ESTRO 36 Abstract Book

S202 ESTRO 36 _______________________________________________________________________________________________

Conclusion We reveal a critical pathway linking ATM and SPOP in regulation of prostate cancer initiation and therapeutic responses to radiation. This also provides the first evidence for a pathophysiological relevant mutation linked to ATM phosphorylation in the DDR.

received an inadequate therapeutic dose. Concerning the low number of locoregional relapses in AC patients after definitive CRT one has to balance increased skin side effects by including the AILD into the standard CTV against a rigid oncological-anatomical interpretation of the local lymphatic drainage.

Award Lecture: Jack Fowler University of Wisconsin Award

Award Lecture: Company Award Lectures

OC-0376 Trajectory Optimization in Radiotherapy Using Sectioning (TORUS) C. Locke 1 , K. Bush 1 1 Stanford Cancer Center, Radiation Oncology, Stanford, USA Purpose or Objective One of the most challenging problems in trajectory optimization for radiotherapy is properly handling the synchronization of the medical accelerator’s dynamic delivery. The initial coarse sampling of control points implemented in a Progressive Resolution Optimization type approach (VMAT) routinely results in MLC aperture forming contention issues as the sampling resolution increases. IMRT based solutions such as 4Pi avoid MLC synchronization issues through use of a static gantry, but inevitably suffer from longer treatment times. This work presents an appoach to optimize continuous, beam-on radiation trajectories thorough exploration of the anatomical topology present in the patient and formation of a novel dual metric graph optimization problem. Material and Methods This work presents a novel perspective on trajectory optimization in radiotherapy using the concept of sectioning (TORUS). TORUS avoids degradation of 3D dose optimization quality by mapping the connectedness of target regions from the BEV perspective throughout the space of deliverable coordinates. This connectedness information is then incorporated into a graph optimization problem to define ideal trajectories. The unique usage of two distance functions in this graph optimization permits the TORUS algorithm to generate efficient dynamic trajectories for delivery while maximing the angular flux through all PTV voxels. 3D dose optimization is performed for trajectories using the Varian’s Photon Optimizer (version 13.6.23). Results The TORUS algorithm is applied to three example treatments: chest-wall, scalp, and the TG-119 C-shape phantom. When restricted to only coplanar trajectories for the chest-wall (dose distributions shown in Figure 1) and scalp cases, the TORUS trajectories are found to outperform both 7 field IMRT and 2 arc VMAT plans in delivery time, organ at risk sparing, conformality, and homogeneity. When the coplanar restriction is removed for the TG-119 phantom and the static non-coplanar trajectories are optimized, TORUS trajectories have superior sparing of the central core avoidance with shorter delivery times, with similar conformality and

OC-0375 Dosimetric quantification of the „true“ ano- inguinal lymphatic drainage of anal cancer patients H. Dapper 1 , G. Habl 1 , M. Mayinger 1 , M. Oechsner 1 , S.E. Combs 1 , D. Habermehl 1 1 Klinikum rechts der Isar der Technischen Universität München, Department of Radiation Oncology, München, Germany Purpose or Objective The ano-inguinal lymphatic drainage (AILD) to the inguinal lymph nodes is located in the subcutaneous adipose tissue on the medial thigh. Even though those node-vessels are very thin and hardly to detect with lymphangiography, this fact is described and shown in standard anatomy atlases. New fluorescence-imaging methods like the indocyanine- green-method corroborate this fact. Anal cancer (AC) patients undergo a combined chemoradiation (CRT) protocol and the clinical target volume (CTV) encompasses the inguinal lymphdrainage because of its affection in about 30% of all patients. Current contouring atlases suggest delineation of the primary tumor region, the mesorectum, inguinal and iliacal lymph nodes but do not advise the inclusion of the true AILD. Aim of this work was the retrospective analysis of the incidental dose to the AILD in an anal cancer patient cohort who underwent definitive CRT with VMAT-IMRT and using structure sets according to current guidelines. Material and Methods VMAT-IMRT plans of 10 anal cancer patients who had been treated with CRT during 2014 and 2016 were analyzed. On these plans we created a new volume, the expected ano- inguinal lymph drain (AILD). Based on anatomic descriptions, we connected the soft tissue between the anus and the inguinal vessels with the following demarcations: The caudal demarcation was defined 2 cm below the tuberculum minus. The cranial end of AILD was at the level of the symphysis (anal) or where no more soft tissue connection between anus and inguinal could be identified (inguinal). Ventral demarcation was the femoral skin, dorsal was the transition of the gluteal muscles to the subcutaneous adipose tissue. The lateral demarcation was the adductor muscles (anal) and the medial femur bone or at least 0.5 cm around femoral vessels (inguinal). We examined dose parameters (minimum, maximum, median, V10, V20, V30, V40, V45, V50) that were delivered to the AILD target volume and the AILD outside of the previous PTV (AILD-PTV) as represented in the dose- volume histogram. Results All of the 10 patients received at least 39.6 Gy to the inguinal lymph nodes, 45 Gy to the iliacal lymph nodes and 50.4 Gy to the primary tumor side. The median volume of AILD and AILD-PTV was 1066 cm3 and 689 cm3, respectively. Mean Dmin, Dmax and Dmean were 5.5 Gy, 58.1 Gy and 38.4 Gy for AILD and 5.5 Gy, 55.2 Gy and 31.1 Gy for AILD-PTV, respectively. Mean V30, V40, V45, V50 for AILD was 71%, 55%, 45% and 31%, respectively. For AILD-PTV it was 57%, 29%, 18% and 5%, respectively. Conclusion At least 71% of the volume of the expected AILD received at least an expected required treatment dose of 30 Gy incidentally. Especially the caudal parts of the created volumes, with a clear distance to the previous PTVs,