2754 - What is the Optimal Fractionation for Image-Based High-Dose-Rate Brachytherapy Boost in Medically Inoperable Endometrial Cancer: 4 vs. 5 Fractions?

10:45am - 12:00pm PT

Hall F

Screen: 10

POSTER

Presenter(s)

Abigail Groszkiewicz, MD - University of Pittsburgh Medical Center, Pittsburgh, PA

A. Groszkiewicz¹, J. Lesnock², P. Sukumvanich², M. Boisen², M. Courtney-Brooks², J. L. Berger², S. Taylor², A. Olawaiye², A. Garrett³, S. Rush³, H. Kim¹, C. J. Houser¹, E. Doraisamy¹, M. Hajduk¹, and J. A. A. Vargo IV¹; ¹Department of Radiation Oncology, University of Pittsburgh Medical Center, Pittsburgh, PA, ²The University of Pittsburgh, Department of Gynecologic Oncology, Pittsburgh, PA, ³Department of Gynecologic Oncology, University of Pittsburgh Medical Center, Pittsburgh, PA

Purpose/Objective(s): For the treatment of medically inoperable endometrial cancer, there is currently no standard recommended fractionation regimen for brachytherapy boost after the completion of external beam radiotherapy (EBRT). This study aims to compare disease control and toxicity with 4 fractions (Fr) versus 5 Fr of brachytherapy.

Materials/Methods: Single institutional retrospective review of medically inoperable patients treated with definitive EBRT followed by high-dose-rate (HDR) brachytherapy boost from January 2010 to June 2024. All disease stages, grades, and histology were included, and patients receiving HDR monotherapy without EBRT were excluded. All brachytherapy was performed using three-dimensional image-based planning with magnetic resonance (MR) image-based planning with each fraction unless contraindication to MR wherein computed tomography-based planning was used. During the study period, the institutional standard transitioned from 25Gy in 5 fractions to 24Gy in 4 fractions. Primary end points of locoregional recurrence (LRR) and late toxicity were calculated using Kaplan-Meier method with comparison between groups performed using log-rank t-test.

Results: 94 patients were included; 68.1% treated with 5 Fr and 31.9% with 4 Fr. There were no significant differences in patient or disease characteristics between fraction groups. Overall median follow-up time was 39.78 months; median follow-up for 5 Fr group 50.57 months and for 4 Fr group 25.83 months. The most common applicator was tandem and cylinder (n=83, 88.3%). The two groups received similar radiation doses in equivalent 2Gy fractions (EQD2) to the high-risk clinical target volume (HRCTV) (5 Fr 74.04Gy vs 4 Fr 71.81Gy, p=0.26), and the 4 Fr group received significantly less radiation dose to critical organs at risk (OARs) including the rectum (EQD2 5 Fr 53.88Gy vs 4 Fr 51.41Gy, p=0.02), sigmoid (EQD2 5 Fr 63.50Gy vs 4 Fr 59.78Gy, p<0.01), and bladder (EQD2 5 Fr 63.21Gy vs 4 Fr 62.04Gy, p=0.02). Comparing the 5 Fr group to the 4 Fr group there was no significant difference in 5-year LRR (5 Fr 11.2% vs 4 Fr 0%, p=0.24) and no significant difference in 5-year overall grade 3+ late toxicity (5 Fr 7.7% vs 4 Fr 5.9%, p=0.81), 5-year gastrointestinal grade 3+ late toxicity (5 Fr 3.7% vs 4 Fr 0%, p=0.39) or 5-year genitourinary grade 3+ late toxicity (5 Fr 2.0% vs 4 Fr 5.9%, p=0.41).

Conclusion: Treatment of inoperable endometrial cancer with 4 Fr brachytherapy boost compared to 5 Fr after EBRT does not compromise cancer control or toxicity however was associated with less dose to OARs. Reduction from 5 Fr to 4 Fr is an important gain in lessening treatment burden for patients and reducing resource strain for health systems.