation. Density overrides for TPS calculations are recommended for correcting attenuation in some sections of the device. High‐density structures like the fixation screw and frame fixation points create high levels of dosimetric uncertainty, and beam entry through those areas has been disallowed.

Direct visualization and correlation of liver stereotactic body radiation therapy treatment delivery accuracy with interfractional motion

This study used the visualization of hypo‐intense regions on liver‐specific MRI to directly quantify stereotactic body radiation therapy (SBRT) spatial delivery accuracy. Additionally, the interfractional motion of the liver region was determined and compared with the MRI‐based evaluation of liver SBRT spatial treatment delivery accuracy. Primovist^®‐enhanced MRI scans were acquired from 17 patients, 8–12 weeks following the completion of liver SBRT treatment. Direct visualization of radiation‐induced focal liver reaction in the form of hypo‐intensity was determined. The auto‐delineation approach was used to localize these regions, and center‐of‐mass (COM) discrepancy was quantified between the MRI hypo‐intensity and the CT‐based treatment plan. To assess the interfractional motion of the liver region, a planning CT was registered to a Cone Beam CT obtained before each treatment fraction. The interfractional motion assessed from this approach was then compared against the localized hypo‐intense MRI regions. The mean ± SD COM discrepancy was 1.4 ± 1.3 mm in the left‐right direction, 2.6 ± 1.8 mm in an anteroposterior direction, and 1.9 ± 2.6 mm in the craniocaudal direction. A high correlation was observed between interfractional motion of visualized hypo‐intensity and interfractional motion of planning treatment volume (PTV); the quantified Pearson correlation coefficient was 0.96. The lack of correlation was observed between Primovist^® MRI‐based spatial accuracy and interfractional motion of the liver, where Pearson correlation coefficients ranged from −0.01 to −0.26. The highest random and systematic errors quantified from interfractional motion were in the craniocaudal direction. This work demonstrates a novel framework for the direct evaluation of liver SBRT spatial delivery accuracy.

Application of discrete cosine transform to assess the effect of tumor motion variations on the definition of ITV in lung and liver SBRT

PURPOSE

To use Discrete Cosine Transform to include tumor motion variations on ITV definition of SBRT patients.

METHODS

Data from 66 patients was collected. 2D planar fluoroscopy images (FI) were available for 54 patients. Daily CBCT projections (CBCTp) from 29 patients were employed to measure interfraction amplitude variability. Systematic amplitude variations were obtained from 17 patients with data from both FI and CBCTp. Tumor motion curves obtained from FI were characterized with a Cosine model (CM), based on cosine functions to the power of 2, 4 or 6, and DCT. Performance of both models was evaluated by means of R² coefficient and by comparing their results on Internal Target Volume (ITV) margins against those calculated from original tumor motion curves. Amplitude variations from CBCTp, as well as estimations of baseline shift variations were added to the DCT model to account for their effect on ITV margins.

RESULTS

DCT replicated tumor motion curves with a mean R² values for all patients of 0.86, 0.91 and 0.96 for the lateral (LAT), anterior-posterior (AP) and cranio-caudal (CC) directions respectively. CM yielded worst results, with R² values of 0.64, 0.61 and 0.74 in the three directions. Interfraction amplitude variation increased ITV margins by a 9%, while baseline shift variability implied a 40% and 80-100% increase for normalized values of baseline shift of 0.2 and 0.4 respectively.

CONCLUSIONS

Probability distribution functions of tumor positions can be successfully characterized with DCT. This permits to include tumor motion variablilities obtained from patient population into patient specific ITVs.

Magnetic Resonance Imaging-Guided Adaptive Radiotherapy for Colorectal Liver Metastases

Technological advances have enabled well tolerated and effective radiation treatment for small liver metastases. Stereotactic ablative radiation therapy (SABR) refers to ablative dose delivery (>100 Gy BED) in five fractions or fewer. For larger tumors, the safe delivery of SABR can be challenging due to a more limited volume of healthy normal liver parenchyma and the proximity of the tumor to radiosensitive organs such as the stomach, duodenum, and large intestine. In addition to stereotactic treatment delivery, controlling respiratory motion, the use of image guidance, adaptive planning and increasing the number of radiation fractions are sometimes necessary for the safe delivery of SABR in these situations. Magnetic Resonance (MR) image-guided adaptive radiation therapy (MRgART) is a new and rapidly evolving treatment paradigm. MR imaging before, during and after treatment delivery facilitates direct visualization of both the tumor target and the adjacent normal healthy organs as well as potential intrafraction motion. Real time MR imaging facilitates non-invasive tumor tracking and treatment gating. While daily adaptive re-planning permits treatment plans to be adjusted based on the anatomy of the day. MRgART therapy is a promising radiation technology advance that can overcome many of the challenges of liver SABR and may facilitate the safe tumor dose escalation of colorectal liver metastases.

Estimation of liver elasticity using the finite element method and four-dimensional computed tomography images as a biomarker of liver fibrosis

PURPOSE

Current radiotherapy planning procedures are generally designed based on anatomical information only and use computed tomography (CT) images that do not incorporate organ-functional information. In this study, we developed a method for estimating liver elasticity using the finite element method (FEM) and four-dimensional CT (4DCT) images acquired during radiotherapy planning, and we subsequently evaluated its feasibility as a biomarker for liver fibrosis.

MATERIALS AND METHODS

Twenty patients who underwent 4DCT and ultrasound-based transient elastography (UTE) were enrolled. All patients had chronic liver disease or cirrhosis. Liver elasticity measurements of the UTE were performed on the right lobe of the patient's liver in 20 patients. The serum biomarkers of the aspartate aminotransferase (AST)-to-platelet ratio index (APRI) and fibrosis-4 index (FIB-4) were available in 18 of the 20 total patients, which were measured within 1 week after undergoing 4DCT. The displacement between the 4DCT images obtained at the endpoints of exhalation and inspiration was determined using the actual (via deformable image registration) and simulated (via FEM) respiration-induced displacement. The elasticity of each element of the liver model was optimized by minimizing the error between the actual and simulated respiration-induced displacement. Then, each patient's estimated liver elasticity was defined as the mean Young's modulus of the liver's right lobe and that of the whole liver using the estimated elasticity map. The estimated liver elasticity was evaluated for correlations with the elasticity obtained via UTE and with two serum biomarkers (APRI and FIB-4).

RESULTS

The mean ± standard deviation (SD) of the errors between the actual and simulated respiration-induced displacement in the liver model was 0.54 ± 0.33 mm. The estimated liver's right lobe elasticity was statistically significantly correlated with the UTE (r = 0.87, P < 0.001). Furthermore, the estimated whole liver elasticity was statistically significantly correlated with the UTE (r = 0.84, P < 0.001), APRI score (r = 0.62, P = 0.005), and FIB-4 score (r = 0.54, P = 0.021).

CONCLUSION

In this study, liver elasticity was estimated through FEM-based simulation and actual respiratory-induced liver displacement obtained from 4DCT images. Furthermore, we assessed that the estimated elasticity of the liver's right lobe was strongly correlated with the UTE. Therefore, the estimated elasticity has the potential to be a feasible imaging biomarker for assessing liver fibrosis using only 4DCT images without additional inspection or equipment costs. Because our results were derived from a limited sample of 20 patients, it is necessary to evaluate the accuracy of elasticity estimation for each liver segment on larger groups of biopsied patients to utilize liver elasticity information for radiotherapy planning.

MRI evaluation of normal tissue deformation and breathing motion under an abdominal compression device

Purpose

Abdominal compression can minimize breathing motion in stereotactic radiotherapy, though it may impact the positioning of dose‐limiting normal tissues. This study quantified the reproducibility of abdominal normal tissues and respiratory motion with the use of an abdominal compression device using MR imaging.

