L. Handsfield

CT-on-rails-guided HDR brachytherapy: single-room, rapid-workflow treatment delivery with integrated image guidance

Brachytherapy is an important component of multidisciplinary cancer care for a variety of solid tumors. Most systems require moving the patient to multiple locations for treatment planning and delivery after the applicator is placed. A dedicated computed tomography (CT)-on-rails brachytherapy suite was installed at our institution to allow image-guided brachytherapy and a rapid scan-plan-treat workflow that is well suited to a busy quaternary care medical center. The suite consists of an OR couch with CT-compatible insert, a CT-on-rails imaging unit, a Varian Varisource iX HDR afterloader and full anesthesia capabilities. The explicit goal was to provide the ability to perform applicator placement, CT-guided treatment planning, and treatment delivery efficiently and without moving the patient. The dedicated CT-on-rails suite for high-dose-rate brachytherapy offers image-guided brachytherapy capabilities with a rapid workflow that lends itself well to efficient, high-quality care that can meet the demands of a large-volume referral center capable of high patient throughput.

SU-D-105-06: A Novel QA Process Based On Monte Carlo 2nd Check and Dose Reconstruction From Machine Log File On TomoTherapy

PURPOSE To develop a QA process that combines the pre-treatment plan verification with post-treatment delivery verification. Individual source of error such as dose calculator, MLC, and gantry angle can be isolated and analyzed. METHODS A Monte-Carlo (MC) package, TomoPen, has been modified for pre-treatment plan verification. During TomoTherapy treatment, information such as ion chamber measured output, gantry and couch position, and MVCT exit-detectors fluence are recorded into a log file (rawdata). The log file is retrieved after each treatment and information about the treatment delivery is extracted. MLC movement is obtained through analysis of the exit-detector fluence. The actual delivery information is fed into MC dose calculation to evaluate the overall impact. 255 treatment deliveries in 38 patient plans were analyzed with this process. RESULTS The average percent difference from the MC 2nd check on planned dose was -1.2%. MLC errors have strong correlation with plans. On average, MLC errors were 0.1+/-2.2%, which will produce negligible error. However, error as big as 1.7+/-0.8% was observed. It is also observed that the MLC errors did not have big variations day-to-day for the same patient plan. The LINAC output has been a big source of error. On average, the output dropped by 0.8% during each treatment, and varies day to day as target degrades. The output error for all treatment during a 7 month span has been 1.3+/-1.1%. MC reconstructed dose is found to correlated with the mean MLC and output errors. The log file analysis can be completed in less than 30 seconds and the MC calculation took 2-4 minutes. CONCLUSION We have developed a QA process capable of detecting errors from the planning to treatment delivery for TomoTherapy. The advantage over the existing phantom based QA method is that it provides more information about discrepancies in planning and delivery. This study is supported in part by UVa George Amorino Pilot Grant.

SU-E-T-332: TomoTherapy Patient Treatment Delivery QA Utilizing Phantom-Less Exit-Detector Patient Delivery Coupled with Monte Carlo Dose Calculations: Validation.

PURPOSE To describe and validate a pre-treatment end-to-end patient dose verification system for TomoTherapy capable of detecting plan transfer, dose calculation, and plan delivery errors and evaluating the dosimetric impact of those errors. METHODS The MCLogQA method for TomoTherapy utilizes a pre-treatment Monte Carlo (MC) dose calculation, post-delivery log file examination and exit-detector based MLC sinogram comparison to confirm intended machine performance. The delivered leaf sinogram is then used with MC for dose reconstruction to evaluate the dosimetric impact of any delivery discrepancies by examining target and OAR DVH metrics. A traditional phantom/ion chamber-based QA plan was created and delivered for ten randomly selected patients to evaluate the accuracy of the MCLogQA algorithms. The ion chamber dose measurements were compared with MC dose calculated using the log file and exit detector data collected during the delivery. Delivered linac output and MLC opening deviations found using the MCLogQA method are reported for 10 patients. RESULTS The MCLogQA reconstructed dose agreed with ion chamber measurements within 1%, while the planned dose deviated from measurement by 2-5%. Analysis of the 10 patients treatment delivery demonstrated that the output during TomoTherapy delivery can vary by approximately 2%. Although patient plans vary from -0.6% to 1.6%, the MLC leaf errors were consistent across fractions for the same patient (excluding one patient). The MCLogQA methods capability of evaluating the impact of deliver errors on patient geometry is demonstrated. CONCLUSION The agreement of MC dose calculations utilizing measured delivered sinograms with ion chamber measurements validates the delivered sinogram based QA method. Our system QAs the on-patient TPS dose computation via MC dose recalculation. Plan transfer and delivery errors are validated by using measured delivered sonograms. Patient dose affects are validated by dose volume metric comparison. Our procedure is an effective and efficient alternative to traditional phantom-based TomoTherapy plan specific QA. Research supported by Funding Opportunity Number CMS-1C1-12-0001 from Centers for Medicare and Medicaid Services, Center for Medicare and Medicaid Innovation. The contents are solely the responsibility of the authors and have not been approved by the Department of Health and Human Services, Centers for Medicare & Medicaid Services.

SU‐E‐T‐567: Evaluation of Contrast Induced CT Artifact for Intracavitary Brachytherapy

PURPOSE Use of heterogeneity correction has only recently been applied in brachytherapy. Vaginal packing with gauze is used in gynecological brachytherapy to physically move the bladder and rectum farther from the applicator, reducing dose to these structures. The Alatus balloon packing system accomplishes the same task, but use of contrast in the balloon can also obscure these structures on CT images. This work was designed, and conducted using a prostate phantom, to determine the optimal contrast concentration in terms of streak artifact and noise and whether patient anatomy would be obscured by high contrast concentration. METHODS A 60 cc syringe was filled with Omnipaque contrast liquid (350 mg I/ml) of varying concentrations (100% contrast to pure water as well as air) and placed in the phantom rectum in a CIRS prostate phantom (used to permit consistent measurement CT scan was then acquired. The mean CT number of a region of interest inside the prostate was determined, along with the percent noise. Additionally, the distance between the rectum and prostate was determined to see if resolution near the high density material was lost. RESULTS The mean CT number of the region of interest did not change from air or water in the syringe through the 100% contrast scan, but the noise increased from 12% to 33%. In all scans the 2 mm distance from the edge of the rectum to the prostate was readily seen. CONCLUSION Previous work has shown that overriding the density in the Alatus balloon packing system for gynecological brachytherapy can lead to additionally radioprotection of the bladder and rectum. This study shows that use of undiluted contrast does not lead to unacceptable streak artifacts that would obscure anatomy, and can be clinically implemented.