Computed Tomography Guided Management of Interfractional Patient Variation

RTOG sarcoma radiation oncologists reach consensus on gross tumor volume and clinical target volume on computed tomographic images for preoperative radiotherapy of primary soft tissue sarcoma of extremity in Radiation Therapy Oncology Group studies

OBJECTIVE

To develop a Radiation Therapy Oncology Group (RTOG) atlas delineating gross tumor volume (GTV) and clinical target volume (CTV) to be used for preoperative radiotherapy of primary extremity soft tissue sarcoma (STS).

METHODS AND MATERIALS

A consensus meeting was held during the RTOG meeting in January 2010 to reach agreement about GTV and CTV delineation on computed tomography (CT) images for preoperative radiotherapy of high-grade large extremity STS. Data were presented to address the local extension of STS. Extensive discussion ensued to develop optimal criteria for GTV and CTV delineation on CT images.

RESULTS

A consensus was reached on appropriate CT-based GTV and CTV. The GTV is gross tumor defined by T1 contrast-enhanced magnetic resonance images. Fusion of magnetic resonance and images is recommended to delineate the GTV. The CTV for high-grade large STS typically includes the GTV plus 3-cm margins in the longitudinal directions. If this causes the field to extend beyond the compartment, the field can be shortened to include the end of a compartment. The radial margin from the lesion should be 1.5 cm, including any portion of the tumor not confined by an intact fascial barrier, bone, or skin surface.

CONCLUSION

The consensus on GTV and CTV for preoperative radiotherapy of high-grade large extremity STS is available as web-based images and in a descriptive format through the RTOG. This is expected to improve target volume consistency and allow for rigorous evaluation of the benefits and risks of such treatment.

Lung tumor reproducibility with active breath control (ABC) in image-guided radiotherapy based on cone-beam computed tomography with two registration methods

PURPOSE

To study the inter- and intrafraction tumor reproducibility with active breath control (ABC) utilizing cone-beam computed tomography (CBCT), and compare validity of registration with two different regions of interest (ROI).

METHODS AND MATERIALS

Thirty-one lung tumors in 19 patients received conventional or stereotactic body radiotherapy with ABC. During each treatment, patients had three CBCT scanned before and after online position correction and after treatment. These CBCT images were aligned to the planning CT using the gray scale registration of tumor and bony registration of the thorax, and tumor position uncertainties were then determined.

RESULTS

The interfraction systematic and random translation errors in the left-right (LR), superior-inferior (SI) and anterior-posterior (AP) directions were 3.6, 4.8, and 2.9mm; 2.5, 4.5, and 3.5mm, respectively, with gray scale alignment; 1.9, 4.3, 2.0mm and 2.5, 4.4, 2.9mm, respectively, with bony alignment. The interfraction systematic and random rotation errors with gray scale and bony alignment groups ranged from 1.4° to 3.0° and 0.8° to 2.3°, respectively. The intrafraction systematic and random errors with gray scale registration in LR, SI, AP directions were 0.9, 2.0, 1.8mm and 1.5, 1.7, 2.9mm, respectively, for translation; 1.5°, 0.9°, 1.0° and 1.2°, 2.2°, 1.8°, respectively, for rotation. The translational errors in SI direction with bony alignment were significantly larger than that of gray scale (p<0.05).

CONCLUSIONS

With CBCT guided online correction the interfraction positioning errors can be markedly reduced. The intrafraction errors were not diminished by the use of ABC. Rotation errors were not very remarkable both inter- and intrafraction. Gray scale alignment of tumor may provide a better registration in SI direction.

Localization accuracy of the clinical target volume during image-guided radiotherapy of lung cancer

PURPOSE

To evaluate the position and shape of the originally defined clinical target volume (CTV) over the treatment course, and to assess the impact of gross tumor volume (GTV)-based online computed tomography (CT) guidance on CTV localization accuracy.

METHODS AND MATERIALS

Weekly breath-hold CT scans were acquired in 17 patients undergoing radiotherapy. Deformable registration was used to propagate the GTV and CTV from the first weekly CT image to all other weekly CT images. The on-treatment CT scans were registered rigidly to the planning CT scan based on the GTV location to simulate online guidance, and residual error in the CTV centroids and borders was calculated.

RESULTS

The mean GTV after 5 weeks relative to volume at the beginning of treatment was 77% ± 20%, whereas for the prescribed CTV, it was 92% ± 10%. The mean absolute residual error magnitude in the CTV centroid position after a GTV-based localization was 2.9 ± 3.0 mm, and it varied from 0.3 to 20.0 mm over all patients. Residual error of the CTV centroid was associated with GTV regression and anisotropy of regression during treatment (p = 0.02 and p = 0.03, respectively; Spearman rank correlation). A residual error in CTV border position greater than 2 mm was present in 77% of patients and 50% of fractions. Among these fractions, residual error of the CTV borders was 3.5 ± 1.6 mm (left-right), 3.1 ± 0.9 mm (anterior-posterior), and 6.4 ± 7.5 mm (superior-inferior).

