ESTRO 2024 - Abstract Book

S2521

Clinical - Urology

ESTRO 2024

Dosimetric coverage of the SV was estimated on a post-fraction CT-scan, on which, after contouring, the adapted plan was rigidly transferred based on a fiducial match with the pre-fraction CT-scan. In case only the base of the SV was part of the target volume, a treatment on the entire SV was simulated and used for analysis. For all participants, SV V95 and relevant OAR-dosimetric parameters were recorded. A treatment was considered successful when, for one patient, a maximum of one out of six fractions showed underdosage of the SV (V95 <99%) on the post-fraction CT-scan. Regarding adaptation, the online adapted plans and the non-adapted baseline plan made during preparation were compared on SV V95 and relevant OAR-dosimetric parameters, after a rigid transfer onto the post-fraction CT-scan based on a fiducial match.

As per trial protocol, genitourinary (GU) and gastrointestinal (GI) toxicities were recorded prior to, directly after and three months after treatment, using CTCAE version 5.0.

Results:

Of the 12 completed treatments analysed, 11/12 were considered successful. One simulated treatment was considered unsuccessful as two out of the six fractions exhibited underdosage of the SV on the post-fraction CT scan. For both fractions, bladder volumes increased substantially (<200 mL pre-fraction to >800 mL post-fraction) resulting in a large posterior/caudal displacement of the SV and subsequently SV V95 of 78.2% and 84.9%, respectively. Of the remaining 11 treatments (consisting of 66 fractions), only two fractions of different patients exhibited SV underdosage on the post-fraction CT-scan. One fraction showed an anterior/cranial displacement of the SV caused by an intrafraction large rectal gas pocket, whereas the second fraction exhibited a posterior/caudal displacement of the SV due to an intrafraction bladder volume increase. Comparing adaptation to the non-adapted treatment plan, an improvement in the population average SV V95 on the post-fraction CT- scan from 97.6% to 99.3% (aim ≥ 99%) was seen, as well as a slight improvement of the OAR dosimetrics, within the constraints. However, on an individual patient level, adaptation resulted in only one unsuccessful treatment compared to four using non-adapted plans. Looking at the individual fractions, adaptation resulted in a total of only four fractions with SV underdosage compared to a total of 18 fractions for the non adapted plans. Furthermore, the adapted plans resulted in OAR doses adhering to the OAR constraints for 13 of the fractions in which the non-adapted plans did not adhere to the constraints.