S. Tenn

Respiratory correlated cone-beam computed tomography on an isocentric C-arm.

A methodology for 3D image reconstruction from retrospectively gated cone-beam CT projection data has been developed. A mobile x-ray cone-beam device consisting of an isocentric C-arm equipped with a flat panel detector was used to image a moving phantom. Frames for reconstruction were retrospectively selected from complete datasets based on the known rotation of the C-arm and a signal from a respiratory monitor. Different sizes of gating windows were tested. A numerical criterion for blur on the reconstructed image was suggested. The criterion is based on minimization of an Ising energy function, similar to approaches used in image segmentation or restoration. It is shown that this criterion can be used for the determination of the optimal gating window size. Images reconstructed from the retrospectively gated projection sequences using the optimal gating window data showed a significant improvement compared to images reconstructed from the complete projection datasets.

Challenges in Linear Accelerator Radiotherapy for Chordomas and Chondrosarcomas of the Skull Base: Focus on Complications

PURPOSE Intracranial chordomas and chondrosarcomas are histologically low-grade, locally invasive tumors that infiltrate the skull base. Currently, consensus therapy includes surgical resection and adjuvant radiotherapy. Radiation delivery is typically limited by the proximity of these tumors to critical skull base structures. METHODS This is a retrospective review of 13 cases of chordomas and 2 cases of chondroid chondrosarcomas of the skull based treated with linear accelerator stereotactic radiotherapy (SRT, n = 10) or stereotactic radiosurgery (SRS, n = 5). The average time to the most recent follow-up visit was 4.5 years. The tumor characteristics, treatment details, and outcomes were recorded. Each radiation plan was reviewed, and the dosage received by the brainstem, optic apparatus, and pituitary was calculated. RESULTS Of the 10 patients treated with SRT, 6 were found to have unchanged or decreased tumor size as determined from radiographic follow-up. Of the 5 patients treated with SRS, 3 were found to have stable or unchanged tumors at follow-up. The complications included 1 SRT patient who developed endocrinopathy, 2 patients (1 treated with SRS and the other with SRT), who developed cranial neuropathy, and 1 SRS patient who developed visual deficits. Additionally, 1 patient who received both SRS and SRT within 2 years for recurrence experienced transient medial temporal lobe radiation changes that resolved. CONCLUSIONS Where proton beam therapy is unavailable, linear accelerator-based SRT or radiosurgery remains a safe option for adjuvant therapy of chordomas and chondrosarcomas of the skull base. The exposure of the optic apparatus, pituitary stalk, and brainstem must be considered during planning to minimize complications. If the optic apparatus is included in the 80% isodose line, it might be best to fractionate therapy. Exposure of the pituitary stalk should be kept to <30 Gy to minimize endocrine dysfunction. Brainstem exposure should be limited to <60 Gy in fractions.

Control of Breathing Motion: Techniques and Models (Gated Radiotherapy)

For continued clinical gains in the practice of radiotherapy, management of breathing motion is essential. The problem of organ motion in radiotherapy is complex; thus, interventions to reduce organ-motion-related uncertainties require effort, expertise, and collaboration from many disciplines. The application of image-guidance techniques, i.e., imageguided radiotherapy, will play an increasing important role in developing new and improved delivery techniques, i.e., adaptive radiotherapy. With some anecdotal clinical evidence and many potentially beneficial but unproven technologies under development and on the horizon, it is essential to place equal emphasis on the planning and implementation of prospective clinical trials.

SU-FF-J-43: Technical Evaluation of Respiration Monitoring Devices and Breathing Training Techniques

