Internal movement, set-up accuracy and margins for stereotactic body radiotherapy using a stereotactic body frame

Are liver contour and bone fusion comparable to fiducials for IGRT in liver SBRT?

Introduction

Liver stereotactic body radiotherapy (SBRT) is increasingly being used to treat tumours. The purpose of this study was to compare the differences in patient positioning when using implanted fiducials as surrogates compared to alternative methods based on liver contour or bone registration.

Material and methods

Eighteen patients treated with SBRT who underwent a fiducial placement procedure were included. Fiducial guidance was our gold standard to guide treatment in this study. After recording the displacements, when fusing the planning CT and CBCT performed in the treatment unit using fiducials, liver contour and bone reference, the differences between fiducials and liver contour and bone reference were calculated. Data from 88 CBCT were analyzed. The correlation between the displacements found with fiducials and those performed based on the liver contour and the nearest bone structure as references was determined. The mean, median, variance, range and standard deviation of the displacements with each of the fusion methods were obtained. μ, Ʃ, and σ values and margins were obtained.

Results

Lateral displacements of less than 3 mm with respect to the gold standard in 92% vs. 62.5% of cases using liver contour and bone references, respectively, with 93.2% vs. 65.9% in the AP axis and SI movement in 69.3% vs. 51.1%. The errors μ, σ and Ʃ of the fusions with hepatic contour and bone reference in SI were 0.26 mm, 4 mm and 3 mm, and 0.8 mm, 5 mm and 3 mm respectively.

Conclusion

Our study showed that displacements were smaller with the use of hepatic contour compared to bone reference and comparable to those obtained with the use of fiducials in the lateral, AP and SI motion axes. This would justify that hepatic contouring can be a guide in the treatment of patients in the absence of fiducials.

Intra- and inter-physician variability in target volume delineation in radiation therapy

Reduction in setup errors is advocated through daily imaging and adaptive therapy, where the target volume is drawn daily. Previous studies suggest that inter-physician volume variation is significant (1.5 cm standard deviation [SD]); however, there are limited data for intra-physician consistency in daily target volume delineation, which is investigated in this study. Seven patients with lung cancer were chosen based on the perceived difficulty of contouring their disease, varying from simple parenchymal lung nodules to lesions with extensive adjacent atelectasis. Four physicians delineated the gross tumor volume (GTV) for each patient on 10 separate days to see the intra- and inter-physician contouring. Isocenter coordinates (x, y and z), target volume (cm3), and largest dimensions on anterior-posterior (AP) and lateral views were recorded for each GTV. Our results show that the variability among the physicians was reflected by target volumes ranging from +109% to -86% from the mean while isocenter coordinate changes were minimal; 3.8, 1.7 and 1.9 mm for x, y and z coordinates, respectively. The orthogonal image (AP and lateral) change varied 16.3 mm and 15.0 mm respectively among days and physicians. We conclude than when performing daily imaging, random variability in contouring resulted in isocenter changes up to ±3.8 mm in our study. The shape of the target varied within ±16 mm. This study suggests that when using daily imaging to track isocenter, target volume, or treatment parameters, physicians should be aware of personal variability when considering margins added to the target volume in daily decision making especially for difficult cases.

Surface guided frameless positioning for lung stereotactic body radiation therapy

Background and purpose

When treating lung tumors with stereotactic body radiation therapy (SBRT), patient immobilization is of outmost importance. In this study, the intra‐fractional shifts of the patient (based on bony anatomy) and the tumor (based on the visible target volume) are quantified, and the associated impact on the delivered dose is estimated for a frameless immobilization approach in combination with surface guided radiation therapy (SGRT) monitoring.

Methods

Cone beam computed tomographies (CBCT) were collected in free breathing prior and after each treatment for 25 patients with lung tumors, in total 137 fractions. The CBCT collected after each treatment was registered to the CBCT collected before each treatment with focus on bony anatomy to determine the shift of the patient, and with focus on the visible target volume to determine the shift of the tumor. Rigid registrations with 6 degrees of freedom were used. The patients were positioned in frameless immobilizations with their position and respiration continuously monitored by a commercial SGRT system. The patients were breathing freely within a preset gating window during treatment delivery. The beam was automatically interrupted if isocenter shifts >4 mm or breathing amplitudes outside the gating window were detected by the SGRT system. The time between the acquisition of the CBCTs was registered for each fraction to examine correlations between treatment time and patient shift. The impact of the observed shifts on the dose to organs at risk (OAR) and the gross tumor volume (GTV) was assessed.