Methods

Twenty healthy volunteers had repeat MR over 3 days under an abdominal compression plate device. Normal tissues were delineated on daily axial T2‐weighted MR and compared on days 2 and 3 relative to day 1, after adjusting for baseline shifts relative to bony anatomy. Inter‐fraction organ deformation was computed using deformable registration of axial T2 images. Deformation > 5 mm was assumed to be clinically relevant. Inter‐fraction respiratory amplitude changes and intra‐fraction baseline drifts during imaging were quantified on daily orthogonal cine‐MR (70 s each), and changes > 3 mm were assumed to be relevant.

Results

On axial MR, the mean inter‐fraction normal tissue deformation was > 5 mm for all organs (range 5.1–13.4 mm). Inter‐fraction compression device misplacements > 5 mm and changes in stomach volume > 50% occurred at a rate of 93% and 38%, respectively, in one or more directions and were associated with larger adjacent organ deformation, in particular for the duodenum. On cine‐MR, inter‐fraction amplitude changes > 3 mm on day 2 and 3 relative to day 1 occurred at a rate of < 12.5% (mean superior–inferior change was 1.6 mm). Intra‐fraction baseline drifts > 3 mm during any cine‐MR acquisition occurred at a rate of 23% (mean superior–inferior changes was 2.4 mm).

Conclusions

Respiratory motion under abdominal compression is reproducible in most subjects within 3 mm. However, inter‐fraction deformations greater than 5 mm in normal tissues were common and larger than inter‐ and intra‐fraction respiratory changes. Deformations were driven mostly by variable stomach contents and device positioning. The magnitude of this motion may impact normal tissue dosimetry during stereotactic radiotherapy.

Quantification of Intrafraction and Interfraction Tumor Motion Amplitude and Prediction Error for Different Liver Tumor Trajectories in Cyberknife Synchrony Tracking

PURPOSE

To research the fiducial-based, real-time tracking intrafraction (during the fraction [intra-]) and interfraction (between fractions [inter-]) tumor respiration amplitude, motion trajectory, and prediction error and quantify their relationships for different types of motion trajectories during Cyberknife-based stereotactic ablation radiotherapy.

METHODS AND MATERIALS

Twelve patients with liver tumors were treated using a Cyberknife system, and 58 fractions were involved in this study. Real-time target motion tracking data were extracted and transformed from the robot coordinate system into the patient coordinate system by the rotation matrix. Only the time sessions of the beam on were studied according to the data information generated from the Cyberknife motion tracking system. The motion correlation model between the external marker signal and internal fiducial position was built to present the type of motion trajectory.

RESULTS

Using the correlation model as a function of external marker signal and internal fiducial position, we knew 4 motion trajectories mainly existed for liver cancer patients as follows: perfect linearity (group I), simple linearity (group II), hysteresis (group III), and area respiratory (group IV) patterns. More than half of the patients had a linear breathing trajectory. Analyzing all patients together, the intra-amplitudes were slightly less than those of the inter-amplitudes. The amplitude from large to small was in the superior-inferior, left-right and anterior-posterior directions, regardless of inter- and intra-amplitudes. Then, patients with a larger peak-to-peak have a larger standard deviation of amplitude and a larger amplitude in all fractions/sessions. The prediction errors of the linear motion trajectory were generally less than 1 mm. The prediction errors of the regular hysteresis breathing model were smaller than those of the irregular hysteresis model. Scattered breathing would result in a larger tracking error, such as the area respiratory trajectory. It was logical that prediction errors were larger for patients who showed much variation in their breathing amplitude.

CONCLUSIONS

This paper showed that the liver motion trajectory model included perfect linearity, sample linearity, hysteresis, and area. The linear motion trajectory presented the minimum tracking error and the best stability, and the hysteresis and area trajectory were the worst. Therefore, breathing management, including respiration training, control, and evaluation of motion trajectory in all directions, was significantly necessary during liver SABR treatment.

Mitigation of motion effects in pencil-beam scanning - Impact of repainting on 4D robustly optimized proton treatment plans for hepatocellular carcinoma

Proton fields delivered by the active scanning technique can be interfered with the intrafractional motion. This in-silico study seeks to mitigate the dosimetric impacts of motion artifacts, especially its interplay with the time-modulated dose delivery. Here four-dimensional (4d) robust optimization and dose repainting, which is the multiple application of the same field with reduced fluence, were combined. Two types of repainting were considered: layered and volumetric repainting. The time-resolved dose calculation, which is necessary to quantify the interplay effect, was integrated into the treatment planning system and validated. Nine clinical cases of hepatocellular carcinoma (HCC) showing motion in the range of 0.4-1.5cm were studied. It was found that the repainted delivery of 4D robustly optimized plans reduced the impact of interplay effect as quantified by the homogeneity index within the clinical target volume (CTV) to a tolerable level. Similarly, the fractional over- and underdosage was reduced sufficiently for some HCC cases to achieve the purpose of motion management. This holds true for both investigated types of repainting with small dosimetric advantages of volume repainting over layered repainting. Volume repainting, however, cannot be applied clinically in proton centers with slow energy changes. Thus, it served as a reference in the in-silico evaluation. It is recommended to perform the dynamic dose calculation for individual cases to judge if robust optimization in conjunction with repainting is sufficient to keep the interplay effect within bounds.

External Beam Radiation Therapy for Liver Metastases

Stereotactic ablative radiotherapy (SABR) commonly is used for small liver metastases. Modern conformal radiotherapy techniques, including 3-dimensional conformal radiotherapy and intensity-modulated radiation therapy, enable the safe delivery of SABR to small liver volumes. For larger tumors, the safe delivery of SABR can be challenging due to a more limited volume of healthy normal liver parenchyma and the proximity of the tumor to radiosensitive organs, such as the stomach, duodenum, and large intestine. Controlling respiratory motion, the use of image guidance, and increasing the number of radiation fractions sometimes are necessary for the safe delivery of SABR in these situations.

The Pivotal Role of the Therapeutic Radiographer/Radiation Therapist in Image-guided Radiotherapy Research and Development

The ability to personalise radiotherapy to fit the individual patient and their diagnosis has been realised through technological advancements. There is now more opportunity to utilise these technologies and deliver precision radiotherapy for more patients. Image-guided radiotherapy (IGRT) has enabled users to safely and accurately plan, treat and verify complex cases; and deliver a high dose to the target volume, while minimising dose to normal tissue. Rapid changes in IGRT have required a multidisciplinary team (MDT) approach, carefully deciding optimum protocols to achieve clinical benefit. Therapeutic radiographer/radiation therapists (RTTs) play a pivotal role in this MDT. There is already a great deal of evidence that illustrates the contribution of RTTs in IGRT development; implementation; quality assurance; and maintaining training and competency programmes. Often this has required the RTT to undertake additional roles and responsibilities. These publications show how the profession has evolved, expanding the scope of practice. There are now more opportunities for RTT-led IGRT research. This is not only undertaken in the more traditional aspects of practice, but in recent times, more RTTs are becoming involved in imaging biomarkers research and radiomic analysis. The aim of this overview is to describe the RTT contribution to the ongoing development of IGRT and to showcase some of the profession's involvement in IGRT research.

Reliability of ITV approach to varying treatment fraction time: a retrospective analysis based on 2D cine MR images

BACKGROUND

Internal Target Volume (ITV) is one of the most common strategies to passively manage tumour motion in Radiotherapy (RT). The reliability of this approach is based on the assumption that the tumour motion estimated during pre-treatment 4D Computed Tomography (CT) acquisition is representative of the motion during the whole RT treatment. With the introduction of Magnetic Resonance-guided RT (MRgRT), it has become possible to monitor tumour motion during the treatment and verify this assumption. Aim of this study was to investigate the reliability of the ITV approach with respect to the treatment fraction time (TFT) in abdominal and thoracic lesions.