CONCLUSIONS

Online guidance based on the visible GTV produces substantial error in CTV localization, particularly for highly regressing tumors. The results of this study will be useful in designing margins for CTV localization or for developing new online CTV localization strategies.

Variation in the gross tumor volume and clinical target volume for preoperative radiotherapy of primary large high-grade soft tissue sarcoma of the extremity among RTOG sarcoma radiation oncologists

PURPOSE

To evaluate variability in the definition of preoperative radiotherapy gross tumor volume (GTV) and clinical target volume (CTV) delineated by sarcoma radiation oncologists.

METHODS AND MATERIALS

Extremity sarcoma planning CT images along with the corresponding diagnostic MRI from two patients were distributed to 10 Radiation Therapy Oncology Group sarcoma radiation oncologists with instructions to define GTV and CTV using standardized guidelines. The CT data with contours were then returned for central analysis. Contours representing statistically corrected 95% (V95) and 100% (V100) agreement were computed for each structure.

RESULTS

For the GTV, the minimum, maximum, mean (SD) volumes (mL) were 674, 798, 752±35 for the lower extremity case and 383, 543, 447±46 for the upper extremity case. The volume (cc) of the union, V95 and V100 were 882, 761, and 752 for the lower, and 587, 461, and 455 for the upper extremity, respectively. The overall GTV agreement was judged to be almost perfect in both lower and upper extremity cases (kappa=0.9 [p<0.0001] and kappa=0.86 [p<0.0001]). For the CTV, the minimum, maximum, mean (SD) volumes (mL) were 1145, 1911, 1605±211 for the lower extremity case and 637, 1246, 1006±180 for the upper extremity case. The volume (cc) of the union, V95, and V100 were 2094, 1609, and 1593 for the lower, and 1533, 1020, and 965 for the upper extremity cases, respectively. The overall CTV agreement was judged to be almost perfect in the lower extremity case (kappa=0.85 [p<0.0001]) but only substantial in the upper extremity case (kappa=0.77 [p<0.0001]).

CONCLUSIONS

Almost perfect agreement existed in the GTV of these two representative cases. There was no significant disagreement in the CTV of the lower extremity, but variation in the CTV of upper extremity was seen, perhaps related to the positional differences between the planning CT and the diagnostic MRI.

Image-guided radiation therapy using computed tomography in radiotherapy

The sharp dose gradients in intensity-modulated radiation therapy increase the treatment sensitivity to various inter- and intra-fractional uncertainties, in which a slight anatomical change may greatly alter the actual dose delivered. Image-guided radiotherapy refers to the use of advanced imaging techniques to precisely track and correct these patient-specific variations in routine treatment. It can also monitor organ changes during a radiotherapy course. Currently, image-guided radiotherapy using computed tomography has gained much popularity in radiotherapy verification as it provides volumetric images with soft-tissue contrast for on-line tracking of tumour. This article reviews four types of computed tomography-based image guidance systems and their working principles. The system characteristics and clinical applications of the helical, megavoltage, computed tomography, and kilovoltage, cone-beam, computed tomography systems are discussed, given that they are currently the most commonly used systems for radiotherapy verification. This article also focuses on the recent techniques of soft-tissue contrast enhancement, digital tomosynthesis, four-dimensional fluoroscopic image guidance, and kilovoltage/megavoltage, in-line cone-beam imaging. These evolving systems are expected to take over the conventional two-dimensional verification system in the near future and provide the basis for implementing adaptive radiotherapy.

Imaging in radiation oncology: a perspective

This paper reviews the integration of imaging and radiation oncology, and discusses challenges and opportunities for improving the practice of radiation oncology with imaging.

An inherent goal of radiation therapy is to deliver enough dose to the tumor to eradicate all cancer cells or to palliate symptoms, while avoiding normal tissue injury. Imaging for cancer diagnosis, staging, treatment planning, and radiation targeting has been integrated in various ways to improve the chance of this occurring. A large spectrum of imaging strategies and technologies has evolved in parallel to advances in radiation delivery. The types of imaging can be categorized into offline imaging (outside the treatment room) and online imaging (inside the treatment room, conventionally termed image-guided radiation therapy). The direct integration of images in the radiotherapy planning process (physically or computationally) often entails trade-offs in imaging performance. Although such compromises may be acceptable given specific clinical objectives, general requirements for imaging performance are expected to increase as paradigms for radiation delivery evolve to address underlying biology and adapt to radiation responses. This paper reviews the integration of imaging and radiation oncology, and discusses challenges and opportunities for improving the practice of radiation oncology with imaging.

Dosimetric evaluation of automatic segmentation for adaptive IMRT for head-and-neck cancer

PURPOSE

Adaptive planning to accommodate anatomic changes during treatment requires repeat segmentation. This study uses dosimetric endpoints to assess automatically deformed contours.