Results

The shift of the patient in the CBCTs was ≤2 mm for 132/137 fractions in the vertical (vrt) and lateral (lat) directions, and 134/137 fractions in the longitudinal (lng) direction and ≤4 mm in 134/137 (vrt) and 137/137 (lat, lng) of the fractions. The shift of the tumor was ≤2 mm in 116/137 (vrt), 123/137 (lat) and 115/137 (lng) fractions and ≤4 mm in 136/137 (vrt), 137/137 (lat), and 135/137 (lng) fractions. The maximal observed shift in the evaluated CBCT data was 4.6 mm for the patient and 7.2 mm for the tumor. Rotations were ≤3.3ᵒ for all fractions and the mean/standard deviation were 0.2/1.0ᵒ (roll), 0.1/0.8ᵒ (yaw), and 0.3/1.0ᵒ (pitch). The SGRT system interrupted the beam due to intra‐fractional isocenter shifts >4 mm for 21% of the fractions, but the patients always returned within tolerance without the need of repositioning. The maximal observed isocenter shift by the SGRT system during the beam holds was 8 mm. For the respiration monitoring, the beam was interrupted at least one time for 54% of the fractions.

The visual tumor was within the planned internal target volume (ITV) for 136/137 fractions in the evaluated CBCT data collected at the end of each fraction. For the fraction where the tumor was outside the ITV, the D_98% for the GTV decreased with 0.4 Gy. For the OARs, the difference between planned and estimated dose from the CBCT data (D_2% or D_mean) was ≤2.6% of the prescribed PTV dose. No correlation was found between treatment time and the magnitude of the patient shift.

Conclusions

Using SGRT for motion management and respiration monitoring in combination with a frameless immobilization is a feasible approach for lung SBRT.

Tumour motion management in lung cancer: a narrative review

Respiratory motion is one of the geometrical uncertainties that may affect the accuracy of thoracic radiotherapy in the treatment of lung cancer. Accounting for tumour motion may allow reducing treatment volumes, irradiated healthy tissue and possibly toxicity, and finally enabling dose escalation. Historically, large population-based margins were used to encompass tumour motion. A paradigmatic change happened in the last decades led to the development of modern imaging techniques during the simulation and the delivery, such as the 4-dimensional (4D) computed tomography (CT) or the 4D-cone beam CT scan, has contributed to a better understanding of lung tumour motion and to the widespread use of individualised margins (with either an internal tumour volume approach or a mid-position/ventilation approach). Moreover, recent technological advances in the delivery of radiotherapy treatments (with a variety of commercial solution allowing tumour tracking, gating or treatments in deep-inspiration breath-hold) conjugate the necessity of minimising treatment volumes while maximizing the patient comfort with less invasive techniques. In this narrative review, we provided an introduction on the intra-fraction tumour motion (in both lung tumours and mediastinal lymph-nodes), and summarized the principal motion management strategies (in both the imaging and the treatment delivery) in thoracic radiotherapy for lung cancer, with an eye on the clinical outcomes.

Image-guided Radiotherapy to Manage Respiratory Motion: Lung and Liver

Organ motion as a result of respiratory and cardiac motion poses significant challenges for the accurate delivery of radiotherapy to both the thorax and the upper abdomen. Modern imaging techniques during radiotherapy simulation and delivery now permit better quantification of organ motion, which in turn reduces tumour and organ at risk position uncertainty. These imaging advances, coupled with respiratory correlated radiotherapy delivery techniques, have led to the development of a range of approaches to manage respiratory motion. This review summarises the key strategies of image-guided respiratory motion management with a focus on lung and liver radiotherapy.