METHODS

A total of 12 thoracic and 15 abdominal lesions was analysed. Before treatment, a 10-phase 4DCT was acquired and ITV margins were estimated considering the envelope of the lesion contoured on the different 4DCT phases. All patients underwent MRgRT treatment in free-breathing, monitoring the tumour position on a sagittal plane with 4 frames per second (sec). ITV margins were projected on the tumour trajectory and the percentage of treatment time in which the tumour was inside the ITV (%TT) was measured to varying of TFT. The ITV approach was considered moderately reliable when %TT ≥ 90% and strongly reliable when %TT ≥ 95%. Additional ITV margins required to achieve %TT ≥ 95% were also calculated.

RESULTS

In the analysed cohort of patients, ITV strategy can be considered strongly reliable only for lung lesions with TFT ≤ 7 min (min). The ITV strategy can be considered only moderately reliable for abdominal lesions, and additional margins are required to obtain %TT ≥ 95%. Considering a TFT ≤ 4 min, additional margins of 2 mm in cranio-caudal (CC) and 1 mm in antero-posterior (AP) are suggested for pancreatic lesions, 3 mm in CC and 2 mm in AP for renal and liver ones.

CONCLUSIONS

On the basis of the analysed cases, the ITV approach appears to be reliable in the thorax, while it results more challenging in the abdomen, due to the higher uncertainty in ITV definition and to the observed larger intra and inter-fraction motion variability. The addition of extra margins based on the TFT may represent a valid tool to compensate such limitations.

Dosimetric assessment of patient-specific breath-hold reproducibility on liver motion for SBRT planning

PURPOSE

To investigate the impact of breath-hold reproducibility on liver motion using a respiratory motion management device.

METHODS

Forty-four patients with hepatic tumors, treated with SBRT with breath-hold, were randomly selected for this study. All patients underwent three consecutive computed tomography (CT) scans using active breath-hold coordinator (ABC) with three repeated single breath-hold during simulation. The three CT scans were labeled as ABC1-CT, ABC2-CT, and ABC3-CT. Displacements of centroids of the entire livers among the three ABC-CTs were measured as a surrogate for intrafractional motion. For each patient, two different treatment plans were prepared: (a) a clinical plan using a 5-mm expansion of an ITV that encompassed all three GTVs from each of the three ABC-CTs, and (b) a research plan using a 5-mm expansion of the GTV from only ABC1-CT to create PTV. The clinical plan acceptance criteria were that 95% of the PTV and 99% of the GTV received 100% of the prescription dose. Dosimetric endpoints were analyzed and compared for the two plans.

RESULTS

All shifts in the medial-lateral direction (range: -3.9 to 2.0 mm) were within 5 mm while 7% of shifts in the anterior-posterior direction (range: -10.5 to 16.7 mm) and 11% of shifts in the superior-inferior direction (range: -17.0 to 8.7 mm) exceeded 5 mm. Six patients (14%) had an intrafraction motion greater than 5 mm in any direction. For these six patients, if a plan was created based on a PTV from a single CT (ex. ABC1-CT), 5 of 12 GTVs captured from other ABC-CTs would fail to meet the clinical acceptance criteria due to poor breath-hold reproducibility.

CONCLUSIONS

Non-negligible intrafractional motion occurs in patients with poor breath-hold reproducibility. To identify this subgroup of patients, acquiring three CTs with active breath-hold during simulation is a feasible practical method.

Effect of setup and inter-fraction anatomical changes on the accumulated dose in CT-guided breath-hold intensity modulated proton therapy of liver malignancies

PURPOSE

To evaluate the effect of setup uncertainties including uncertainties between different breath holds (BH) and inter-fractional anatomical changes under CT-guided BH with intensity-modulated proton therapy (IMPT) in patients with liver cancer.

METHODS AND MATERIALS

This retrospective study considered 17 patients with liver tumors who underwent feedback-guided BH (FGBH) IMRT treatment with daily CT-on-rail imaging. Planning CT images were acquired at simulation using FGBH, and FGBH CT-on-rail images were also acquired prior to each treatment. Selective robust IMPT plans were generated using planning CT and re-calculated on each daily CT-on-rail image. Subsequently, the fractional doses were deformed and accumulated onto the planning CT according to the deformable image registration between daily and planning CTs. The doses to the target and organs at risk (OARs) were compared between IMRT, planned IMPT, and accumulated IMPT doses.

RESULTS

For IMPT plans, the mean of D_98% of CTV for all 17 patients was slightly reduced from the planned dose of 68.90 ± 1.61 Gy to 66.48 ± 1.67 Gy for the accumulated dose. The target coverage could be further improved by adjusting planning techniques. The dose-volume histograms of both planned and accumulated IMPT doses showed better sparing of OARs than that of the IMRT.

CONCLUSIONS

IMPT with FGBH and CT-on-rail guidance is a robust treatment approach for liver tumor cases.

Performance of gimbal-based dynamic tumor tracking for treating liver carcinoma

BACKGROUND

Since the introduction of tumor tracking in radiotherapy, it is possible to ensure a precise irradiation of moving targets. To follow the tumor movement, most systems rely on the detection of implanted markers and correlation models between the internal and external patient movement. This study reports the clinical workflow and first results of the dynamic tumor tracking (DTT) performance for patients with liver carcinoma at the Vero SBRT system of the University Hospital Erlangen regarding the detection of the internal marker and the changes of the determined correlation models.

METHODS

So far 13 liver patients were treated with DTT. For each patient, two fiducial markers (FM), which are monitored with X-rays during treatment, were implanted in the vicinity of the tumor. All patients received a fraction dose of 4-6 Gy with 8 to 12 fractions. Treatment and patient data is evaluated by processing the acquired log-files of the DTT treatment. Based on this, the marker detection and the changes of the correlation model between the internal and external movement is investigated.

RESULTS

The median treatment time was 19:42 min. During treatment a median of 173 X-ray stereoscopic images were acquired. The marker detection was successful in 64.6% of the images. The FM detection is independent of the relative angle between the marker and the imager, but shows a dependency on the average intensity surrounding the FM position within the kV images. The number of correlation models needed during treatment increases in the presence of baseline shifts. The comparison of the correlation models shows large differences in the internal-external correlation between the different models acquired for one patient.

CONCLUSION

Thirteen liver patients were treated with DTT at the Vero SBRT system and the marker detection was analyzed. Furthermore, the importance of regularly monitoring the internal target motion could be shown, since the correlation between the internal and external motion changes considerably over the course of the treatment.

Effect of a Device-Free Compressed Shell Fixation Method on Hepatic Respiratory Movement: Analysis for Respiratory Amplitude of the Liver and Internal Motions of a Fiducial Marker

PURPOSE

Suppression of respiratory movement of the liver would be desirable for high-precision radiation therapy for liver tumors. We aimed to investigate the effect of our original device-free compressed shell fixation method and breathing instruction on suppression of respiratory movement. The characteristics of liver motion based on the movement of a fiducial marker were also analyzed.

METHODS AND MATERIALS

First, respiratory amplitudes of the liver with the device-free compressed shell were analyzed from the data of 146 patients. The effect of this shell fixing method on liver movement was evaluated. Second, as another cohort study with 166 patients, interfractional internal motion of the liver for patients fixed in the shell was calculated using the fiducial marker coordinate data of images for position setting before daily irradiation. Third, in another 12 patients, intrafractional internal motion was calculated from the fiducial marker coordinate data using x-ray images before and after irradiation.

RESULTS

The median respiratory movement without the shell, after fixing with the shell, and after instructing on the breathing method with the shell was 14.2 (interquartile range, 10.7-19.8), 11.5 (8.6-17.5), and 10.4 mm (7.3-15.8), respectively. Systematic and random errors of interfractional internal motion were all ≤2 mm in the left-right and anteroposterior directions and 3.7 and 3.0 mm, respectively, in the craniocaudal direction. Systematic and random errors of intrafractional internal motion were all ≤1.3 mm in the left-right and anteroposterior directions and 0.8 and 2.4 mm, respectively, in the craniocaudal direction.