METHODS AND MATERIALS

Sixteen patients with head-and-neck cancer had adaptive plans because of anatomic change during radiotherapy. Contours from the initial planning computed tomography (CT) were deformed to the mid-treatment CT using an intensity-based free-form registration algorithm then compared with the manually drawn contours for the same CT using the Dice similarity coefficient and an overlap index. The automatic contours were used to create new adaptive plans. The original and automatic adaptive plans were compared based on dosimetric outcomes of the manual contours and on plan conformality.

RESULTS

Volumes from the manual and automatic segmentation were similar; only the gross tumor volume (GTV) was significantly different. Automatic plans achieved lower mean coverage for the GTV: V95: 98.6 +/- 1.9% vs. 89.9 +/- 10.1% (p = 0.004) and clinical target volume: V95: 98.4 +/- 0.8% vs. 89.8 +/- 6.2% (p < 0.001) and a higher mean maximum dose to 1 cm(3) of the spinal cord 39.9 +/- 3.7 Gy vs. 42.8 +/- 5.4 Gy (p = 0.034), but no difference for the remaining structures.

CONCLUSIONS

Automatic segmentation is not robust enough to substitute for physician-drawn volumes, particularly for the GTV. However, it generates normal structure contours of sufficient accuracy when assessed by dosimetric end points.

GPU-based ultra-fast dose calculation using a finite size pencil beam model

Online adaptive radiation therapy (ART) is an attractive concept that promises the ability to deliver an optimal treatment in response to the inter-fraction variability in patient anatomy. However, it has yet to be realized due to technical limitations. Fast dose deposit coefficient calculation is a critical component of the online planning process that is required for plan optimization of intensity-modulated radiation therapy (IMRT). Computer graphics processing units (GPUs) are well suited to provide the requisite fast performance for the data-parallel nature of dose calculation. In this work, we develop a dose calculation engine based on a finite-size pencil beam (FSPB) algorithm and a GPU parallel computing framework. The developed framework can accommodate any FSPB model. We test our implementation in the case of a water phantom and the case of a prostate cancer patient with varying beamlet and voxel sizes. All testing scenarios achieved speedup ranging from 200 to 400 times when using a NVIDIA Tesla C1060 card in comparison with a 2.27 GHz Intel Xeon CPU. The computational time for calculating dose deposition coefficients for a nine-field prostate IMRT plan with this new framework is less than 1 s. This indicates that the GPU-based FSPB algorithm is well suited for online re-planning for adaptive radiotherapy.

[Potential place of FDG-PET for the GTV delineation in head and neck and lung cancers]

The recent progresses performed in imaging, computational and technological fields bring new opportunities to achieve high precision radiation dose delivery. However, IMRT requires a particular attention at the target delineation step to avoid inadequate dosage to TVs/OARs. In this context, the biological information provided by PET might advantageously complete CT-Scan to refine the target delineation in HNSCC and lung cancer. Integrating PET into the treatment planning however requires the use and validation of accurate and reproducible segmentation methods, which adequately integrate the PET image properties such as the blur effect and the high level of noise. In this context, we developed specific tools, i.e. edge-preserving filters for denoising and deconvolution algorithms for deblurring that allowed the detection of gradient intensity peaks. Our gradient-based method has been validated on phantom and patient materials, and proved to be more accurate than threshold-based approaches. With this tool in hand, we demonstrated that the use of FDG-PET resulted in smaller TVs than the CT-based TVs, on both pre- and per-treatment images, and significantly improved the dose distributions to the TVs/OARs. This opens avenues for dose escalation strategies that might potentially improve the tumor local control.

Real-time motion-adaptive-optimization (MAO) in TomoTherapy

IMRT delivery follows a planned leaf sequence, which is optimized before treatment delivery. However, it is hard to model real-time variations, such as respiration, in the planning procedure. In this paper, we propose a negative feedback system of IMRT delivery that incorporates real-time optimization to account for intra-fraction motion. Specifically, we developed a feasible workflow of real-time motion-adaptive-optimization (MAO) for TomoTherapy delivery. TomoTherapy delivery is characterized by thousands of projections with a fast projection rate and ultra-fast binary leaf motion. The technique of MAO-guided delivery calculates (i) the motion-encoded dose that has been delivered up to any given projection during the delivery and (ii) the future dose that will be delivered based on the estimated motion probability and future fluence map. These two pieces of information are then used to optimize the leaf open time of the upcoming projection right before its delivery. It consists of several real-time procedures, including 'motion detection and prediction', 'delivered dose accumulation', 'future dose estimation' and 'projection optimization'. Real-time MAO requires that all procedures are executed in time less than the duration of a projection. We implemented and tested this technique using a TomoTherapy research system. The MAO calculation took about 100 ms per projection. We calculated and compared MAO-guided delivery with two other types of delivery, motion-without-compensation delivery (MD) and static delivery (SD), using simulated 1D cases, real TomoTherapy plans and the motion traces from clinical lung and prostate patients. The results showed that the proposed technique effectively compensated for motion errors of all test cases. Dose distributions and DVHs of MAO-guided delivery approached those of SD, for regular and irregular respiration with a peak-to-peak amplitude of 3 cm, and for medium and large prostate motions. The results conceptually proved that the proposed method is applicable for real-time motion compensation in TomoTherapy delivery. Extension of the method to real-time adaptive radiation therapy (ART) that compensates for all kinds of delivery errors was proposed. Further validation and clinical implementation is underway.

