ESTRO 2022 - Abstract Book

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Abstract book

ESTRO 2022

registration to the pCT, for each robustness scenario a dose summation was created, deforming dose from reCT to pCT. Robust dose summation was performed by summing each scenario over all reCTs. Subsequently, voxel-wise minimum and maximum summed dose distributions were derived. Diaphragm positions in inhale and exhale phases of the 4D CT were extracted by measuring the diaphragm-lung intersection at the axial center of each lung. Therefrom a baseline diaphragm position (i.e. the average between inhale and exhale) and a diaphragm amplitude (i.e. the difference between inhale and exhale diaphragm position) were derived. Results Diaphragm average amplitude [standard deviation (SD)] on the pCT was 1.4 [0.8] cm. The amplitude change during treatment evaluated on each reCT was on average 0.2 [0.5] cm. The average baseline shift observed on the repeat CTs was -0.2 [0.5] cm (Figure 1). The average difference in voxel-wise minimum ITV D98% [SD] on repeat CTs with respect to the planning CT was -2.7 [9.4] %. We found a weak negative correlation between changes in ITV D98% and baseline shift (Pearson’s R = 0.54) and no correlation with changes in amplitude. After robust dose summation, the ITV D98% of the summed dose distributions is >94% for 32 of 35 evaluated patients, in accordance with the 90% aimed at in the margin recipe (Figure 2).

Conclusion Despite changes in diaphragm amplitude up to 2.5cm and in baseline position up to 2 cm during patient treatment, robust dose summation showed that ITV coverage in proton therapy for esophageal cancer remained sufficient using a posterior beam setup.

PD-0397 Influence of manual segmentation in DIR on accumulated dose evaluation for cervical cancer