ESTRO 2022 - Abstract Book

S192

Abstract book

ESTRO 2022

Results The median (range) amplitude of respiratory motion measured at sternum position in the axial plane on the 4DCT scan was 2 mm (1 to 4 mm). Dosimetric results from the nominal plan and the 4D evaluation can be seen Table 1 and an example of a dose-volume histogram for a patient can be seen in Figure 1. CTV coverage was below 98% for level 4 in one nominal plan and three 4D evaluations, for IMN only in one 4D evaluation and for breast/chest wall in one nominal plan. In the 4D evaluation, mean dose to the heart increased to 0.3 Gy RBE (from 1.9 to 2.2 Gy RBE) and decreased to 0.5 Gy RBE (from 1.6 to 1.1 Gy RBE), and mean dose to the ipsilateral lung increased to 1.0 Gy RBE (from 9.8 to 10.8 Gy RBE) and decreased to 0.8 Gy RBE (from 12.7 to 11.9 Gy RBE).

Conclusion Respiratory motion has a modest impact on the dose distribution in proton therapy plans for breast cancer when using robust and single-field optimisation and an en face beam arrangement.

PD-0232 Large intra-fractional tumor position variations in deep-inspiration breath-hold lung SBRT

W. Ottosson 1 , N.C. Rand Momsen 1 , S. Fortin Jørgensen 1 , S.N. Bekke 1 , P. Sibolt 1 , C.P. Behrens 1 , G. Fredberg Persson 1

1 Copenhagen University Hospital – Herlev and Gentofte, Department of Oncology, Copenhagen, Denmark

Purpose or Objective Deep-inspiration breath-hold (DIBH) stereotactic body radiotherapy (SBRT) is an option for very mobile or very small lung tumors. Inter-fractional tumor position variation is minimized by daily cone-beam CT (CBCT), but the extent of intra- fractional tumor position variation is difficult to assess. The aim of this study was to estimate the intra-fractional tumor position variation based on CBCTs acquired before and mid-treatment for patients receiving lung SBRT in DIBH and free breathing (FB). Additionally, investigate what impact two different patient cohorts have on the resulting setup margins. Materials and Methods Two patient cohorts (MM1 (from April 2018 to March 2019), and MM2 (from October 2020 to September 2021)), in total 84 consecutive patients scheduled for lung SBRT (67.5 Gy / 3 fractions, 45 Gy / 3 fractions, or 50 Gy / 5 fractions) were retrospectively included in this study. Both cohorts included 21 DIBH and 21 FB patients each. Patients were selected for