CONCLUSIONS

The device-free compressed shell fixation method was effective in suppressing the respiratory movement of the liver. Irradiation position matching using the fiducial marker can correct the interfractional internal motion on each day, which would contribute to the reduction of the margin to be given around the target.

A Multi-Institutional Experience of MR-Guided Liver Stereotactic Body Radiation Therapy

Purpose

Daily magnetic resonance (MR)–guided radiation has the potential to improve stereotactic body radiation therapy (SBRT) for tumors of the liver. Magnetic resonance imaging (MRI) introduces unique variables that are untested clinically: electron return effect, MRI geometric distortion, MRI to radiation therapy isocenter uncertainty, multileaf collimator position error, and uncertainties with voxel size and tracking. All could lead to increased toxicity and/or local recurrences with SBRT. In this multi-institutional study, we hypothesized that direct visualization provided by MR guidance could allow the use of small treatment volumes to spare normal tissues while maintaining clinical outcomes despite the aforementioned uncertainties in MR-guided treatment.

Methods and materials

Patients with primary liver tumors or metastatic lesions treated with MR-guided liver SBRT were reviewed at 3 institutions. Toxicity was assessed using National Cancer Institute Common Terminology Criteria for Adverse Events Version 4. Freedom from local progression (FFLP) and overall survival were analyzed with the Kaplan-Meier method and χ² test.

Results

The study population consisted of 26 patients: 6 hepatocellular carcinomas, 2 cholangiocarcinomas, and 18 metastatic liver lesions (44% colorectal metastasis). The median follow-up was 21.2 months. The median dose delivered was 50 Gy at 10 Gy/fraction. No grade 4 or greater gastrointestinal toxicities were observed after treatment. The 1-year and 2-year overall survival in this cohort is 69% and 60%, respectively. At the median follow-up, FFLP for this cohort was 80.4%. FFLP for patients with hepatocellular carcinomas, colorectal metastasis, and all other lesions were 100%, 75%, and 83%, respectively.

Conclusions

This study describes the first clinical outcomes of MR-guided liver SBRT. Treatment was well tolerated by patients with excellent local control. This study lays the foundation for future dose escalation and adaptive treatment for liver-based primary malignancies and/or metastatic disease.

Predicting tumour motion during the whole radiotherapy treatment: a systematic approach for thoracic and abdominal lesions based on real time MR

INTRODUCTION

Aim of this study was to investigate the ability of pre-treatment four dimensional computed tomography (4DCT) to capture respiratory-motion observed in thoracic and abdominal lesions during treatment. Treatment motion was acquired using full-treatment cine-MR acquisitions. Results of this analysis were compared to the ability of 30 seconds (s) cine Magnetic Resonance (MR) to estimate the same parameters.

METHODS

A 4DCT and 30 s cine-MR (ViewRay, USA) were acquired on the simulation day for 7 thoracic and 13 abdominal lesions. Mean amplitude, intra- and inter-fraction amplitude variability, and baseline drift were extracted from the full treatment data acquired by 2D cine-MR, and correlated to the motion on pre-treatment 30 s cine-MR and 4DCT. Using the full treatment data, safety margins on the ITV, necessary to account for all motion variability from 4DCT observed during treatment, were calculated. Mean treatment amplitudes were 2 ± 1 mm and 5 ± 3 mm in the anteroposterior (AP) and craniocaudal (CC) direction, respectively. Differences between mean amplitude during treatment and amplitude on 4DCT or during 30 s cine-MR were not significant, but 30 s cine-MR was more accurate than 4DCT. Intra-fraction amplitude variability was positively correlated with both 30 s cine-MR and 4DCT amplitude. Inter-fraction amplitude variability was minimal.

RESULTS

Mean baseline drift over all fractions and patients equalled 1 ± 1 mm in both CC and AP direction, but drifts per fraction up to 16 mm (CC) and 12 mm (AP) were observed. Margins necessary on the ITV ranged from 0 to 8 mm in CC and 0 to 5 mm in AP direction. Neither amplitude on 4DCT nor during 30 s cine MR is correlated to the magnitude of drift or the necessary margins in both directions.

CONCLUSION

Lesions moving with small amplitude show limited amplitude variability throughout treatment, making passive motion management strategies seem adequate. However, other variations such as baseline drifts and shifts still cause significant geometrical uncertainty, favouring real-time monitoring and an active approach for all lesions influenced by respiratory motion.

A Self-Sorting Coronal 4D-MRI Method for Daily Image Guidance of Liver Lesions on an MR-LINAC

PURPOSE

Novel hybrid MR-LINAC devices provide MRI's superior soft-tissue contrast in the treatment room and thus have the potential to increase accuracy of liver stereotactic body radiation therapy (SBRT). Requirements for daily position verification using 4-dimensional MRI include tumor visibility and short acquisition-reconstruction time (preferably <5 min). The proposed method provides fast acquisition-reconstruction time and the flexibility to vary T1- and T2-weighting, using standard imaging sequences for straightforward implementation on an MR-LINAC.

METHODS AND MATERIALS

Images were acquired using a coronal 2-dimensional, multislice, single-shot turbo spin-echo (TSE) and turbo field-echo (TFE) sequence, which were repeated 30 times. An image-based self-sorting signal (ImS) was extracted from the data, and rigid registration of the diaphragm per slice position was performed and corrected for amplitude variation in the anteroposterior direction. Data were sorted into 10 bins according to amplitude and phase. ImS was validated in 4 healthy volunteers against a navigator signal. Positional variations within bins, missing data, and smoothness of the liver dome were compared between amplitude and phase binning in 10 volunteers. Tumor contrast and registration were investigated in 3 patients.

RESULTS

Each ImS was found to be in excellent agreement with the navigator signal with a correlation coefficient of >0.95 and binning differences of <1 bin. Better liver dome smoothness per bin in case of amplitude binning compared with that in phase binning (2.0-2.6 mm vs 2.4-3.7 mm, respectively) is a tradeoff for more missing data (3.5%-17.5% vs 3.5%-4.7%, respectively). Liver lesions were visible in almost all coronal TSE and TFE images, but the lesion boundary was better defined in the TSE images. Rigid registrations could be performed on the tumor area.

CONCLUSIONS

An efficient self-sorted 4-dimensional MRI method was developed and validated using standard sequences and fast reconstruction on a LINAC-integrated MRI scanner providing good tumor visibility for daily image-guided liver stereotactic body radiation therapy.

Quantitative analysis of respiration-induced motion of each liver segment with helical computed tomography and 4-dimensional computed tomography

Background

To analyze the respiratory-induced motion of each liver segment using helical computed tomography (helical CT) and 4-dimensional computed tomography (4DCT), and to establish the individual segment expansion margin of internal target volume (ITV) to facilitate target delineation of tumors in different liver segments.

Methods

Twenty patients who received radiotherapy with CT-simulation scanning of the whole liver in both helical CT and 10-phase-gated 4DCT were investigated, including 2 patients with esophagus cancer, 4 with lung cancer, 10 with breast cancer, 2 with liver cancer, 1 with thymoma, and 1 with gastric diffuse large B-cell lymphoma (DLBCL). For each patient, 9 representative points were drawn on the helical CT images of liver segments 1, 2, 3, 4a, 4b, 5, 6, 7, and 8, respectively, and adaptively deformed to 2 phases of the 4DCT images at the end of inspiration (phase 0 CT) and expiration (phase 50 CT) in the treatment planning system. Using the amplitude of each point between phase 0 CT and phase 50 CT, we established quantitative data for the respiration-induced motion of each liver segment in 3-dimensional directions. Moreover, using the amplitude between the original helical CT and both 4DCT images, we rendered the individual segment expansion margin of ITV for hepatic target delineation to cover more than 95% of each tumor.

