Abstract Book

S1241

ESTRO 37

Results In this experiment, the variation of TLD measurements presented within 3% of standard deviation. Percentage of dose different between TG-43 and TLD measurements were found in the range of 7.58% to -21.21%, and -1.26% to 6.61% for Acuros TM BV. In homogeneous medium, dose calculation algorithm based-on TG-43 and Acuros TM BV presented the accurate dosimetric results with the dose different less than 4% when compared to the TLD. However, measuring point in the heterogeneous as behind the cortical bone showed the maximum dose different at -21.21% in TG-43, and was decreased to be 6.61% with Acuros TM BV. About the gamma evaluation with criteria of 3% dose difference and 3mm DTA as well as 10% threshold, the data showed the gamma passing rate at 97.3% for TG-43, and 98.3% for Acuros TM BV when compared to film measurement. Conclusion This investigation showed the accurate dosimetric results from both of TPS dose calculation algorithms in homogeneous region. However, heterogeneity correction algorithm as Varian Acuros TM BV showed significantly improve the accuracy of TPS dose calculation in heterogeneous medium when compared with the measurement. EP-2245 Precision IORT – Dosimetric and geometric evaluation of 10 Kypho-IORTs F. Schneider 1 , K. Steinbach 2 , F. Bludau 2 , S. Clausen 1 , F. Giordano 1 , F. Wenz 1 , U. Obertacke 2 , Y. Abo-Madyan 1 1 University Medical Center Mannheim, Department of Radiation Oncology, Mannheim, Germany 2 University Medical Center Mannheim, Department for Orthopaedics and Trauma Surgery, Mannheim, Germany Purpose or Objective Kypho-IORT is a treatment option for patients with spinal column metastases in which a minimally invasive surgical stabilisation of the vertebra (kyphoplasty) is combined with a sterilising dose of intraoperative radiation therapy (IORT).[1] This technique is geometrically limited by the trajectory through the pedicel to the metastasis inside the vertebra the surgeon can use to insert the kyphoplasty equipment and the needle applicator for IORT. It is dosimetrically limited by the dose to the spinal cord. Because of the lack of a treatment planning system (TPS) taking tissue heterogeneities into account, table based water prescription dose and depth were escalated in a clinical trial. [2] In this study a new Monte Carlo based TPS was retrospectively used to geometrically and dosimetrically evaluate 10 patients treated with Kypho-IORT with its aim to individualize this kind of treatment. Material and Methods 10 patients underwent standard Kypho-IORT in vertebrae ranging from Th8 – L5 using the Intrabeam system including a dedicated Needle Applicator (Carl Zeiss Meditec AG, Jena, Germany). To measure the distance from applicator tip to the spinal canal intraoperative cone beam CTs (CBCT) using Artis Zeego (Siemens, Erlangen, Germany) were performed with the applicator in place. Post-operatively these CBCTs including each contoured applicator tip were registered with preoperative CTs including the contoured spinal canal and the metastasis as described in Schneider et. al. [3] Dose calculations were performed in Radiance (GMV, Madrid, Spain) using a hybrid Monte Carlo algorithm simulating dose in heterogeneous tissue. Geometrically the difference between applicator tip and center of each metastasis was evaluated. Dosimetrically the 8Gy coverage of each metastasis and the maximum dose to the spinal canal were evaluated.

Results The geometrical distance from the tip of the applicator to the ideal center of the metastases, ranging from 1.14cm³ to 17.84cm³ in size, was 6.4mm in average (2.4mm – 13.1mm). Dosimetrically 44.9% - 99.9% of the metastases were covered with 8Gy. The maximum dose to the spinal canal was within 1.5Gy and 7.2Gy. Conclusion Because of the high attenuation in the cortical bone the maximum dose to the spinal canal was less than expected. The 8Gy coverage is rather driven by the applicator position than the size of the metastasis. So a pre-operative treatment planning followed by an image guided surgery might help defining the trajectory and to optimize the treatment. 1. Wenz, F., et al., Kypho-IORT--a novel approach of intraoperative radiotherapy during kyphoplasty for vertebral metastases. Radiat Oncol, 2010. 5: p. 11. 2. Bludau, F., et al., Phase I/II trial of combined kyphoplasty and intraoperative radiotherapy in spinal metastases. Spine J, 2017. 3. Schneider, F., et al., Precision IORT - Image guided intraoperative radiation therapy (igIORT) using online treatment planning including tissue heterogeneity correction. Phys Med, 2017. 37: p. 82-87. EP-2246 Generating and navigating a multi-criteria trade-off surface in high dose-rate brachytherapy C. Deufel 1 , V. Wu 2 , M. Epelman 2 , M. Sir 3 , K. Pasupathy 3 , M. Herman 1 1 Mayo Clinic MN, Radiation Oncology, Rochester, USA 2 University of Michigan, Department of Industrial and Operations Engineering, Ann Arbor, USA 3 Mayo Clinic MN, Health Sciences Research, Rochester, USA Purpose or Objective A method for generating and navigating the full range of dosimetric trade-offs between the target(s) and healthy tissue(s) in a high dose-rate (HDR) brachytherapy treatment is presented. This approach aims to improve HDR treatment planning efficiency and quality. Material and Methods The method generates a multi-criteria trade-off surface from an extensive library of high-quality HDR candidate plans, each obtained by solving an instance of a constrained optimization. Triangulation is further used to fill-in the surface between optimized plans by interpolating the dwell times of relevant library plans in each region. The trade-off surface is navigated with a graphical user interface (GUI) that enables the user to explore improving any structure at the expense of one or more other structures. The GUI displays pair-wise dose volume histogram (DVH) tradeoffs among all structures and has a selection tool for controlling the magnitude of improvement for the selected structure. The GUI also provides pairwise metric tradeoff ratios (e.g. slopes) local to the current plan. Results Proof-of-principle was demonstrated using a clinical case for the prostate treatment site and five DVH metrics, PTVD95, PTVD10, UrethraD10, BladderD1cc, and RectumD1cc. 625 candidate plans that were distributed over the trade-off surface were generated. The candidate plans were generated by varying the optimization constraints for four of the metrics (all of the metrics except for PTVD95, which was the optimization objective) by ± 10% variation in 5% increments. Plans chosen using the GUI were deliverable, i.e. it was composed of feasible dwell times that produce the displayed plan doses. The loss of quality from interpolation was less than 1%. The user may elect to perform a final optimization after navigation if coarser plan spacing in the library generation or less quality loss