ESTRO 2020 Abstract Book

S1060 ESTRO 2020

The results confirm that the application of registration guidance results in a significant improvement in inter- observer agreement for MR-CT image registration in the SI and AP directions. This strategy is expected to facilitate efficient integration of image registration for cervix plan of the day MGgRT and of other treatment sites following further validation. PO-1900 Is the 4D planning CT representative for breathing motion of esophageal tumors during treatment? Q. Roos 1 , Z. Faiz 2 , M. Dieters 1 , H.P. Van der Laan 1 , L.A. Den Otter 1 , J.T.M. Plukker 2 , S. Both 1 , J.A. Langendijk 1 , A. Knopf 1 , C.T. Muijs 1 , N.M. Sijtsema 1 1 University Medical Centre Groningen, Radiation Oncology, Groningen, The Netherlands ; 2 University Medical Centre Groningen, Surgical Oncology, Groningen, The Netherlands Purpose or Objective Radiotherapy for distal esophageal cancer (EC) is challenging because of high mobility of the tumor due to respiratory motion. When using a bony anatomy match for position verification, the tumor can be missed. Also, the position of the diaphragm with respect to the treatment beams may have a major influence on the radiological equivalent path lengths and therefore, on the delivered dose. Therefore, the aim of the current study was to evaluate the magnitude and inter-fractional variation of the breathing motion and position of EC with respect to 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