ESTRO 2020 Abstract Book

S900 ESTRO 2020

exceeding 97.2% (3%/2mm). DVHs and dose-volume metrics were also in satisfying agreement. Mean doses measured as compared to calculated in the treatment planning system agreed within 0.4%. Targets larger than 10 mm showed a decrease in D95 of less than 2.7% relative to the planning system. The D95 degradation was more pronounced with decreasing target size. The 3D nature of gel dosimetry allows us to perform spatial accuracy analysis of the delivered dose to the targets. A dose distribution center of mass comparison for each of the targets revealed a spatial accuracy of delivery ranging from 0.4 – 1.3 mm. Conclusion Overall the results of this work suggest that surface imaging can be both an accurate and efficient way to verify table positioning for multi-focal mono-isocentric stereotactic radiosurgery on the Elekta VersaHD utilizing HDRS. PO-1642 CBCT Padding for Full Field of View Daily Dose Accumulation and Head and Neck Adaptive Radiotherapy K. Brock 1 , A. Ohrt 1 , G. Cazoulat 1 , M. McCulloch 1 , P. Balter 2 , J. Ohrt 2 , S. Svensson 3 , R. Nilsson 3 , S. Andersson 3 , A. Mohamed 4 , H. Bahig 4 , Y. Ding 4 , J. Wang 2 , B. McDonald 2 , J. Yang 2 , S. Vedam 2 , B. Elgohari 4 , A. Sen 1 , C. Fuller 4 1 The University of Texas MD Anderson Cancer Center, Imaging Physics, Houston, USA ; 2 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA ; 3 RaySearch Laboratories, Research Department, Stockholm, Sweden ; 4 The University of Texas MD Anderson Cancer Center, Radiation Oncology, Houston, USA Purpose or Objective Daily CBCT for dose accumulation in head and neck (HN) radiotherapy (RT) is challenged by the limited field of view (FOV) of the CBCT, which excludes critical normal tissues and CTVs. The goal of this work is to validate the use of CBCT padding to enable daily dose accumulation. Material and Methods Planning CT (pCT), weekly CTs (wCT), and daily CBCTS were obtained for patients enrolled on an institutional review board approved clinical trial for adaptive RT. CBCT padding was achieved using ANACONDA DIR between the pCT and wCT and the CBCTs images. Only intensity information was used to drive the deformation. Outside of the CBCT FOV, where information to drive the DIR is not present, a smooth transition between the DVF in the region of the CBCT and a purely rigid registration at the boundary of the CT was achieved through iteratively increasing the regularization until a non-inverted DVF was achieved. Using the DVF, a padded CBCT was generated by augmenting the CBCT data with the transformed CT data. The DVF was used also for propagating the pCT contours onto the padded CBCT. To be able to compute dose on the images, the CBCT was corrected for shading artefacts and converted to HU using an algorithm available in the research version of RayStation. Dose was then calculated on the CBCT, pCt padded CBCT, and wCT padded CBCT. Qualitative evaluation was performed to assess the transition zone of the padded CBCT. CT to CBCT DIR accuracy was assessed by evaluating the Dice Similarity Coefficient (DSC) of all structures. Clinical dose metrics were computed and compared between the same day wCT and the CBCT, CBCT padded with wCT, and CBCT padded with pCT. Results CBCT padding has been evaluated for 42 images on 7 patients to date. Qualitative evaluation demonstrated smooth transitions with minimal artifacts using the pCT and wCT for padding. DIR-based contour propagation resulted in DSC greater than intra-observer contour variation (0.8) for all structures (OARs, GTV, CTV, and

No correlation is found between bladder volumes and delivered maximum dose. However, in patients who suffer a considerable weight loss during the treatment, the maximum delivered dose in the bladder is higher (CI = - 0.32; p <0.01) as well as the maximum delivered dose in the body (CI = -0.55; p <0.01). Conclusion Daily anatomy of rectum and bladder has a considerable impact on delivered dose. The patient preparation protocol makes the plan reproducible with regard to the rectum, but not to the bladder in which delivered doses are greater than plan doses over the entire dose range. PO-1641 Role of surface imaging for verification of mono-isocentric multi-focal stereotactic radiosurgery D. Saenz 1 , V. Bry 1 , K. Zourari 2 , E. Zoros 2 , E. Pappas 3 , K. Rasmussen 1 , N. Papanikolaou 1 1 UT Health San Antonio, Radiation Oncology, San Antonio, USA ; 2 National and Kapodistrian University of Athens, Medical Physics Laboratory, Athens, Greece ; 3 University of West Attica, Department of Biomedical Sciences Radiology & Radiotherapy Sector, Athens, Greece Purpose or Objective Stereotactic radiosurgery (SRS) requires high accuracy and precision when targeting intracranial lesions. Cone-beam CT has been shown to be effective for initial setup and alignment but cannot be conducted at table positions other than 0°. We aimed to demonstrate that surface imaging can be used to provide the necessary verification of patient positioning at other table angles when other x- ray-based imaging technologies are not available. Material and Methods An SRS treatment plan was devised on an RTsafe Prime phantom using Elekta high definition dynamic radiosurgery (HDRS) in Monaco. Five targets of varying diameter (7-20 mm) were targeted with a single isocenter (targets 12-38 mm from isocenter) to 8 Gy in a 3D printed polymer gel phantom matching patient anatomy. The phantom was setup on the Elekta VersaHD linear accelerator at table angle 0° in a frameless SRS mask and aligned to the isocenter based on CBCT and a tabletop capable of six- degree of freedom corrections. A reference image was then created using the CRAD Catalyst HD three-camera surface imaging system. Subsequent arcs treated at different angles were verified making corrections as suggested by surface imaging (shifts > 0.7 mm). The irradiated phantom was subsequently scanned on a 1.5 T MRI unit using a 2D multi-slice, multi-echo PD to T2- weighted sequence using a head coil with 2 mm slice thickness. In the resulting scans, the absorbed dose is directly proportional to the relaxation rate. Results were compared with the planned dose distribution and assessed using 3D gamma analysis, target mean dose and D95, and geometric offset. Results 3D GI (gamma index) comparison between the calculated and measured dose distributions resulted in passing rates