Results

The average amplitude (mean ± standard deviation) was 0.6 ± 3.0 mm in the left-right (LR) direction, 2.3 ± 2.4 mm in the anterior-posterior (AP) direction, and 5.7 ± 3.4 mm in the superior-inferior (SI) direction, respectively. All of the segments moved posteriorly and superiorly during expiration. Segment 7 had the largest amplitude in the SI direction, at 8.6 ± 3.4 mm. Otherwise, the segments over the lateral side, including segments 2, 3, 6, and 7, had greater excursion in the SI direction compared to the medial segments. To cover more than 95% of each tumor, the required expansion margin of ITV in the LR, AP, and SI directions were at least 2.5 mm, 2.5 mm, and 5.0 mm on average, respectively, with variations between different segments.

Conclusions

The greatest excursion occurred in liver segment 7, followed by the segments over the lateral side in the SI direction. The individual segment expansion margin of ITV is required to delineate targets for each segment and direction.

Evaluation of the intra- and interfractional tumor motion and variability by fiducial-based real-time tracking in liver stereotactic body radiation therapy

PURPOSE

Tumor motion amplitude varies during treatment. The purpose of the study was to evaluate the intra- and interfraction tumor motion and variability in patients with liver cancer treated with fiducial-based real-time tracking stereotactic body radiotherapy (SBRT).

METHODS

Fourteen liver patients were treated with SBRT using a CyberKnife. Two to four fiducial markers implanted near the tumor were used for real-time monitoring using the Synchrony system. The tumor motion information during treatment was extracted from the log files recorded by the Synchrony system. Logfile-based amplitudes in the superior-posterior (SI), left-right (LR) and anterior-posterior (AP) directions were compared to the 4DCT-based amplitudes. The intra- and interfraction amplitude variations and the incidence of baseline shifts were analyzed for 66 fractions administered to 14 patients.

RESULTS

The median (range) logfile-based liver motion amplitudes for all patients were 11.9 (5.1-17.3) mm, 1.3 (0.4-4) mm and 3.8 (0.9-7.7) mm in the SI, LR and AP directions, respectively. Compared with the logfile-based amplitude, the 4DCT-based amplitude was underestimated (P < 0.05). The median (range) intra- and interfraction liver motion amplitude variations were 4.3 (1.6-6.0) mm (SI), 0.5 (0.2-2.2) mm(LR) and 1.5 (0.3-3.3) mm (AP) and 1.7 (0.5-4.6) mm (SI), 0.3 (0.1-3.0) mm (LR) and 0.7 (0.3-2.7) mm (AP), respectively. Baseline shifts exceeding 2 mm, 3 mm and 5 mm were observed in 27.3%, 7.6% and 3% of the measurements, respectively, within 10 min, and in 66.7%, 38.1% and 19%, respectively, within 30 min for the square root of the sum of the squares of the distances in the SI, LR and AP directions (3D). The tumor motion amplitude was found to be correlated with the baseline shift.

CONCLUSIONS

Most patients showed significant intra- and interfraction liver motion amplitude variations over the entire course of radiation. More caution is needed for patients with large tumor motion amplitudes.

Interfractional diaphragm changes during breath-holding in stereotactic body radiotherapy for liver cancer

Aim and background

IGRT based on bone matching may produce a large target positioning error in terms of the reproducibility of expiration breath-holding on SBRT for liver cancer. We evaluated the intrafractional and interfractional errors using the diaphragm position at the end of expiration by utilising Abches and analysed the factor of the interfractional error.

Materials and methods

Intrafractional and interfractional errors were measured using a couple of frontal kV images, planning computed tomography (pCT) and daily cone-beam computed tomography (CBCT). Moreover, max-min diaphragm position within daily CBCT image sets with respect to pCT and the maximum value of diaphragm position difference between CBCT and pCT were calculated.

Results

The mean ± SD (standard deviation) of the intra-fraction diaphragm position variation in the frontal kV images was 1.0 ± 0.7 mm in the C-C direction. The inter-fractional diaphragm changes were 0.4 ± 4.6 mm in the C-C direction, 1.4 ± 2.2 mm in the A-P direction, and -0.6 ± 1.8 mm in the L-R direction. There were no significant differences between the maximum value of the max-min diaphragm position within daily CBCT image sets with respect to pCT and the maximum value of diaphragm position difference between CBCT and pCT.

Conclusions

Residual intrafractional variability of diaphragm position is minimal, but large interfractional diaphragm changes were observed. There was a small effect in the patient condition difference between pCT and CBCT. The impact of the difference in daily breath-holds on the interfractional diaphragm position was large or the difference in daily breath-holding heavily influenced the interfractional diaphragm change.

Image quality of four-dimensional cone-beam computed tomography obtained at various gantry rotation speeds for liver stereotactic body radiation therapy with fiducial markers

In this study, qualities of 4D cone-beam CT (CBCT) images obtained using various gantry rotation speeds (GRSs) for liver stereotactic body radiation therapy (SBRT) with fiducial markers were quantitatively evaluated. Abdominal phantom containing a fiducial marker was moved along a sinusoidal waveform, and 4D-CBCT images were acquired with GRSs of 50-200° min^-1. We obtained the 4D-CBCT projection data from six patients who underwent liver SBRT and generated 4D-CBCT images at GRSs of 67-200° min^-1, by varying the number of projection data points. The image quality was evaluated based on the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and structural similarity index (SSIM). The fiducial marker positions with different GRSs were compared with the setup values and a reference position in the phantom and clinical studies, respectively. The root mean square errors (RMSEs) were calculated relative to the reference positions. In the phantom study, the mean SNR, CNR, and SSIM decreased from 37.6 to 10.1, from 39.8 to 10.1, and from 0.9 to 0.7, respectively, as the GRS increased from 50 to 200° min^-1. The fiducial marker positions were within 2.0 mm at all GRSs. Similarly, in the clinical study, the mean SNR, CNR, and SSIM decreased from 50.4 to 13.7, from 24.2 to 6.0, and from 0.92 to 0.73, respectively. The mean RMSEs were 2.0, 2.1, and 3.6 mm for the GRSs of 67, 100, and 200° min^-1, respectively. We conclude that GRSs of 67 and 85° min^-1 yield images of acceptable quality for 4D-CBCT in liver SBRT with fiducial markers.

Observation of different tumor motion magnitude within liver and estimate of internal motion margins in postoperative patients with hepatocellular carcinoma

Aims

To assess motion magnitude in different parts of the liver through surgical clips in postoperative patients with hepatocellular carcinoma and to examine the correlation between the clip and diaphragm motion.

Methods

Four-dimensional computed tomography images from 30 liver cancer patients under thermoplastic mask immobilization were selected for this study. Three to seven surgical clips were placed in the resection cavity of each patient. The liver volume on computed tomography image was divided into the right upper (RU), right middle (RM), right lower (RL), hilar, and left lobes. Agreement between the clip and diaphragm motion was assessed by calculating intraclass correlation coefficient, and Bland–Altman analysis (Diff). Furthermore, population-based and patient-specific margins for internal motion were evaluated.

Results

The clips located in the RU lobe showed the largest motion, (7.5±1.6) mm, which was significantly more than in the RM lobe (5.7±2.8 mm, p=0.019), RL lobe (4.8±3.3 mm, p=0.017), and hilar lobe (4.7±2.7 mm, p<0.001) in the cranial–caudal direction. The mean intraclass correlation coefficient values between the clip and diaphragm motion were 0.915, 0.735, 0.678, 0.670, and the mean Diff values between them were 0.1±0.8 mm, 2.3±1.4 mm, 3.1±2.0 mm, 2.4±1.5 mm, when clips were located in the RU lobe, RM lobe, RL lobe, and hilar lobe, respectively. The clip and diaphragm motions had high concordance when clips were located in the RU lobe. Internal margin can be reduced from 5 mm in the cranial–caudal direction based on patient population average and to 3 mm based on patient-specific margins.