Evaluation of Inter-fractional Setup Shifts for Site-specific Helical Tomotherapy Treatments

This paper proposes to summarize and analyze the daily patient setup shifts based on megavoltage computed tomography (MVCT) image registration results for Helical TomoTherapy® (HT) treatment. One hundred and fifty-five consecutive treatment plans for a total of 137 patients delivered by the HT unit through one year were collected in this study. The patient data included pelvis (26%), abdomen (23%), lung (21%), head and neck (10%), prostate (8%), and others (12%). All the translational and roll rotational shifts made via auto MVCT and kilovoltage computed tomography (kVCT) image registration were recorded at each fraction. Manual fine-tuning was followed if automatic registration result was not satisfactory. The mean shift ± one standard deviation (1 SD) was calculated for each patient based on the entire treatment course. For each treatment site, the average shift was analyzed as well as displacement in 3D vector. Statistical tests were performed to analyze the relationship of patient-specific, tumor site-specific, and fraction number association with the patient setup shifts. For all the treatment sites, the largest average shift was found in the anterior-posterior direction. The population standard deviations were between 1.2 and 5.6 mm for the X, Y, and Z directions and ranged from 0.2 to 0.6 degrees for the roll rotational correction. The largest standard deviations of the setup reproducibility in X, Y, and Z directions were found in lung patients (4.2 mm), abdomen, lung and spine patients (4.4 mm), and prostate patients (5.6 mm), respectively. The maximum 3D displacement was 10.9 mm for prostate patients' setup. ANOVA tests demonstrated the setup shifts were statistically different between patients even for those that were treated at the same tumor site in the translational directions. No strong correlation between the setup and the fraction number was found. In conclusion, the MVCT guided function in the HT treatment enables us to generate relatively accurate daily setup through registration with KVCT data sets. Our results indicate that lung, prostate, and abdominal patients are more prone to setup uncertainty and should be carefully evaluated.

A comprehensive assessment by tumor site of patient setup using daily MVCT imaging from more than 3,800 helical tomotherapy treatments

PURPOSE

To assess patient setup corrections based on daily megavoltage CT (MVCT) imaging for four anatomic treatment sites treated on tomotherapy.

METHOD AND MATERIALS

Translational and rotational setup corrections, based on registration of daily MVCT to planning CT images, were analyzed for 1,179 brain and head and neck (H&N), 1,414 lung, and 1,274 prostate treatment fractions. Frequencies of three-dimensional vector lengths, overall distributions of setup corrections, and patient-specific distributions of random and systematic setup errors were analyzed.

RESULTS

Brain and H&N had lower magnitude positioning corrections and smaller variations in translational setup errors but were comparable in roll rotations. Three-dimensional vector translational shifts of larger magnitudes occurred more frequently for lung and prostate than for brain and H&N treatments, yet this was not observed for roll rotations. The global systematic error for prostate was 4.7 mm in the vertical direction, most likely due to couch sag caused by large couch extension distances. Variations in systematic errors and magnitudes of random translational errors ranged from 1.6 to 2.6 mm for brain and H&N and 3.2 to 7.2 mm for lung and prostate, whereas roll rotational errors ranged from 0.8 degrees to 1.2 degrees for brain and H&N and 0.5 degrees to 1.0 degrees for lung and prostate.

CONCLUSIONS

Differences in setup were observed between brain, H&N, lung, and prostate treatments. Patient setup can be improved if daily imaging is performed. This analysis can assess the utilization of daily image guidance and allows for further investigation into improved anatomic site-specific and patient-specific treatments.

The adaptation of megavoltage cone beam CT for use in standard radiotherapy treatment planning

Potential areas where megavoltage computed tomography (MVCT) could be used are second- and third-phase treatment planning in 3D conformal radiotherapy and IMRT, adaptive radiation therapy, single fraction palliative treatment and for the treatment of patients with metal prostheses. A feasibility study was done on using MV cone beam CT (CBCT) images generated by proprietary 3D reconstruction software based on the FDK algorithm for megavoltage treatment planning. The reconstructed images were converted to a DICOM file set. The pixel values of megavoltage cone beam computed tomography (MV CBCT) were rescaled to those of kV CT for use with a treatment planning system. A calibration phantom was designed and developed for verification of geometric accuracy and CT number calibration. The distance measured between two marker points on the CBCT image and the physical dimension on the phantom were in good agreement. Point dose verification for a 10 cm x 10 cm beam at a gantry angle of 0 degrees and SAD of 100 cm were performed for a 6 MV beam for both kV and MV CBCT images. The point doses were found to vary between +/-6.1% of the dose calculated from the kV CT image. The isodose curves for 6 MV for both kV CT and MV CBCT images were within 2% and 3 mm distance-to-agreement. A plan with three beams was performed on MV CBCT, simulating a treatment plan for cancer of the pituitary. The distribution obtained was compared with those corresponding to that obtained using the kV CT. This study has shown that treatment planning with MV cone beam CT images is feasible.

