ESTRO 2020 Abstract book

S952 ESTRO 2020

PTVs) except cord (0.79), esophagus (0.79), brachial plexes (0.62), and cochleas (0.60). The average difference between the same day wCT and the CBCT padded with wCT was -0.11 cGy (0% of planned fraction dose, SD 0.99 cGY); CBCT padded with pCT was -0.08 cGy (0%, SD 0.73 cGy); CBCT without padding was 3.75 cGy (0.25%, SD 2.03 cGy). Although padding with the wCT had a smaller dosimetric difference, time trends were evaluated using the Mann- Kendall test at a significance level of 0.05. None of the tests produced significant p-values, which points to the absence of trends in the dose differences over time between padding with the pCT or wCT. Conclusion DIR can be accurately performed between CT and CBCT. CBCT padding with planning or weekly CT images enables accurate dose calculations over the full FOV. Combined together, these tools enable full FOV dose accumulation over the course of treatment using CBCT to support adaptive RT in HN. PO-1643 Factors associated with image guided radiation therapy image rejection in a multi-site institution B. Traube 1 , M. Khan 2 , R. Kumar 2 , G. WALKER 2 1 University of Arizona, Medicine, Phoenix, USA ; 2 Banner MD Anderson Cancer Center, Radiation Oncology, Gilbert, USA Purpose or Objective The study evaluated the factors that influenced the approval vs rejection of image-guided radiation therapy images (IGRT) in a large practice. Material and Methods The approval and rejection incidents of IGRT recorded within an electronic imaging system was obtained from July 1, 2016-June 30, 2018. Variables included: the attending physician of the patient, the physician reviewing the image, total images reviewed the physician that day, time of day, day of week, treatment site, imaging modality, whether review occurred prior to a holiday, and degree of shift made. Logistic multivariate analysis was performed to determine factors associated with IGRT rejection rate controlling for treatment site and physician variability in approval rates. Results There were 51,797 image records obtained, of which 881 (1.70%) were rejected and 50,916 (98.30%) were approved. Multivariate analysis showed that the following factors were associated with an increased rate of IGRT rejection: images reviewed after regular clinic hours (OR 1.32, p=0.025), by physicians with high rejection rates (OR 3.41, p<0.001), by non-treating physician (OR 1.25, p=0.002), by physician reviewing less number of IGRT images (OR 0.99, p=0.009), by specific anatomical sites and cone beam computed tomography (CBCT) images (OR 1.62, p<0.001) (Table 1).

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