MRI Assessment of Hepatocellular Carcinoma after Local-Regional Therapy: A Comprehensive Review

Nearly 80% of cirrhotic patients diagnosed with hepatocellular carcinoma (HCC) are not eligible for surgical resection and instead undergo local-regional treatment. After therapy for HCC, patients undergo imaging surveillance to assess treatment efficacy and identify potential sites of progressive tumor elsewhere within the liver. Accurate interpretation of posttreatment imaging is essential for guiding further management decisions, and radiologists must understand expected treatment-specific imaging findings for each of the local-regional therapies. Of interest, expected imaging findings seen after radiation-based therapies (transarterial radioembolization and stereotactic body radiation therapy) are different than those seen after thermal ablation and transarterial chemoembolization. Given differences in expected posttreatment imaging findings, the current radiologic treatment response assessment algorithms used for HCC (modified Response Evaluation Criteria in Solid Tumors classification, European Association for the Study of Liver Diseases criteria, and Liver Imaging and Reporting Data System Treatment Response Algorithm) must be applied cautiously for radiation-based therapies in which persistent arterial phase hyperenhancement in the early posttreatment period is common and expected. This article will review the concept of tumor response assessment for HCC, the forms of local-regional therapy for HCC, and the expected posttreatment findings for each form of therapy. Keywords: Abdomen/GI, Liver, MR-Imaging, Treatment Effects, Tumor Response © RSNA, 2020.

Questionnaire survey on treatment planning techniques for lung stereotactic body radiotherapy in Japan

This study aimed to obtain details regarding treatment planning techniques for lung stereotactic body radiation therapy (SBRT) employed at each institution in Japan by using a questionnaire survey. An Internet questionnaire survey on SBRT procedures performed in 2016 was conducted by the QA/QC committee of the Japan Society of Medical Physics from April to June 2017. The questionnaire assessed two aspects: the environment for SBRT at each institution and the treatment planning techniques with and without respiratory motion management techniques (RMMT). Of the 309 evaluated responses, 218 institutions had performed SBRT. A total of 186 institutions performed SBRT without RMMT and 139 institutions performed SBRT with RMMT. When respiratory motion was ≥10 mm, 69 institutions applied RMMT. The leading RMMT were breath holding (77 institutions), respiratory gating (49 institutions) and real-time tumor tracking (11 institutions). The most frequently used irradiation technique was 3D conformal radiotherapy, which was used in 145 institutions without RMMT and 119 institutions with RMMT. Computed tomography (CT) images acquired under free breathing were mostly used for dose calculation for patients treated without RMMT. The usage ratio of IMRT/VMAT to SBRT is low in Japan, compared to elsewhere in the world (<20% vs ≥70%). Among the available dose calculation algorithms, superposition convolution was the most frequently used regardless of RMMT; however, 2% of institutions have not yet made heterogeneity corrections. In the prescription setting, about half of the institutions applied point prescriptions. The survey results revealed the most frequently used conditions, which may facilitate standardization of treatment techniques in lung SBRT.

Evaluation of the use of abdominal compression of the lung in stereotactic radiation therapy

The goal of this retrospective study was to determine the benefit in using abdominal compression to reduce tumor motion for patients treated with lung stereotactic body radiotherapy. Forty-four lung lesions (n = 44) from 37 patients (N = 37) treated at the University of Toledo's Dana Cancer Center were assessed by determining the overall tumor displacement along with possible surrogates such as change in tidal volume and diaphragm displacement, with and without abdominal compression. Measurements of lung capacity were acquired from the 4DCT at maximum and minimum respiration in order to determine the tidal volume, with and without abdominal compression. Tumor centroid and diaphragm apex motion was then assessed in 3 dimensions from phase 0 to phase 50. This was measured in centimeters using the ruler method on MIM software, both with and without the compression belt. Change in overall tumor movement was 0.61 cm ± 0.09 cm with compression, and 0.60 cm ± 0.09 cm without the compression belt. Delta tumor motion was reduced in 5 cases, increased (made worse) in 6 cases, and did not clinically impact the remaining 33 cases. Average tidal volume with abdominal compression was 379.7 mL or 12.0% ± 0.724% of total lung volume while average tidal volume without abdominal compression was 337.7 mL or 10.5% ± 0.649% of total lung volume. Change in diaphragm position throughout the breathing cycle was 1.21 cm ± 0.10 cm with compression, and 1.28 ± 0.13 cm without the compression belt. These findings indicate that abdominal compression may not be an effective method in the reduction of respiratory motion, and can even negatively impact tumor motion by increasing its displacement. Compression decreased tumor motion in 5 out of the 44 cases studied. The 5 cases that benefitted tended to be lesions close to the diaphragm but these 5 corresponded to less than half of the inferior lesions, suggesting that even inferior lung lesions may not be prime candidates for abdominal compression.