Performance evaluation of automatic anatomy segmentation algorithm on repeat or four-dimensional computed tomography images using deformable image registration method

PURPOSE

Auto-propagation of anatomic regions of interest from the planning computed tomography (CT) scan to the daily CT is an essential step in image-guided adaptive radiotherapy. The goal of this study was to quantitatively evaluate the performance of the algorithm in typical clinical applications.

METHODS AND MATERIALS

We had previously adopted an image intensity-based deformable registration algorithm to find the correspondence between two images. In the present study, the regions of interest delineated on the planning CT image were mapped onto daily CT or four-dimensional CT images using the same transformation. Postprocessing methods, such as boundary smoothing and modification, were used to enhance the robustness of the algorithm. Auto-propagated contours for 8 head-and-neck cancer patients with a total of 100 repeat CT scans, 1 prostate patient with 24 repeat CT scans, and 9 lung cancer patients with a total of 90 four-dimensional CT images were evaluated against physician-drawn contours and physician-modified deformed contours using the volume overlap index and mean absolute surface-to-surface distance.

RESULTS

The deformed contours were reasonably well matched with the daily anatomy on the repeat CT images. The volume overlap index and mean absolute surface-to-surface distance was 83% and 1.3 mm, respectively, compared with the independently drawn contours. Better agreement (>97% and <0.4 mm) was achieved if the physician was only asked to correct the deformed contours. The algorithm was also robust in the presence of random noise in the image.

CONCLUSION

The deformable algorithm might be an effective method to propagate the planning regions of interest to subsequent CT images of changed anatomy, although a final review by physicians is highly recommended.

Emerging role of intensity-modulated radiation therapy in anorectal cancer

Although radiation therapy has an established role to play in the management of rectal and anal tumors, there are often treatment-related morbidities that negatively impact on patients. There is a long-standing interest in radiation oncology on maximizing treatment efficacy while minimizing treatment-related toxicities, which can be pronounced in the treatment of pelvic malignancies. Intensity-modulated radiation therapy is a recently introduced technology that has the potential to increase the efficacy:toxicity ratio. It has been implemented in the treatment of prostate and head and neck tumors with success. This article reviews the rationale for its use in treating anorectal tumors and discusses early clinical data supporting its continued investigation.

Conventionally-fractionated image-guided intensity modulated radiotherapy (IG-IMRT): a safe and effective treatment for cancer spinal metastasis

BACKGROUND

Treatments for cancer spinal metastasis were always palliative. This study was conducted to investigate the safety and effectiveness of IG-IMRT for these patients.

METHODS

10 metastatic lesions were treated with conventionally-fractionated IG-IMRT. Daily kilovoltage cone-beam computed tomography (kV-CBCT) scan was applied to ensure accurate positioning. Plans were evaluated by the dose-volume histogram (DVH) analysis.

RESULTS

Before set-up correction, the positioning errors in the left-right (LR), superior-inferior (SI) and anterior-posterior (AP) axes were 0.3 +/- 3.2, 0.4 +/- 4.5 and -0.2 +/- 3.9 mm, respectively. After repositioning, those errors were 0.1 +/- 0.7, 0 +/- 0.8 and 0 +/- 0.7 mm, respectively. The systematic/random uncertainties ranged 1.4-2.3/3.0-4.1 before and 0.1-0.2/0.7-0.8 mm after online set-up correction. In the original IMRT plans, the average dose of the planning target volume (PTV) was 61.9 Gy, with the spinal cord dose less than 49 Gy. Compared to the simulated PTVs based on the pre-correction CBCT, the average volume reduction of PTVs was 42.3% after online correction. Also, organ at risk (OAR) all benefited from CBCT-based set-up correction and had significant dose reduction with IGRT technique. Clinically, most patients had prompt pain relief within one month of treatment. There was no radiation-induced toxicity detected clinically during a median follow-up of 15.6 months.

CONCLUSION

IG-IMRT provides a new approach to treat cancer spinal metastasis. The precise positioning ensures the implementation of optimal IMRT plan, satisfying both the dose escalation of tumor targets and the radiation tolerance of spinal cord. It might benefit the cancer patient with spinal metastasis.