Conclusions

The motion magnitude of clips varied significantly depending on their location within the liver. The diaphragm was a more appropriate surrogate for tumor located in the RU lobe than for other lobes.

Experience with an abdominal compression band for radiotherapy of upper abdominal tumours

Introduction

Radiotherapy outcomes are influenced by treatment delivery geometric accuracy and organ‐at‐risk dose. The location of abdominal structures such as the liver, kidneys and tumour volumes can be strongly influenced by respiratory motion. This increases geometric uncertainty and dose to organs‐at‐risk. One common method of minimising respiratory motion is abdominal compression (AC).

Methods

Fifteen patients being treated for radiotherapy to upper abdominal tumours were analysed. Each patient underwent 2 four‐dimensional computerised tomography (4D‐CT) scans, one with and one without AC with a pneumatic compression belt. Liver and kidney positions were measured on the 4DCT scans at the peak inspiratory and expiratory respiratory phases. The patient received radiation therapy treatment planned on the CT data set with the technique (compression or no compression) that provided the least respiratory motion.

Results

There was no statistically significant motion difference over the sample population with AC for the kidneys or liver. Of the 14 evaluable patients, 4, 6 and 6 saw reduction in superior‐inferior motion for left kidney, right kidney and liver respectively. The remainder either had negligible (<2 mm) or increase in motion with AC. For anterior‐posterior motion, 2, 2 and 1 saw a reduction for left‐kidney, right‐kidney and liver respectively.

Conclusion

AC through the use of a pneumatic compression belt was found to result in inconsistent reduction in kidney and liver respiratory motion. It is recommended that the effect of AC is evaluated on a per‐patient basis.

Intra- and inter-fractional liver and lung tumor motions treated with SBRT under active breathing control

Purpose

To assess intra‐ and inter‐fractional motions of liver and lung tumors using active breathing control (ABC).

Methods and Materials

Nineteen patients with liver cancer and 15 patients with lung cancer treated with stereotactic body radiotherapy (SBRT) were included in this retrospective study. All patients received a series of three CTs at simulation to test breath‐hold reproducibility. The centroids of the whole livers and of the lung tumors from the three CTs were compared to assess intra‐fraction variability. For 15 patients (8 liver, 7 lung), ABC‐gated kilovoltage cone‐beam CTs (kV‐CBCTs) were acquired prior to each treatment, and the centroids of the whole livers and of the lung tumors were also compared to those in the planning CTs to assess inter‐fraction variability.

Results

Liver intra‐fractional systematic/random errors were 0.75/0.39 mm, 1.36/0.97 mm, and 1.55/1.41 mm at medial‐lateral (ML), anterior‐posterior (AP), and superior‐inferior (SI) directions, respectively. Lung intra‐fractional systematic/random errors were 0.71/0.54 mm (ML), 1.45/1.10 mm (AP), and 3.95/1.93 mm (SI), respectively. Substantial intra‐fraction motions (>3 mm) were observed in 26.3% of liver cancer patients and in 46.7% of lung cancer patients. For both liver and lung tumors, most inter‐fractional systematic and random errors were larger than the corresponding intra‐fractional errors. However, these inter‐fractional errors were mostly corrected by the treatment team prior to each treatment based on kV CBCT‐guided soft tissue alignment, thereby eliminating their effects on the treatment planning margins.

Conclusions

Intra‐fractional motion is the key to determine the planning margins since inter‐fractional motion can be compensated based on daily gated soft tissue imaging guidance of CBCT. Patient‐specific treatment planning margins instead of recipe‐based margins were suggested, which can benefit mostly for the patients with small intra‐fractional motions.

Tumor motion changes in stereotactic body radiotherapy for liver tumors: an evaluation based on four-dimensional cone-beam computed tomography and fiducial markers

Background

For stereotactic body radiation therapy (SBRT) of liver tumors, tumor motion induced by respiration must be taken into account in planning and treatment. We evaluated whether liver tumor motion at the planning simulation represents liver tumor motion during SBRT, and estimated inter- and intrafractional tumor motion changes in patients undergoing liver SBRT.

Methods

Ten patients underwent four-dimensional cone-beam computed tomography (4D-CBCT) image-guided liver SBRT with abdominal compression (AC) and fiducial markers. 4D-CBCT was performed to evaluate liver tumor motion at the planning simulation, pre-, and post-SBRT. The translational distances at the center position of the fiducial markers from all 10 phases on the 4D-CBCT images were measured as the extent of the liver tumor motion in the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions. Pearson correlation coefficients were calculated to evaluate the correlation between liver tumor motion of the planning simulation and the mean liver tumor motion of the pre-SBRT. Inter- and intrafractional liver tumor motion changes were measured based on the 4D-CBCT of planning simulation, pre-, and post-SBRT. Significant inter- and intrafractional changes in liver tumor motion were defined as a change of >3 mm.

Results

The mean (± SD) liver tumor motion of the planning simulation 4D-CBCT was 1.7 ± 0.8 mm, 2.4 ± 2.2 mm, and 5.3 ± 3.3 mm, in the LR, AP, and SI directions, respectively. Those of the pre-SBRT 4D-CBCT were 1.2 ± 0.7 mm, 2.3 ± 2.3 mm, and 4.5 ± 3.8 mm, in the LR, AP, and SI directions, respectively. There was a strong significant correlation between liver tumor motion of the planning simulation and pre-SBRT in the LR (R = 0.7, P < 0.01), AP (R = 0.9, P < 0.01), and SI (R = 0.9, P < 0.01) directions. Significant inter- and intrafractional liver tumor motion changes occurred in 10 and 2% of treatment fractions, respectively.

Conclusions

Liver tumor motion at the planning simulation represents liver tumor motion during SBRT. Inter- and intrafractional liver tumor motion changes were small in patients with AC.

4D-CT scans reveal reduced magnitude of respiratory liver motion achieved by different abdominal compression plate positions in patients with intrahepatic tumors undergoing helical tomotherapy

PURPOSE

While abdominal compression (AC) can be used to reduce respiratory liver motion in patients receiving helical tomotherapy for hepatocellular carcinoma, the nature and extent of this effect is not well described. The purpose of this study was to evaluate the changes in magnitude of three-dimensional liver motion with abdominal compression using four-dimensional (4D) computed tomography (CT) images of several plate positions.

METHODS

From January 2012 to October 2015, 72 patients with intrahepatic carcinoma and divided into four groups underwent 4D-CT scans to assess respiratory liver motion. Of the 72 patients, 19 underwent abdominal compression of the cephalic area between the subxiphoid and umbilicus (group A), 16 underwent abdominal compression of the caudal region between the subxiphoid area and the umbilicus (group B), 11 patients underwent abdominal compression of the caudal umbilicus (group C), and 26 patients remained free breathing (group D). 4D-CT images were sorted into ten-image series, according to the respiratory phase from the end inspiration to the end expiration, and then transferred to treatment planning software. All liver contours were drawn by a single physician and confirmed by a second physician. Liver relative coordinates were automatically generated to calculate the liver respiratory motion in different axial directions to compile the 10 ten contours into a single composite image. Differences in respiratory liver motion were assessed with a one-way analysis of variance test of significance.