Consensus on Stereotactic Body Radiation Therapy for Small-Sized Hepatocellular Carcinoma at the 7th Asia-Pacific Primary Liver Cancer Expert Meeting

BACKGROUND

Stereotactic body radiation therapy (SBRT) is an advanced technique of external beam radiation therapy that delivers large ablative doses of radiation. In the past decade, many cancer centers have adopted SBRT as one mode of radically treating small-sized hepatocellular carcinoma (HCC), based on encouraging clinical outcomes. SBRT thus seems reasonable as first-line treatment of inoperable HCC confined to the liver. However, most of the clinical studies to date have been retrospective in nature, with key issues still under investigation.

SUMMARY

The above-mentioned publications were subjected to scrutiny, fueling discussions at the 7th Asia-Pacific Primary Liver Cancer Expert (APPLE 2016) Meeting on various clinical variables, such as indications for SBRT, therapeutic outcomes, treatment-related toxicities, doses prescribed, and specific techniques. The consensus reached should be of interest to all professionals active in the treatment of HCC, especially radiation oncologists.

KEY MESSAGES

SBRT is a safe and effective therapeutic option for patients with small-sized HCC, offering substantial local control, improved overall survival, and low toxicity.

[Stereotactic body radiotherapy for liver tumors: State of the art]

Thanks to the improvement in radiotherapy physics, biology, computing and imaging, patients presenting with liver tumors can be efficiently treated by radiation. Radiotherapy has been included in liver tumors treatment guidelines at all disease stages. Liver stereotactic radiotherapy has to be preferred to standard fractionated radiotherapy whenever possible, as potentially more efficient because of higher biological equivalent dose. Liver stereotactic radiotherapy planning and delivery require extensive experience and optimal treatment quality at every step, thus limiting its availability to specialized centres. Multicentre studies are difficult to develop due to a large technical heterogeneity. Respiratory management, image guidance and immobilization are considerations as important as machine type. The use of multimodal planning imaging is compulsory to achieve expected contouring quality. Treatment efficacy is difficult to assess following liver stereotactic radiotherapy, but local control is high and toxicity unusual. As a consequence, liver stereotactic radiotherapy is part of multimodal and multidisciplinary management of liver tumors.

Cone beam CT-based set-up strategies with and without rotational correction for stereotactic body radiation therapy in the liver

BACKGROUND

Accurate patient positioning is crucial in stereotactic body radiation therapy (SBRT) due to a high dose regimen. Cone-beam computed tomography (CBCT) is often used for patient positioning based on radio-opaque markers. We compared six CBCT-based set-up strategies with or without rotational correction.

MATERIAL AND METHODS

Twenty-nine patients with three implanted markers received 3-6 fraction liver SBRT. The markers were delineated on the mid-ventilation phase of a 4D-planning-CT. One pretreatment CBCT was acquired per fraction. Set-up strategy 1 used only translational correction based on manual marker match between the CBCT and planning CT. Set-up strategy 2 used automatic 6 degrees-of-freedom registration of the vertebrae closest to the target. The 3D marker trajectories were also extracted from the projections and the mean position of each marker was calculated and used for set-up strategies 3-6. Translational correction only was used for strategy 3. Translational and rotational corrections were used for strategies 4-6 with the rotation being either vertebrae based (strategy 4), or marker based and constrained to ±3° (strategy 5) or unconstrained (strategy 6). The resulting set-up error was calculated as the 3D root-mean-square set-up error of the three markers. The set-up error of the spinal cord was calculated for all strategies.

RESULTS

The bony anatomy set-up (2) had the largest set-up error (5.8 mm). The marker-based set-up with unconstrained rotations (6) had the smallest set-up error (0.8 mm) but the largest spinal cord set-up error (12.1 mm). The marker-based set-up with translational correction only (3) or with bony anatomy rotational correction (4) had equivalent set-up error (1.3 mm) but rotational correction reduced the spinal cord set-up error from 4.1 mm to 3.5 mm.