Purpose: To compare external respiration monitoring devices and breathing training techniques for use in gated imaging and radiotherapy. Method and Materials: Respiration was monitored on four humans with three devices: a spirometer, a pressure gauge inside an adjustable belt, and an infrared marker tracking system. Monitoring was performed under free breathing, with audible instruction, with visual instruction, and with both audible and visual instruction. Each test was executed for two minutes. Belt placement was tested on both the chest and abdomen, while marker placement was on the abdomen alone. Period and amplitude variability was analyzed on all traces to determine optimal belt placement and instruction technique. Individual cycle periods were compared between the three devices to test their correlation. Results: Abdominal belt placement was optimal in all aspects. The uncertainty in periodicity with all devices was 4.6%, 15.3%, 18.6%, and 16.7% for free breathing, visual instruction, audible instruction, and both instruction techniques, respectively. The most consistent relative amplitudes were found using both instruction techniques. Significant drift in amplitude was observed under free breathing conditions. Comparison of individual cycle periods showed that the pressure gauge and infrared marker systems agreed on average within 10 ms, while the spirometer data differed from the others by 60 ms on average. Conclusion: If using a pressure gauge system, the belt should be placed around the patients abdomen. The most consistent periodicity results from free breathing, while the most consistent amplitude results from visual and audible instruction. The differences in period measurement between the devices can be attributed to the differences in sampling interval, while the consistently larger difference found with the spirometric data most likely is due to the calculation of volume from measured flow. This work is supported in part by Grant ♯ 03-028-01-CCE from the American Cancer Society and Siemens Medical Solutions.

Comparison of lung tumor motion measured using a model-based 4DCT technique and a commercial protocol

PURPOSE To compare lung tumor motion measured with a model-based technique to commercial 4-dimensional computed tomography (4DCT) scans and describe a workflow for using model-based 4DCT as a clinical simulation protocol. METHODS AND MATERIALS Twenty patients were imaged using a model-based technique and commercial 4DCT. Tumor motion was measured on each commercial 4DCT dataset and was calculated on model-based datasets for 3 breathing amplitude percentile intervals: 5th to 85th, 5th to 95th, and 0th to 100th. Internal target volumes (ITVs) were defined on the 4DCT and 5th to 85th interval datasets and compared using Dice similarity. Images were evaluated for noise and rated by 2 radiation oncologists for artifacts. RESULTS Mean differences in tumor motion magnitude between commercial and model-based images were 0.47 ± 3.0, 1.63 ± 3.17, and 5.16 ± 4.90 mm for the 5th to 85th, 5th to 95th, and 0th to 100th amplitude intervals, respectively. Dice coefficients between ITVs defined on commercial and 5th to 85th model-based images had a mean value of 0.77 ± 0.09. Single standard deviation image noise was 11.6 ± 9.6 HU in the liver and 6.8 ± 4.7 HU in the aorta for the model-based images compared with 57.7 ± 30 and 33.7 ± 15.4 for commercial 4DCT. Mean model error within the ITV regions was 1.71 ± 0.81 mm. Model-based images exhibited reduced presence of artifacts at the tumor compared with commercial images. CONCLUSION Tumor motion measured with the model-based technique using the 5th to 85th percentile breathing amplitude interval corresponded more closely to commercial 4DCT than the 5th to 95th or 0th to 100th intervals, which showed greater motion on average. The model-based technique tended to display increased tumor motion when breathing amplitude intervals wider than 5th to 85th were used because of the influence of unusually deep inhalations. These results suggest that care must be taken in selecting the appropriate interval during image generation when using model-based 4DCT methods.

SU-E-T-481: Dosimetric Comparison of Acuros XB and Anisotropic Analytic Algorithm with Commercial Monte Carlo Based Dose Calculation Algorithm for Stereotactic Body Radiation Therapy of Lung Cancer

PURPOSE To evaluate performance of three commercially available treatment planning systems for stereotactic body radiation therapy (SBRT) of lung cancer using the following algorithms: Boltzmann transport equation based algorithm (AcurosXB AXB), convolution based algorithm Anisotropic Analytic Algorithm (AAA); and Monte Carlo based algorithm (XVMC). METHODS A total of 10 patients with early stage non-small cell peripheral lung cancer were included. The initial clinical plans were generated using the XVMC based treatment planning system with a prescription of 54Gy in 3 fractions following RTOG0613 protocol. The plans were recalculated with the same beam parameters and monitor units using AAA and AXB algorithms. A calculation grid size of 2mm was used for all algorithms. The dose distribution, conformity, and dosimetric parameters for the targets and organs at risk (OAR) are compared between the algorithms. RESULTS The average PTV volume was 19.6mL (range 4.2-47.2mL). The volume of PTV covered by the prescribed dose (PTV-V100) were 93.97±2.00%, 95.07±2.07% and 95.10±2.97% for XVMC, AXB and AAA algorithms, respectively. There was no significant difference in high dose conformity index; however, XVMC predicted slightly higher values (p=0.04) for the ratio of 50% prescription isodose volume to PTV (R50%). The percentage volume of total lungs receiving dose >20Gy (LungV20Gy) were 4.03±2.26%, 3.86±2.22% and 3.85±2.21% for XVMC, AXB and AAA algorithms. Examination of dose volume histograms (DVH) revealed small differences in targets and OARs for most patients. However, the AAA algorithm was found to predict considerable higher PTV coverage compared with AXB and XVMC algorithms in two cases. The dose difference was found to be primarily located at the periphery region of the target. CONCLUSION For clinical SBRT lung treatment planning, the dosimetric differences between three commercially available algorithms are generally small except at target periphery. XVMC and AXB algorithms are recommended for accurate dose estimation at tissue boundaries.