Retrospective IMRT dose reconstruction based on cone-beam CT and MLC log-file

PURPOSE

Head-and-neck (HN) cone-beam computed tomography (CBCT) can be exploited to probe the IMRT dose delivered to a patient taking into account the interfraction anatomic variation and any potential inaccuracy in the IMRT delivery. The aim of this work is to reconstruct the intensity-modulated radiation therapy dose delivered to an HN patient using the CBCT and multileaf collimator (MLC) log-files.

METHODS AND MATERIALS

A cylindrical CT phantom was used for calibrating the electron density and validating the procedures of the dose reconstruction. Five HN patients were chosen, and for each patient, CBCTs were performed on three separate fractions spaced every 2 weeks starting from the first fraction. The respective MLC log-files were retrieved and converted into fluence maps. The dose was then reconstructed on the corresponding CBCT with the regenerated fluence maps. The reconstructed dose distribution, dosimetric endpoints, and DVHs were compared with that of the treatment plan.

RESULTS

Phantom study showed that HN CBCT can be directly used for dose reconstruction. For most treatment sessions, the CBCT-based dose reconstructions yielded DVHs of the targets close (within 3%) to that of the original treatment plans. However, dosimetric changes (within 10%) due to anatomic variations caused by setup inaccuracy, organ deformation, tumour shrinkage, or weight loss (or a combination of these) were observed for the critical organs.

CONCLUSIONS

The methodology we established affords an objective dosimetric basis for the clinical decision on whether a replanning is necessary during the course of treatment and provides a valuable platform for adaptive therapy in future.

Planning in the IGRT context: closing the loop

Technological developments in image-guided radiotherapy systems have introduced new considerations to the treatment-planning process. These include more rational assessment and reduction of treatment margins; adaptation of treatment plans according to information gathered as treatment progresses; and facilitation of treatments involving the delivery of large, highly focused doses of radiation to tumors. We examine the performance of different treatment-room image-guidance systems in terms of target position accuracy; such information is important for determining treatment margins and deciding on an appropriate correction strategy. Some clinical situations may warrant a modification to a patient's treatment plan part way through a course of treatment, such as tumor shrinkage in response to treatment and daily organ variation. We discuss the challenges and review proposed strategies for treatment-plan adaptation. Image guidance in combination with 3-dimensional conformal and intensity-modulated radiotherapy has provided the tools for clinical trials of single-dose and hypofractionated treatment as an alternative to standard fractionation. We discuss the clinical realization of this treatment paradigm in various disease sites.

Imaging doses from the Elekta Synergy X-ray cone beam CT system

The Elekta Synergy is a radiotherapy treatment machine with integrated kilovoltage (kV) X-ray imaging system capable of producing cone beam CT (CBCT) images of the patient in the treatment position. The aim of this study is to assess the additional imaging dose. Cone beam CT dose index (CBDI) is introduced and measured inside standard CTDI phantoms for several sites (head: 100 kV, 38 mAs, lung: 120 kV, 152 mAs and pelvis: 130 kV, 456 mAs). The measured weighted doses were compared with thermoluminescent dosimeter (TLD) measurements at various locations in a Rando phantom and at patients' surfaces. The measured CBDIs in-air at the isocentre were 9.2 mGy 100 mAs(-1), 7.3 mGy 100 mAs(-1) and 5.3 mGy 100 mAs(-1) for 130 kV, 120 kV and 100 kV, respectively. The body phantom weighted CBDI were 5.5 mGy 100 mAs(-1) and 3.8 mGy 100 mAs(-1 )for 130 kV and 120 kV. The head phantom weighted CBDI was 4.3 mGy 100 mAs(-1) for 100 kV. The weighted doses for the Christie Hospital CBCT imaging techniques were 1.6 mGy, 6 mGy and 22 mGy for the head, lung and pelvis. The measured CBDIs were used to estimate the total effective dose for the Synergy system using the ImPACT CT Patient Dosimetry Calculator. Measured CBCT doses using the Christie Hospital protocols are low for head and lung scans whether compared with electronic portal imaging (EPI), commonly used for treatment verification, or single and multiple slice CT. For the pelvis, doses are similar to EPI but higher than CT. Repeated use of CBCT for treatment verification is likely and hence the total patient dose needs to be carefully considered. It is important to consider further development of low dose CBCT techniques to keep additional doses as low as reasonably practicable.