CONCLUSIONS

Marker-based set-up was substantially better than bony-anatomy set-up. Rotational correction may improve the set-up, but further investigations are required to determine the optimal correction strategy.

Evaluation of initial setup errors of two immobilization devices for lung stereotactic body radiation therapy (SBRT)

The aim of this study was to investigate the accuracy and efficacy of two commonly used commercial immobilization systems for stereotactic body radiation therapy (SBRT) in lung cancer. This retrospective study assessed the efficacy and setup accuracy of two immobilization systems: the Elekta Body Frame (EBF) and the Civco Body Pro‐Lok (CBP) in 80 patients evenly divided for each system. A cone beam CT (CBCT) was used before each treatment fraction for setup correction in both devices. Analyzed shifts were applied for setup correction and CBCT was repeated. If a large shift (>5 mm) occurred in any direction, an additional CBCT was employed for verification after localization. The efficacy of patient setup was analyzed for 105 sessions (48 with the EBF, 57 with the CBP). Result indicates that the CBCT was repeated at the 1^st treatment session in 22.5% and 47.5% of the EBF and CBP cases, respectively. The systematic errors {left–right (LR), anterior–posterior (AP), cranio‐caudal (CC), and 3D vector shift: (LR² + AP² + CC²)^1/2 (mm)}, were {0.5 ± 3.7, 2.3 ± 2.5, 0.7 ± 3.5, 7.1 ± 3.1} mm and {0.4 ± 3.6, 0.7 ± 4.0, 0.0 ± 5.5, 9.2 ± 4.2} mm, and the random setup errors were {5.1, 3.0, 3.5, 3.9} mm and {4.6, 4.8, 5.4, 5.3} mm for the EBF and the CBP, respectively. The 3D vector shift was significantly larger for the CBP (P < 0.01). The setup time was slightly longer for the EBF (EBF: 15.1 min, CBP: 13.7 min), but the difference was not statistically significant. It is concluded that adequate accuracy in SBRT can be achieved with either system if image guidance is used. However, patient comfort could dictate the use of CBP system with slightly reduced accuracy.

SBRT planning for liver metastases: A focus on immobilization, motion management and planning imaging techniques

AIM

To evaluate the different techniques used for liver metastases Stereotactic Body Radiation Therapy (SBRT) planning. We especially focused on immobilization devices, motion management and imaging used for contouring.

BACKGROUND

Although some guidelines exist, there is no consensus regarding the minimal requirements for liver SBRT treatments.

MATERIALS AND METHODS

We reviewed the main liver metastases SBRT publications and guidelines; and compared the techniques used for immobilization, motion management, margins and imaging.

RESULTS

There is a wide variety of techniques used for immobilization, motion management and planning imaging.

CONCLUSIONS

We provide a subjective critical analysis of minimal requirements and ideal technique for liver SBRT planning.

Stereotactic body radiotherapy: an effective local treatment modality for hepatocellular carcinoma

Although liver-directed therapies such as surgery or ablation can cure hepatocellular carcinoma, few patients are eligible due to advanced disease or medical comorbidities. In advanced disease, systemic therapies have yielded only incremental survival benefits. Historically, radiotherapy for liver cancer was dismissed due to concerns over unacceptable toxicities from even moderate doses. Although implementation requires more resources than standard radiotherapy, stereotactic body radiotherapy can deliver reproducible, highly conformal ablative radiotherapy to tumors while minimizing doses to nearby critical structures. Trials of stereotactic body radiotherapy for hepatocellular carcinoma have demonstrated promising local control and survival results with low levels of toxicity in Child-Pugh class A patients. We review the published literature and make recommendations for the future of this emerging modality.

Integrating respiratory-gated PET-based target volume delineation in liver SBRT planning, a pilot study

Background

To assess the feasibility and benefit of integrating four-dimensional (4D) Positron Emission Tomography (PET) – computed tomography (CT) for liver stereotactic body radiation therapy (SBRT) planning.