SU‐E‐J‐242: Post‐Treatment Planning Tool for Estimating Dose Distribution Delivered to Spinal Radiosurgery Patients Based On Measured Intra‐Fraction Positional Data

PURPOSE To demonstrate feasibility and utility of a planning tool for post treatment reconstruction of delivered dose distribution based on measured patient motion. The spinal cord dose limits in clinical use are based on planned dosimetric data which can significantly differ from delivered dose. This tool can be utilized to correlate clinical outcomes to delivered dose METHODS AND MATERIALS: ur previous studies have shown that without intrafraction motion management, patients can move up to 3mm. The current institutional protocol requires intrafraction motion management by stereoscopic imaging prior to each treatment field. Patients are repositioned when 1mm tolerance is exceeded. Dosimetric effects of 62 spinal radiosurgery patients were investigated by simulating patient motion. For each patient plan, three additional plans were created, where the planning isocenter was shifted towards the cord by 1mm, 2mm and 3mm, for a total of 248 plans. D95% was evaluated for the target volume and D0.035cc and V10Gy were evaluated for the spinal cord. RESULTS D95% decreased up to 3%, 9% and 15% for 1mm, 2mm and 3mm shifts correspondingly. These shifts resulted in D0.035cc exceeding 12Gy in 26%, 55% and 76% of patients, respectively, while previous work revealed that patients with a 1mm shift exceeded the 12Gy dose Dmax constraint 51% of the time. These respective shifts also resulted in 53%, 65% and 74% exceeding the constraint limit of 10Gy dose to 10% of the spinal cord outlined 6 mm above and below the target volume. CONCLUSION We have shown that delivered dose distributions can significantly differ from planned dose distributions. Spinal cord dose limits in clinical use are based on planned data, assuming no patient motion. This tool can be utilized to estimate delivered dose distribution and correlate clinical outcomes to delivered dose data.

TU‐G‐103‐07: Quantifying the Dose Distribution of OBI CBCT Using Monte Carlo Simulations with a Measurement‐Based Source Model

PURPOSE To test the feasibility of using a measurement-based source model in Monte Carlo simulations to calculate in-phantom 3D dose distributions, with the application to kV-CBCT dose quantification on a Varian OBI platform. METHODS A 3-part nested CTDI phantom was used in this investigation. It contains 7 tracks in the x-y plane that can be probed. The central reference point of the phantom was aligned at machine isocenter, and using standard head and pelvis CBCT protocols, point dose measurements were made at various 3D locations with a 0.6cc thimble ionization chamber by moving the phantom along the z-direction and changing the ionization chamber position among the tracks under repetitive scans. We have generated a measurement-based source model for the OBI system and incorporated it with the Monte Carlo software MCNPX to model each CBCT protocol under examination. The in-phantom CBCT dose (mGy per particle) was simulated at all 3D measurement locations and converted to absolute dose (mGy) by normalizing with the ratio between measured and Monte Carlo simulated dose at the reference center. The calculated results were compared with the physical measurement and their discrepancies were assessed quantitatively. RESULTS Agreement was observed between Monte Carlo simulation results and in-phantom measurements for both the OBI CBCT head and pelvis protocols. For the head protocol, the average agreement was 3.40% with a maximum and minimum disagreement of 5.72% and 1.61%, respectively. For the pelvis protocol, the average agreement was 3.18% with a maximum and minimum disagreement of 4.29% and 1.87%, respectively. CONCLUSION We have demonstrated the accuracy of a measurement-based OBI source model to reproduce an array of measured point doses in phantom when using the CBCT mode of operation. These results serve as the basis for using this source model to investigate OBI dosimetry with anthropomorphic phantoms and patient specific models. (a) Dr. McNitt-Gray: Institutional research agreement, Siemens AG;Recipient research support Siemens AG; Consultant, Flaherty Sensabaugh Bonasso PLLC; Consultant, Fulbright and Jaworski, LLC; (b) Dr. Dan Ruan: Recipient research support AACR; Recipient research support TRDRP; (c) K McMillan: Research partially supported by a grant from Siemens AG; Research partially supported by a grant from AACR; Research partially supported by a grant from TRDRP.