RESULTS

The average respiratory liver motion in the Y axial direction was 4.53 ± 1.16, 7.56 ± 1.30, 9.95 ± 2.32, and 9.53 ± 2.62 mm in groups A, B, C, and D, respectively, with a significant change among the four groups (p < 0.001). Abdominal compression was most effective in group A (compression plate on the subxiphoid area), with liver displacement being 2.53 ± 0.93, 4.53 ± 1.16, and 2.14 ± 0.92 mm on the X-, Y-, and Z-axes, respectively. There was no significant difference in respiratory liver motion between group C (displacement: 3.23 ± 1.47, 9.95 ± 2.32, and 2.92 ± 1.10 mm on the X-, Y-, and Z-axes, respectively) and group D (displacement: 3.35 ± 1.55, 9.53 ± 2.62, and 3.35 ± 1.73 mm on the X-, Y-, and Z-axes, respectively). Abdominal compression was least effective in group C (compression on caudal umbilicus), with liver motion in this group similar to that of free-breathing patients (group D).

CONCLUSIONS

4D-CT scans revealed significant liver motion control via abdominal compression of the subxiphoid area; however, this control of liver motion was not observed with compression of the caudal umbilicus. The authors, therefore, recommend compression of the subxiphoid area in patients undergoing external radiotherapy for intrahepatic carcinoma.

Target position uncertainty during visually guided deep-inspiration breath-hold radiotherapy in locally advanced lung cancer

PURPOSE

The purpose of this study was to estimate the uncertainty in voluntary deep-inspiration breath-hold (DIBH) radiotherapy for locally advanced non-small cell lung cancer (NSCLC) patients.

METHODS

Perpendicular fluoroscopic movies were acquired in free breathing (FB) and DIBH during a course of visually guided DIBH radiotherapy of nine patients with NSCLC. Patients had liquid markers injected in mediastinal lymph nodes and primary tumours. Excursion, systematic- and random errors, and inter-breath-hold position uncertainty were investigated using an image based tracking algorithm.

RESULTS

A mean reduction of 2-6mm in marker excursion in DIBH versus FB was seen in the anterior-posterior (AP), left-right (LR) and cranio-caudal (CC) directions. Lymph node motion during DIBH originated from cardiac motion. The systematic- (standard deviation (SD) of all the mean marker positions) and random errors (root-mean-square of the intra-BH SD) during DIBH were 0.5 and 0.3mm (AP), 0.5 and 0.3mm (LR), 0.8 and 0.4mm (CC), respectively. The mean inter-breath-hold shifts were -0.3mm (AP), -0.2mm (LR), and -0.2mm (CC).

CONCLUSION

Intra- and inter-breath-hold uncertainty of tumours and lymph nodes were small in visually guided breath-hold radiotherapy of NSCLC. Target motion could be substantially reduced, but not eliminated, using visually guided DIBH.

Magnitude and influencing factors of respiration-induced liver motion during abdominal compression in patients with intrahepatic tumors

PURPOSE

The purpose of this study was to use 4-dimensional-computed tomography (4D-CT) to evaluate respiration-induced liver motion magnitude and influencing factors in patients with intrahepatic tumors undergoing abdominal compression.

METHODS

From January 2012 to April 2016, 99 patients with intrahepatic tumors were included in this study. They all underwent 4D-CT to assess respiratory liver motion. This was performed during abdominal compression in 53 patients and during free-breathing (no abdominal compression) in 46 patients. We defined abdominal compression as being effective in managing the breath amplitude if respiration-induced liver motion in the cranial-caudal (CC) direction during compression was ≤5 mm and as being ineffective if >5 mm of motion was observed. Gender, age, body mass index (BMI), transarterial chemoembolization history, liver resection history, tumor area, tumor number, and tumor size (diameter) were determined. Multivariate logistic regression analysis was used to analyze influencing factors associated with a breath amplitude ≤5 mm in the CC direction.

RESULTS

The mean respiration-induced liver motion during abdominal compression in the left-right (LR), CC, anterior-posterior (AP), and 3-dimensional vector directions was 2.9 ± 1.2 mm, 5.3 ± 2.2 mm, 2.3 ± 1.1 mm and 6.7 ± 2.1 mm, respectively. Univariate analysis indicated that gender and BMI significantly affected abdominal compression effectiveness (both p < 0.05). Multivariate analysis confirmed these two factors as significant predictors of effective abdominal compression: gender (p = 0.030) and BMI (p = 0.006). There was a strong correlation between gender and compression effectiveness (odds ratio [OR] = 7.450) and an even stronger correlation between BMI and compression effectiveness (OR = 10.842).

CONCLUSIONS

The magnitude of respiration-induced liver motion of patients with intrahepatic carcinoma undergoing abdominal compression is affected by gender and BMI, with abdominal compression being less effective in men and overweight patients.

Evaluation of reproducibility of tumor repositioning during multiple breathing cycles for liver stereotactic body radiotherapy treatment

AIM

To evaluate the tumor repositioning during gated volumetric modulated arc therapy (VMAT) for liver stereotactic body radiotherapy(SBRT) treatment using implanted fiducial markers and intrafraction kilovoltage (kV) images acquired during dose delivery.

MATERIALS AND METHODS

Since 2012, 47 liver cancer patients with implanted fiducial markers were treated using the gated VMAT technique with a Varian Truebeam STx linear accelerator. The fiducial markers were implanted inside or close to the tumor target before treatment simulation. They were defined at the maximum inhalation and exhalation phases on a 4-dimensionnal computed tomography (4DCT) acquisition. During the treatment, kV images were acquired just before the beam-on at each breathing cycle at maximum exhalation and inhalation phases to verify the fiducial markers positions. For the five first fractions of treatment in the first ten consecutive patients, a total of 2705 intrafraction kV images were retrospectively analyzed to assess the differences between expected and actual positions of the fiducial markers along the cranio-caudal (CC) direction during the exhalation phase.

RESULTS

The mean absolute intrafractional fiducial marker deviation along the CC direction was 1.0 mm at the maximum exhalation phase. In 99%, 95% and 90% cases, the fiducial marker deviations were ≤4.5 mm, 2.8 mm and 2.2 mm, respectively.

CONCLUSION

Intrafraction kV images allowed us to ensure the consistency of tumor repositioning during treatment. In 99% cases, the fiducial marker deviations were ≤4.5 mm corresponding to our 5 mm treatment margin. This margin seems to be well-adapted to the gated VMAT SBRT treatment in liver disease.

Cone-Beam Computed Tomography Internal Motion Tracking Should Be Used to Validate 4-Dimensional Computed Tomography for Abdominal Radiation Therapy Patients

PURPOSE

To demonstrate that fiducial tracking during pretreatment Cone-Beam CT (CBCT) can accurately measure tumor motion and that this method should be used to validate 4-dimensional CT (4DCT) margins before each treatment fraction.

METHODS AND MATERIALS

For 31 patients with abdominal tumors and implanted fiducial markers, tumor motion was measured daily with CBCT and fluoroscopy for 202 treatment fractions. Fiducial tracking and maximum-likelihood algorithms extracted 3-dimensional fiducial trajectories from CBCT projections. The daily internal margin (IM) (ie, range of fiducial motion) was calculated for CBCT and fluoroscopy as the 5th-95th percentiles of displacement in each cardinal direction. The planning IM from simulation 4DCT (IM4DCT) was considered adequate when within ±1.2 mm (anterior-posterior, left-right) and ±3 mm (superior-inferior) of the daily measured IM. We validated CBCT fiducial tracking as an accurate predictive measure of intrafraction motion by comparing the daily measured IMCBCT with the daily IM measured by pretreatment fluoroscopy (IMpre-fluoro); these were compared with pre- and posttreatment fluoroscopy (IMfluoro) to identify those patients who could benefit from imaging during treatment.

RESULTS

Four-dimensional CT could not accurately predict intrafractional tumor motion for ≥80% of fractions in 94% (IMCBCT), 97% (IMpre-fluoro), and 100% (IMfluoro) of patients. The IMCBCT was significantly closer to IMpre-fluoro than IM4DCT (P<.01). For patients with median treatment time t < 7.5 minutes, IMCBCT was in agreement with IMfluoro for 93% of fractions (superior-inferior), compared with 63% for the t > 7.5 minutes group, demonstrating the need for patient-specific intratreatment imaging.