Methods

8 patients with 14 metastases were accrued in the study. They all underwent a non-gated PET and a 4D PET centered on the liver. The same CT scan was used for attenuation correction, registration, and considered the planning CT for SBRT planning. Six PET phases were reconstructed for each 4D PET. By applying an individualized threshold to the 4D PET, a Biological Internal Target Volume (BITV) was generated for each lesion. A gated Planning Target Volume (PTVg) was created by adding 3 mm to account for set-up margins. This volume was compared to a manual Planning Target Volume (PTV) delineated with the help of a semi-automatic Biological Target Volume (BTV) obtained from the non-gated exam. A 5 mm radial and a 10 mm craniocaudal margins were applied to account for tumor motion and set-up margins to create the PTV.

Results

One undiagnosed liver metastasis was discovered thanks to the 4D PET. The semi-automatic BTV were significantly smaller than the BITV (p = 0.0031). However, after applying adapted margins, 4D PET allowed a statistically significant decrease in the PTVg as compared to the PTV (p = 0.0052).

Conclusions

In comparison to non-gated PET, 4D PET may better define the respiratory movements of liver targets and improve SBRT planning for liver metastases. Furthermore, non respiratory-gated PET exams can both misdiagnose liver metastases and underestimate the real internal target volumes.

Digital tomosynthesis for respiratory gated liver treatment: clinical feasibility for daily image guidance

PURPOSE

Breath-hold (BH) treatment minimizes internal target volumes (ITV) when treating sites prone to motion. Digital tomosynthesis (DTS) imaging has advantages over cone-beam CT (CBCT) for BH imaging: BH-DTS scan can be completed during a single breath-hold, whereas BH-CBCT is usually acquired by parsing the gantry rotation into multiple BH segments. This study evaluates the localization accuracy of DTS for BH treatment of liver tumors.

METHODS

Both planning CT and on-board DTS/CBCT images were acquired under BH, using the planning CT BH window as reference. Onboard imaging data sets included two independent DTS orientations (coronal and sagittal), and CBCT images. Soft tissue target positioning was measured by each imaging modality and translated into couch shifts. Performance of the two DTS orientations was evaluated by comparing target positioning with the CBCT benchmark, determined by two observers.

RESULTS

Image data sets were collected from thirty-eight treatment fractions (14 patients). Mean differences between the two DTS methods and the CBCT method were <1 mm in all directions (except the lateral direction with sagittal-DTS: 1.2 mm); the standard deviation was in the range of 2.1-3.5 mm for all techniques. The Pearson correlation showed good interobserver agreement for the coronal-DTS (0.72-0.78). The interobserver agreement for the sagittal-DTS was good for the in-plane directions (0.70-0.82), but poor in the out-of-plane direction (lateral, 0.26).

CONCLUSIONS

BH-DTS may be a simpler alternative to BH-CBCT for onboard soft tissue localization of the liver, although the precision of DTS localization appears to be somewhat lower because of the presence of subtle out-of-plane blur.

Potentials and limitations of guiding liver stereotactic body radiation therapy set-up on liver-implanted fiducial markers

PURPOSE

We investigated the potentials and limitations of guiding liver stereotactic body radiation therapy (SBRT) set-up on liver-implanted fiducial markers.

METHODS AND MATERIALS

Twelve patients undergoing compression-supported SBRT in a stereotactic body frame received fluoroscopy at treatment preparation and before each treatment fraction. In fluoroscopic videos we localized the markers and diaphragm tip at expiration and the spine (measurements on free-breathing and abdominal compression). Day-to-day displacements, rotations (markers only), and deformations were determined. Marker guidance was compared to conventional set-up strategies in treatment set-up simulations.

RESULTS

For compression, day-to-day motion of markers with respect to their centers of mass (COM) was sigma = 0.9 mm (random error SD), Sigma = 0.4 mm (systematic error SD), and <2.1 mm (maximum). Consequently, assuming that markers were closely surrounding spherical tumors, marker COM-guided set-up would have required safety margins of approximately 2 mm. Using marker COM as the gold standard, other set-up methods (using no correction, spine registration, and diaphragm tip craniocaudal registration) resulted in set-up errors of 1.4 mm < sigma < 2.8 mm, 2.6 mm < Sigma < 5.1 mm, and 6.3 mm < max < 12.4 mm. Day-to-day intermarker motion of <16.7%, 2.2% median, and rotations between 3.5 degrees and 7.2 degrees were observed. For markers not surrounding the tumor, e.g., 5 cm between respective COMs, these changes could effect residual tumor set-up errors up to 8.4 mm, 1.1 mm median (deformations), and 3.1 mm to 6.3 mm (rotations). Compression did not systematically contribute to deformations and rotations, since similar results were observed for free-breathing.