Effect of the first day correction on systematic setup error reduction

Treatment simulation is usually performed with a conventional simulator using kV X-rays or with a computed tomography (CT) simulator before the treatment course begins. The purpose is to verify patient setup under the same conditions as for treatment planning. Systematic (preparation) setup errors can be introduced by this process. The purpose of this study is to characterize the setup errors using electronic portal image (EPI) analyses and to propose a method to reduce the systematic component by performing simulation and patient preparation on the treatment machine. In this study, the first four or five days EPIs were analyzed from a total of 533 prostate cancer patients who were simulated on conventional simulators. We characterized setup errors using four parameters: {M(microi), Sigma (microi), RMS(microi), sigma (sigmai)}, where microi and sigmai are individual patient mean and standard deviation, M, Sigma, and RMS are the mean, standard deviation, and root-mean-square of underlying variables (microi and sigmai). We have performed a simulation of removing systematic components by correcting the first day setup error. As a comparison, we also carried out a similar analyses for patients simulated on a CT simulator and patients treated on a linac with an on-board kV CT imaging system, although a limited number of patients were available in these two samples. We found that Sigma (/ui)=(2.6,3.4,2.4) mm, and RMS(sigmai)=(1.5,1.9,1.0) mm in lateral, anterior/posterior, and cranial/caudal directions, indicating that systematic errors are much larger than random errors. Strong correlations were found between measurement on the first day and microi, implying the first day's measurement is a good predictor for microi. The same parameters were also computed for days 2-4, with and without the first day correction. Without correction, M(microi)2-4=(0.7,1.6,-1.0) mm, and Sigma(microi)2-4=(2.6,3.5,2.4) mm. With correction, M(microi)2-4=(0.0,0.4,0.4) mm, much closer to zero, and Sigma(microi)2-4=(1.8,2.2, 1.2) mm, also much smaller. While the use of a CT simulator can reduce the systematic errors, the benefits of first day correction can still be observed, although at a smaller magnitude. Therefore, the systematic setup error can be significantly reduced if the patient is marked and fields are verified on the treatment machine on the first fraction, preferably with an on-board kV imaging system.

Commissioning experience with cone-beam computed tomography for image-guided radiation therapy

This paper reports on the commissioning of an Elekta cone‐beam computed tomography (CT) system at one of the first U.S. sites to install a “regular,” off‐the‐shelf Elekta Synergy (Elekta, Stockholm, Sweden) accelerator system. We present the quality assurance (QA) procedure as a guide for other users. The commissioning had six elements: (1) system safety, (2) geometric accuracy (agreement of megavoltage and kilovoltage beam isocenters), (3) image quality, (4) registration and correction accuracy, (5) dose to patient and dosimetric stability, and (6) QA procedures. The system passed the safety tests, and agreement of the isocenters was found to be within 1 mm. Using a precisely moved skull phantom, the reconstruction and alignment algorithm was found to be accurate within 1 mm and 1 degree in each dimension. Of 12 measurement points spanning a 9×9×15‐cm volume in a Rando phantom (The Phantom Laboratory, Salem, NY), the average agreement in the x, y, and z coordinates was 0.10 mm, −0.12 mm, and 0.22 mm [standard deviations (SDs): 0.21 mm, 0.55 mm, 0.21 mm; largest deviations: 0.6 mm, 1.0 mm, 0.5 mm] respectively. The larger deviation for the y component can be partly attributed to the CT slice thickness of 1 mm in that direction. Dose to the patient depends on the machine settings and patient geometry. To monitor dose consistency, air kerma (output) and half‐value layer (beam quality) are measured for a typical clinical setting. Air kerma was 6.3 cGy (120 kVp, 40 mA, 40 ms per frame, 360‐degree scan, S20 field of view); half value layer was 7.1 mm aluminum (120 kV, 40 mA).

We suggest performing items 1, 2, and 3 monthly, and 4 and 5 annually. In addition, we devised a daily QA procedure to verify agreement of the megavoltage and kilovoltage isocenters using a simple phantom containing three small steel balls. The frequency of all checks will be reevaluated based on data collected during about 1 year.

PACS number: 87.53.Xd

Interfractional Variations in Patient Setup and Anatomic Change Assessed by Daily Computed Tomography

PURPOSE

To analyze the interfractional variations in patient setup and anatomic changes at seven anatomic sites observed in image-guided radiotherapy.

METHODS AND MATERIALS

A total of 152 patients treated at seven anatomic sites using a Hi-Art helical tomotherapy system were analyzed. Daily tomotherapy megavoltage computed tomography images acquired before each treatment were fused to the planning kilovoltage computed tomography images to determine the daily setup errors and organ motions and deformations. The setup errors were corrected before treatment and were used, along with the organ motions, to determine the clinical target volume/planning target volume margins. The organ motions and deformations for 3 representative patient cases (pancreas, uterus, and soft-tissue sarcoma) and for 14 kidneys of 7 patients are presented.

RESULTS

Interfractional setup errors in the skull, brain, and head and neck are significantly smaller than those in the chest, abdomen, pelvis, and extremities. These site-specific relationships are statistically significant. The margins required to account for these setup errors range from 3 to 8 mm for the seven sites. The margin to account for both setup errors and organ motions for kidney is 16 mm. Substantial interfractional anatomic changes were observed. For example, the pancreas moved up to +/-20 mm and volumes of the uterus and sarcoma varied <or=30% and 100%, respectively.