CONCLUSIONS

Tumor motion determined from 4DCT simulation does not accurately predict the daily motion observed on CBCT or fluoroscopy. Cone-beam CT could replace fluoroscopy for pretreatment verification of simulation IM4DCT, reducing patient setup time and imaging dose. Patients with treatment time t > 7.5 minutes could benefit from the addition of intratreatment imaging.

Intravenous contrast-enhanced cone beam computed tomography (IVCBCT) of intrahepatic tumors and vessels

Purpose

Liver tumors are challenging to visualize on cone beam computed tomography (CBCT) without intravenous (IV) contrast. Image guidance for liver cancer stereotactic body ablative radiation therapy (SABR) could be improved with the direct visualization of hepatic tumors and vasculature. This study investigated the feasibility of the use of IV contrast-enhanced CBCT (IV-CBCT) as a means to improve liver target visualization.

Methods and Materials

Patients on a liver SABR protocol underwent IV-CBCT before 1 or more treatment fractions in addition to a noncontrast CBCT. Image acquisition was initiated 0 to 30 seconds following injection and acquired over 60 to 120 seconds. “Stop and go” exhale breath-hold CBCT scans were used whenever feasible. Changes in mean CT number in regions of interest within visible vasculature, tumor, and adjacent liver were quantified between CBCT and IV-CBCT.

Results

Twelve pairs of contrast and noncontrast CBCTs were obtained in 7 patients. Intravenous-CBCT improved hepatic tumor visibility in breath-hold scans only for 3 patients (2 metastases, 1 hepatocellular carcinoma). Visible tumors ranged in volume from 124 to 564 mL. Small tumors in free-breathing patients did not show enhancement on IVCBT.

Conclusions

Intravenous-CBCT may enhance the visibility of hepatic vessels and tumor in CBCT scans obtained during breath hold. Optimization of IV contrast timing and reduction of artifacts to improve tumor visualization warrant further investigation.

A single reference measurement can predict liver tumor motion during respiration

AIM

To evaluate liver tumor motion and how well reference measurement predicts motion during treatment.

MATERIAL AND METHODS

This retrospective study included 20 patients with colorectal cancer that had metastasized to the liver who were treated with stereotactic ablative radiotherapy. An online respiratory tumor tracking system was used. Tumor motion amplitudes in the superior-inferior (SI), latero-lateral (LL), and anterior-posterior (AP) directions were collected to generate patient-specific margins. Reference margins were generated as the mean motion and 95th percentile of motion from measurements recorded for different lengths of time (1, 3, and 5 min). We analyzed the predictability of tumor motion in each axis, based on the reference measurement and intra-/interfraction motions.

RESULTS

About 96,000 amplitudes were analyzed. The mean tumor motions were 9.9 ± 4.2 mm, 2.6 ± 0.8 mm, and 4.5 ± 1.8 mm in the SI, LL, and AP directions, respectively. The intrafraction variations were 3.5 ± 1.8 mm, 0.63 ± 0.35 mm, and 1.4 ± 0.65 mm for the SI, LL, and AP directions, respectively. The interfraction motion variations were 1.32 ± 0.79 mm, 0.31 ± 0.23 mm, and 0.68 ± 0.62 mm for the SI, LL, and AP directions, respectively. The Pearson's correlation coefficients for margins based on the reference measurement (mean motion or 95th percentile) and margins covering 95% of the motion during the whole treatment were 0.8-0.91, 0.57-0.7, and 0.77-0.82 in the SI, LL, and AP directions, respectively.

CONCLUSION

Liver tumor motion in the SI direction can be adequately represented by the mean tumor motion amplitude generated from a single 1 min reference measurement. Longer reference measurements did not improve results for patients who were well-educated about the importance of regular breathing. Although the study was based on tumor tracking data, the results are useful for ITV delineation when tumor tracking is not available.

Changes in Liver Volume Observed Following Sorafenib and Liver Radiation Therapy

PURPOSE

The purpose of this study was to quantify unexpected liver volume reductions in patients treated with sorafenib prior to and during liver radiation therapy (RT).

METHODS AND MATERIALS

Fifteen patients were treated in a phase 1 study of sorafenib for 1 week, followed by concurrent sorafenib-RT (in 6 fractions). Patients had either focal cancer (treated with stereotactic body RT [SBRT]) or diffuse disease (treated with whole-liver RT). Liver volumes were contoured and recorded at planning (day 0) from the exhale CT. After 1 week of sorafenib (day 8), RT image guidance at each fraction was performed using cone beam CT (CBCT). Planning liver contours were propagated and modified on the reconstructed exhale CBCT. This was repeated in 12 patients treated with SBRT alone without sorafenib. Three subsequent patients (2 sorafenib-RT and 1 non-sorafenib) were also assessed with multiphasic helical breath-hold CTs.

RESULTS

Liver volume reductions on CBCT were observed in the 15 sorafenib-RT patients (median decrease of 68 cc, P=.02) between day 0 and 8; greater in the focal (P=.025) versus diffuse (P=.52) cancer stratum. Seven patients (47%) had reductions larger than the 95% intraobserver contouring error. Liver reductions were also observed from multiphasic CTs in the 2 additional sorafenib-RT patients between days 0 and 8 (decreases of 232.5 cc and 331.7 cc, respectively) and not in the non-sorafenib patient (increase of 92 cc). There were no significant changes in liver volume between planning and first RT in 12 patients with focal cancer treated with SBRT alone (median increase, 4.8 cc, P=.86).

CONCLUSIONS

Liver volume reductions were observed after 7 days of sorafenib, prior to RT, most marked in patients with focal liver tumors, suggesting an effect of sorafenib on normal liver. Careful assessment of potential liver volume changes immediately prior to SBRT may be necessary in patients in sorafenib or other targeted therapies.

Lipiodol versus diaphragm in 4D-CBCT-guided stereotactic radiotherapy of hepatocellular carcinomas

PURPOSE

The purpose of this work was to investigate the potential of lipiodol as a direct tumor surrogate alternative to the diaphragm surrogate on four-dimensional cone-beam computed tomography (4D-CBCT) image guidance for stereotactic radiotherapy of hepatocellular carcinomas.

METHODS

A total of 29 hepatocellular carcinomas (HCC) patients treated by stereotactic radiotherapy following transarterial chemoembolization (TACE) with homogeneous or partial defective lipiodol retention were included. In all, 4-7 pretreatment 4D-CBCT scans were selected for each patient. For each scan, either lipiodol or the diaphragm was used for 4D registration. Resulting lipiodol/diaphragm motion ranges and position errors relative to the reconstructed midventilation images were analyzed to obtain the motion variations, and group mean (ΔM), systematic (Σ), and random (σ) errors of the treatment setup.

RESULTS

Of the lipiodolized tumors, 55 % qualified for direct localization on the 4D-CBCT. Significant correlations of lipiodol and diaphragm positions were found in the left-right (LR), craniocaudal (CC), and anteroposterior (AP) directions. ΔM and σ obtained with lipiodol and diaphragm were similar, agreed to within 0.5 mm in the LR and AP, and 0.3 mm in the CC directions, and Σ differed by 1.4 (LR), 1.1 (CC), and 0.6 (AP) mm. Variations of diaphragm motion range > 5 mm were not observed with lipiodol and in one patient with diaphragm. The margin required for the tumor prediction error using the diaphragm surrogate was 6.7 (LR), 11.7 (CC), and 4.1 (AP) mm.

CONCLUSION

Image-guidance combining lipiodol with 4D-CBCT enabled accurate localization of HCC and thus margin reduction. A major limitation was the degraded lipiodol contrast on 4D-CBCT.