SU-E-T-108: EBT2 Film Based Dosimetry Incorporating Correction for Spatial-Temporal Dose Response Variation

Purpose: Implementation of radiochromic film, such as GafChromic EBT2 film, as a quantitative 2‐dimensional dosimeter requires an array of equipment calibrations and corrections. We have developed a holistic model‐based calibration/correction mechanism for radiochromic film dosimetry. Methods: One challenge of using radiochromic film is that most of the densitometry scanners in use are designed for qualitative, not quantitative use. We performed careful characterization and quantification of the systematic and random components of all temporal and spatial optical density variations. In the proposed workflow, the spatial‐dependent dose response is estimated in the calibration stage under a novel model‐based optimization framework. A benchmark calibration process has been developed for the full (optical density, space) calibration map. For improved robustness, a simplified model that assumes separability of spatial variation and dose dependence was developed. The corresponding QA tool utilizes a consistent correction principle and maps the observed pixel intensity value based on the estimated spatially varying calibration map. The developed method has been tested on seven plans (both IMRT and VMAT) and compared with commercial software correction results. Dosimetry accuracy was validated against EDR2 film by comparison with treatment planning system calculated dose distribution using 3%‐3mm gamma criterion.Results: Under equivalent test conditions, the proposed method achieves gamma passing rates with EBT2 that are comparable to or better than a correction method of a commercial counterpart. Such corroboration is consistent for both IMRT and VMAT plans under gamma tests Conclusions: We have developed a systematic approach to comprehensively calibrate and use EBT2 film, accounting for temporal and spatial dose response variations. Specifically, a formal generative model and an estimation methodology was developed to achieve good balance between measurement sensitivity and robustness. This process has been implemented with in‐house software. Research supported in part by AACR career development award.

SU‐GG‐T‐573: Total Scalp Irradiation: Comparison between Volumetric Modulated Arc Therapy, Helical Tomotherapy and Conventional Electron and Photon Field Combination

Purpose: Homogeneous irradiation of the scalp is challenging due to relatively complex and superficial treatment volume. Conventional treatment approach of combining matched electron and photon fields produces dose heterogeneity, especially at matched lines. Fixed gantry IMRT techniques have not been able to provide clinically acceptable plans but helical tomotherapy has been demonstrated to be an alternative option. We have implemented total scalp treatment with volumetric modulated arc therapy (VMAT) and compared the dosimetric characteristics to tomotherapy and conventional method. Materials and Methods: Two patients have been planned for total scalp irradiation using RapidArc VMAT, helical tomotherapy and conventional electron and photon field combination. The same anatomy and structure sets have been used for the planning purposes. After each operator achieved clinically acceptable results for the particular method, the dose distributions and dose volume histograms were compared. Results: VMAT and tomotherapy provide significantly more homogeneous dose distribution and target coverage. Maximum PTV dose with VMAT and tomotherapy were 20% and 30% less than the conventional method respectively. Although, the conventional method provided 2/3 less dose to brain at D50% and D75%, tomotherapy and VMAT gave lower dose to the brain in high‐dose regions. Brainstem was best spared by conventional method with 1/4 of the maximum dose delivered with tomotherapy and VMAT. Tomotherapy provided more homogeneous dose distribution than VMAT but the brain and brainstem sparing were comparable. Conclusions: VMAT for total scalp irradiation is a clinically acceptable and comparable to tomotherapy. VMAT provides significantly more homogeneous distribution than conventional method. VMAT and Tomotherapy give lower dose to the brain in high‐dose regions than conventional method. Moreover, VMAT and tomotherapy plans can be delivered more reproducibly and reliably than conventional treatments. Average beam on time for VMAT and tomotherapy were 2.5 mins and 7.7 mins respectively. Research sponsored by Varian Medical Systems.