CONCLUSIONS

If markers can be implanted near and around the tumor, residual set-up errors by marker guidance are small compared to those of conventional set-up methods, allowing high-precision tumor radiation set-up. However, substantial errors may result if markers are not implanted precisely, requiring further research to obtain adequate safety margins.

On-line image guidance for frameless stereotactic radiotherapy of lung malignancies by cone beam CT: comparison between target localization and alignment on bony anatomy

INTRODUCTION

Free-breathing stereotactic radiotherapy for lung malignancies requires reliable prediction of respiratory motion and accurate target localization. A protocol was adopted for reproducibility and reduction of respiratory motion and for target localization by CBCT image guidance. Tumor respiratory displacements and tumor positioning errors relative to bony anatomy alignment are analyzed.

MATERIALS AND METHOD

Image guided SRT was performed for 99 lung malignancies. Two groups of patients were considered: group A did not perform any breathing control; group B controlled visually their respiratory cycle and volumes on an Active Breathing Coordinator (ABC) monitor during the acquisition of simulation CT and CBCT, and treatment delivery. GTV on end inhale and exhale CT data sets were fused in an ITV and the extent of tumor motion evaluated between these 2 phases. A pre-treatment CBCT was acquired and aligned to the reference CT using bony anatomy; for tumor positioning the ITV contour on the reference CT was matched to the visible tumor on CBCT. Interobserver variability of tumor positioning was evaluated. ITV and CBCT tumor dimensions were compared.

RESULTS

3D tumor breathing displacement (mean+/-SD) was significantly higher for group A (14.7+/-9.9 mm) than for group B (4.7+/-3.1 mm). The detected differences between tumor and bony structure alignment below 3 mm were 68% for group B and 45% for group A, reaching statistical significance. Interobserver variability was 1.7+/-1.1 mm (mean+/-SD). Dimensions of tumor image on CBCT were consistent with ITV dimensions for group B (max difference 14%).

CONCLUSIONS

The adopted protocol seems effective in reducing respiratory internal movements and margin. Tumor positioning errors relative to bony anatomy are also reduced. However bony anatomy as a surrogate of the target may still lead to some relevant positioning errors. Target visualization on CBCT is essential for an accurate localization in lung SRT.

Refinement of treatment setup and target localization accuracy using three-dimensional cone-beam computed tomography for stereotactic body radiotherapy

PURPOSES

To quantitatively compare two-dimensional (2D) orthogonal kV with three-dimensional (3D) cone-beam CT (CBCT) for target localization; and to assess intrafraction motion with kV images in patients undergoing stereotactic body radiotherapy (SBRT).

METHODS AND MATERIALS

A total of 50 patients with 58 lesions received 178 fractions of SBRT. After clinical setup using in-room lasers and skin/cradle marks placed at simulation, patients were imaged and repositioned according to orthogonal kV/MV registration of bony landmarks to digitally reconstructed radiographs from the planning CT. A subsequent CBCT was registered to the planning CT using soft tissue information, and the resultant "residual error" was measured and corrected before treatment. Posttreatment 2D kV and/or 3D CBCT images were compared with pretreatment images to determine any intrafractional position changes. Absolute averages, statistical means, standard deviations, and root mean square (RMS) values of observed setup error were calculated.

RESULTS

After initial setup to external marks with laser guidance, 2D kV images revealed vector mean setup deviations of 0.67 cm (RMS). Cone-beam CT detected residual setup deviations of 0.41 cm (RMS). Posttreatment imaging demonstrated intrafractional variations of 0.15 cm (RMS). The individual shifts in three standard orthogonal planes showed no obvious directional biases.

CONCLUSIONS

After localization based on superficial markings in patients undergoing SBRT, orthogonal kV imaging detects setup variations of approximately 3 to 4 mm in each direction. Cone-beam CT detects residual setup variations of approximately 2 to 3 mm.