CONCLUSION

The interfractional variations in patient setup and in shapes, sizes, and positions of both targets and normal structures are site specific and may be used to determine the site-specific margins. The data presented in this work dealing with seven anatomic sites may be useful in developing adaptive radiotherapy.

Direct aperture optimization for online adaptive radiation therapy

This paper is the first investigation of using direct aperture optimization (DAO) for online adaptive radiation therapy (ART). A geometrical model representing the anatomy of a typical prostate case was created. To simulate interfractional deformations, four different anatomical deformations were created by systematically deforming the original anatomy by various amounts (0.25, 0.50, 0.75, and 1.00 cm). We describe a series of techniques where the original treatment plan was adapted in order to correct for the deterioration of dose distribution quality caused by the anatomical deformations. We found that the average time needed to adapt the original plan to arrive at a clinically acceptable plan is roughly half of the time needed for a complete plan regeneration, for all four anatomical deformations. Furthermore, through modification of the DAO algorithm the optimization search space was reduced and the plan adaptation was significantly accelerated. For the first anatomical deformation (0.25 cm), the plan adaptation was six times more efficient than the complete plan regeneration. For the 0.50 and 0.75 cm deformations, the optimization efficiency was increased by a factor of roughly 3 compared to the complete plan regeneration. However, for the anatomical deformation of 1.00 cm, the reduction of the optimization search space during plan adaptation did not result in any efficiency improvement over the original (nonmodified) plan adaptation. The anatomical deformation of 1.00 cm demonstrates the limit of this approach. We propose an innovative approach to online ART in which the plan adaptation and radiation delivery are merged together and performed concurrently-adaptive radiation delivery (ARD). A fundamental advantage of ARD is the fact that radiation delivery can start almost immediately after image acquisition and evaluation. Most of the original plan adaptation is done during the radiation delivery, so the time spent adapting the original plan does not increase the overall time the patient has to spend on the treatment couch. As a consequence, the effective time allotted for plan adaptation is drastically reduced. For the 0.25, 0.5, and 0.75 cm anatomical deformations, the treatment time was increased by only 2, 4, and 6 s, respectively, as compared to no plan adaptation. For the anatomical deformation of 1.0 cm the time increase was substantially larger. The anatomical deformation of 1.0 cm represents an extreme case, which is rarely observed for the prostate, and again demonstrates the limit of this approach. ARD shows great potential for an online adaptive method with minimal extension of treatment time.

Advanced radiation therapy technologies in the treatment of rectal and anal cancer: intensity-modulated photon therapy and proton therapy

Intensity-modulated photon radiation therapy (RT; IMRT) and proton therapy are advanced radiation technologies that permit improved conformation of radiation dose to target structures while limiting irradiation of surrounding normal tissues. Application of these technologies in the treatment of rectal and anal cancer is attractive, based on the potential reduction in radiation treatment toxicities that are frequently incurred in the pelvis and perineum. Furthermore, conformal RT might also allow for dose escalation to target areas, leading to improved tumor control. This review discusses the underlying principles of IMRT. In addition, the rationale and clinical data regarding the efficacy of radiation dose escalation for rectal and anal cancer will be highlighted, as well as tolerance of pelvic organs to RT and chemotherapy. Finally, preliminary results of IMRT in the treatment of lower gastrointestinal tract cancers will be reviewed. The potential and rationale for proton therapy in treatment of these malignancies are also discussed.

Imaging and alignment for image-guided radiation therapy

Image-guided radiation therapy is an exciting new area that focuses heavily on the potential benefit of advanced imaging and image registration to improve precision, thus limiting morbidity and potentially allowing for safe delivery of increased dose. This review explores the issues surrounding the use of imaging and image registration for treatment planning and verification, with emphasis on the underlying patient model and alignment algorithms.

Advances in image-guided radiation therapy

Imaging is central to radiation oncology practice, with advances in radiation oncology occurring in parallel to advances in imaging. Targets to be irradiated and normal tissues to be spared are delineated on computed tomography (CT) scans in the planning process. Computer-assisted design of the radiation dose distribution ensures that the objectives for target coverage and avoidance of healthy tissue are achieved. The radiation treatment units are now recognized as state-of-the-art robotics capable of three-dimensional soft tissue imaging immediately before, during, or after radiation delivery, improving the localization of the target at the time of radiation delivery, to ensure that radiation therapy is delivered as planned. Frequent imaging in the treatment room during a course of radiation therapy, with decisions made on the basis of imaging, is referred to as image-guided radiation therapy (IGRT). IGRT allows changes in tumor position, size, and shape to be measured during the course of therapy, with adjustments made to maximize the geometric accuracy and precision of radiation delivery, reducing the volume of healthy tissue irradiated and permitting dose escalation to the tumor. These geometric advantages increase the chance of tumor control, reduce the risk of toxicity after radiotherapy, and facilitate the development of shorter radiotherapy schedules. By reducing the variability in delivered doses across a population of patients, IGRT should also improve interpretation of future clinical trials.