ESTRO 2020 Abstract book

S1114 ESTRO 2020

the bony anatomy from repeat 4D computed tomography scans (4D CT) and to show the potential impact of these inter-fractional variations in state of the art Volumetric Modulated Arc Therapy (VMAT) and Intensity Modulated Proton Therapy (IMPT) plans. Material and Methods A total of 20 EC patients were included in this study. Based on the close anatomic relationship between the gastroesophageal junction and the left diaphragm, the left diaphragm was used as a surrogate for the target and was delineated in all phases of the 4D CT scans to establish the maximum expiration and inspiration phase. The maximum diaphragm expiration (DE) and diaphragm inspiration (DI) delineations were transferred to the average CT-scans where the breathing amplitude was determined by the distance between the DE and DI delineations. To establish the location of the diaphragm with respect to the bony anatomy, the linear superior-inferior (SI) distances from the top of the twelfth thoracic vertebra to the top of the DE and DI delineations were measured on baseline (pCT) and repeat CT-scans (rCTs). The off-set was defined by the measured difference in DE and DI diaphragm locationon the rCTs with respect to the pCT. The potential dosimetric impact of respiration motion was evaluated for a 2 arc VMAT and an 2 posterior fields IMPT proton therapy plan on an average repeat CT with a large off-set and one with a small off-set. Results Differences in diaphragm amplitudes of the repeat CT- scans compared to the planning CT-scan were relatively small and ranged from 0 – 0.8 cm (figure 1). However, the established off-sets were larger, ranging from -2.1 to 1.9 cm. Of the 70 repeat CT-scans, the off-set exceeded the ITV-PTV margin of 0.8 cm in expiration in 4 CT-scans (5.7%) and in inspiration in 13 CT-scans (18.6%). The dosimetric validation in two example patients showed hotspots up to 119.5% of the prescribed dose (figure 2). Consequently, for the IMPT plan the mean heart dose increased from 11.3 Gy on the pCT to 15.1 Gy on the rCT. For the VMAT plan it increased from 19.8 Gy to 20.9 Gy, respectively.

Conclusion This study demonstrates that despite relatively constant breathing amplitudes, positions of the diaphragm with respect to the bony anatomy, and consequently tumor positions, were large. These motion effects may result in missing the target volume or dose deviations in terms of hot or cold spots. PO-1901 Rectal volumes comparison in prostate cancer radiotherapy treated before and after noon in 1140 CBCT P. Umbarkar 1 , R. Harjani hinduja 1 , S. Deshpande 1 , S. Naidu 1 , O. Jadhav 1 , S. Ullagaddi 1 , S. Shinde 1 , N. Parmar 1 , A. Jejurkar 1 , P. Kamble 1 , V. Anand 1 , R. Bajpai 1 , R. Kabre 1 , P. Alurkar 1 , W. Mistari 1 , V. Kannan 1 1 P D Hinduja Hospital, Radiation oncology, Mumbai, India Purpose or Objective Accuracy of radiation therapy delivery for prostate cancer depends on consistent rectal and bladder filling. Daily variations in rectal filling can significantly alter the delivered dose distribution as compared to that intended. A number of factors have been postulated to impact the rectal volume. The objective of this study was to assess whether time of treatment delivery has any impact on rectal filling. Material and Methods This is a comparative study of 1140 CBCT image sets where the rectal volume was compared based on the time of treatment. Thirty-eight patients with localized prostate cancer who were treated with radical radiotherapy to prostate ± seminal vesicles were included in this retrospective study. The patients were treated using arc technique. 1140 cone beam computerized tomography (CBCT) image-sets were obtained as each patient received radiation treatment with daily CBCT verification. The bowel preparation consisted of dietary advice to all patients along with a laxative to those deemed necessary (after assessing planning CT images, which were also repeated if necessary) as decided by the treating physician. The rectum was contoured on all CBCT image-