ESTRO 2025 - Abstract Book

S219

Brachytherapy - Gynaecology

ESTRO 2025

Purpose/Objective: This study assesses dosimetric variations between the initial dosimetric CT and a control CT scan performed mid treatment in patients undergoing single-application HDR brachytherapy for locally advanced cervical cancer (LACC). Material/Methods: In this retrospective study, 24 patients treated with HDR brachytherapy for cervical or endometrial cancer (with cervical involvement) from November 2021 to August 2022 were included. Patients received four fractions of 7 Gy using vaginal mold, the Utrecht or Venezia applicator, with a control CT scan performed before the third fraction. The control scan aimed at evaluating applicator stability and dose consistency, using rigid and deformable registration techniques to map initial dosimetry onto the control scan. Dose variations in target volumes and organs at risk (OARs) were calculated in both absolute (Gy) and relative terms values (%). Results: The median absolute dose variation for D90 of HR CTV in EQD2 was 0.6 Gy (range: -4.1 to +5.0 Gy), while D98 of IR CTV showed a median variation of 1.2 Gy (range: -1.1 to +9.7 Gy). OAR dose variations were also reported, with the bladder D2cm3 in EQD2, showing a median variation of -1.4 Gy and the rectum of -2.8 Gy. Out of 24 patients, the soft constraint for HR CTV (D90 > 85 Gy) was achieved in 9 patients on the dosimetric scan and 11 on the control scan. The median dose to the HR CTV was 81.7 Gy [68.4;87.0]. For the IR CTV, 14 and 15 patients, respectively, met the target (D98 > 60 Gy) on the dosimetric and control scans (Fig1). Most patients met the bladder and rectum dose constraints across both scans, although minor shifts in dose compliance were observed (Fig2).