ESTRO 2022 - Abstract Book

S269

Abstract book

ESTRO 2022

Conclusion This study shows that the overall 3D shift corrections for lung patients initially aligned with surface guidance were significantly smaller than those aligned without surface guidance.Surface imaging systems can be considered a good option for initial patient setup and are preferable to external marks for lung cancer patients. There is no significant difference between SGRT systems for lung patients treated on the same immobilization device. The positioning of SBRT patients on the Orfit base plate ™ resulted in a significantly smaller residual translational error compared to the Thorax Support ™.

MO-0307 Deep inspiration breath hold and online CBCT verification in gastric lymphoma patients

F. Ong 1 , S. Bouwhuis-Scholten 1 , C. Oude Hesselink 1 , Y. Reinders 1 , E. van Dieren 1 , L. Zwart 1

1 Medisch Spectrum Twente, Radiotherapy, Enschede, The Netherlands

Purpose or Objective Deep inspiration breath hold (DIBH) techniques for gastric lymphoma patients are used to decrease mean heart dose. However, reliable online verification is complicated as CBCTs of the abdomen are hard to interpret and generally two breath holds are needed. The aims of this study are to assess the quality of online verification, the adequacy of CTV coverage using standard margins, and confirm heart sparing as compared to free breathing (FB). Materials and Methods Four consecutive patients irradiated for st. I gastric lymphoma (MALT lymphoma or diffuse large cell B-cel lymphoma) were treated in DIBH with a dose of 30 Gy in 15 fractions to the whole stomach (in 1 patient followed by a boost of 6 Gy in 3 fractions). As comparison, for three gastric or pancreatic carcinoma patients treated with free breathing (FB) the stomach was retrospectively contoured on all phases of a 4D-CT, an ITV and PTV were generated and replanned to 30 Gy in 15 fractions. For all patients an isotropic PTV margin of 1.5 cm around the CTV or ITV was used. To check the reliability of the online soft tissue match, we contoured the stomach on the CBCT. The CBCTs were then matched to the planning-CT and the gastric volume on the CBCT outside of the 95% isodose was calculated both for the retrospective check as for the online match. In addition the mean heart dose between patients treated in DIBH and FB was compared. Results Of 60 CBCTs in DIBH 2 were 4D-CBCTs and not used, 2 were excluded because of insufficient quality (artifacts due to bowel gas) and on 13 CBCTs the extreme cranial or caudal edges of the stomach were not captured, leaving 43/60 CBCTs for analysis. In 41/43 fractions (95%) CTV coverage was excellent (<3% of stomach volume outside of the 95% isodose), both for the online match as for the retrospective soft tissue match using the contoured stomach on the CBCT. For FB 5 fractions per patients were analyzed. For the online match excellent CTV coverage was found in 11/15 fractions (73%), however, in the retrospective check, this was 13/15 (87%), with CTV coverage deemed excellent in 2 more patients. Concordance of the retrospective check with the online match was therefore 56/58 CBCTs (96.6%). Mean heart dose was considerably lower for DIBH (mean 3.75 Gy, range 1.18-5.8) than FB (mean 7.42 Gy, range 6.3-8.9).