ESTRO 2022 - Abstract Book

S1627

Abstract book

ESTRO 2022

Before the planning-CT scan a short CT scan was performed. In sagittal and transverse views it was examined if the bite block was properly positioned. If the bite block was not positioned correctly, it was repositioned, and the short scan was performed again. When the RTTs were not satisfied, a radiation oncologist was consulted for adjustment of the bite block. 30 patients were included in this study. For the first 10 patients the short CT scan was assessed by 4 RTTs with different expertise (CT, mouldroom, IGRT). After this initial training phase the short CT scans of the remaining 20 patients were judged by 2 CT RTTs. Prior to the daily treatment for each patient an online CBCT scan was made and registered to the planning-CT scan. At the end of treatment all CBCT scans were analyzed and checked whether the position of the bite block was equal to the planning-CT scan. Results In 7 out of 30 planning-CT scans (23%) the bite block was repositioned due to the short CT scan. For these planning-CT scans with problems , in 83 out of 161 CBCTs (51%), the bite block was incorrectly positioned. For the 23 planning-CT scans without problems in only 7 out of 642 CBCTs (1%) the bite block was incorrectly positioned. We conclude that, when the RTTs had to reposition the bite block at the planning-CT scan, the RTTs had to reposition the bite block at the linac many times, leading to additional CBCTs. A frequently found problem was that due to the lack of teeth, patients had no grip on the bite block and the bite block could be positioned in various ways. Also it was found that if the bite block was difficult to place at the planning-CT scan, it appeared that during treatment it was not possible anymore due to toxicity. Conclusion By introducing the short CT scan for head-and-neck cancer patients, the RTTs at the planning-CT became more aware and critical with regard to placing and checking the position of the bite block.Also adjustment of the bite block at the planning- CT gave a better reproducibility at the linac. This caused less differences between planning-CT and CBCT scans. The position of the bite block became more representative for the whole treatment. In addition we have gained better insight and understanding when to expect difficulties with the bite block at the linac. These patients are closely monitored by the RTTs. Purpose or Objective To investigate the contour agreement and dosimetric accuracy in the utilization of different registration methods and anatomical landmarks for cone beam computed tomography (CBCT) verification of primary radical prostate cancer radiotherapy. Materials and Methods 15 prostate cancer patients, each with their own 9 sets of CBCT images, were retrospectively recruited. Their planning Computed Tomography (pCT) were deformably registered to their pre-treatment CBCT to construct deformed planning CT (dpCT) images. Therefore, the characteristics of the CBCT images were preserved in the dpCT. All available dpCT images (n=135) were then registered to the pCT using 3D translations and yaw-rotation. The registration was conducted three times for each set of dpCT, using (1) automatic whole image registration (all slices in the range of dpCT dataset), (2) automatic planning target volume (PTV) based registration, and (3) manual registration carried out by an experienced oncologist. Per registration approach, dose distribution in each registered dpCT dataset was calculated with an identical setting to the original treatment plan. Contour agreement between registered dpCT and pCT images was measured for targets and organ at risks (OARs) in terms of dice similarity coefficient (DSC). Results Statistically significant differences in D2% (dose received by 2% volume) and D95% (dose received by 95% volume) PTV, V50Gy (volume in % receiving 50Gy) and V70Gy (volume in % receiving 70Gy) rectum, V55Gy (volume in % receiving 55Gy) and V70Gy bladder, DSC PTV and DSC rectum were found among three registration techniques. The whole image registration significantly reduced the target hotspots (D2% PTV), compared with the PTV-based and manual registrations. Superior PTV and rectal alignments (DSC PTV and DSC rectum) were also observed in the whole image registration than manual registration, and higher performance in bladder preservation (V55Gy and V70Gy) than the PTV-based registration. However, its target coverage (D95% PTV) was inferior to manual registration. The PTV-based registration showed significantly higher D95% PTV, DSC PTV, and DSC rectum with lower rectal dose (V50Gy and V70Gy) than other registration methods. For the manual registration, it had the highest achievement in bladder sparing (V55Gy and V70Gy) among all techniques. Conclusion Dosimetric and anatomic improvements can be achieved with a PTV-based registration for prostate cancer patients, increasing target coverage with superior contour agreement and rectal preservation. Target hotspots can be reduced by a whole image registration while manual registration can be employed for a higher level of bladder sparing. PO-1831 Different ROIs in CBCT Verification for Evaluation of Anatomic and Dosimetric Impacts in Prostate RT H. Pang 1 1 Pamela Youde Nethersole Eastern Hospital, Clinical Oncology, Hong Kong, Hong Kong (SAR) China

PO-1832 Quantification of fat on MRI and impact on effectiveness of abdominal compression for radiotherapy