3789 - Can Treatment Response to Radiation In Non-Operable Endometrial Cancer Predict Outcomes?

Purpose/Objective(s): In medically inoperable endometrial cancer (MIEC), definitive treatment consists of external beam radiation (EBRT) with brachytherapy. While radiographic treatment response predicts outcomes in other malignancies, its role in MIEC remains unexplored. This study evaluates whether imaging response correlates with outcomes in MIEC.

Materials/Methods: Patients with MIEC who were treated with pelvic +/- paraaortic EBRT and triple tandem brachytherapy (TTB) and had pre-treatment, post-EBRT (+/- TTB applicator in place), or post-treatment imaging were included. Primary tumor dimensions were measured on MRI using RECIST 1.1 criteria (anteroposterior, transverse; plus craniocaudal); change in maximum SUV was used for PET imaging. Treatment response was calculated as percent change from pre-RT baseline. Kaplan Meier survival curves were used to evaluate those who had at least 50% post-EBRT response and 100% (complete) post-treatment response and its effects on PFS and OS. Further, a multivariate analysis on overall treatment response rate (post-EBRT or post-treatment if available) was performed to assess the relationship with PFS while accounting for patient demographics (age, race, BMI, FIGO stage, and histologic type).

Results: From 2017-2025, 17 patients (median age 65y, range 42-82) with FIGO Stage IA1-IVB (I: 41.2%, II: 0%, III: 35.3%, IV: 11.8%) MIEC were treated with EBRT (median 45 Gy, range 42-50.4) +/- nodal simultaneous boost (n=9, range 44-60 Gy) and TTB (median HR-CTV D90 EQD2 75.5 Gy, range 52.2-101.4 Gy). 4 patients received concurrent chemotherapy. With median follow up of 13.6 mos (range 0-74 mos), median PFS was 17 mos (range 0-74 mos). With median time from pre- to post-treatment imaging of 3 mos, post-EBRT imaging (n=11, all MRI) showed a median response of 57.1% (range 10.2-100%), while post-treatment imaging (n=6; 5 MRI, 1 PET) showed a median response of 100% (range 63.6-100%). Two patients with the lowest post-EBRT response rates (10, 19%) developed both local recurrence and distant metastases (at 1, 3 mos, respectively). Two patients failed distantly alone (at 1, 18 mos) with mixed post-EBRT response rates (33, 100%). With Kaplan Meier analyses, post-EBRT response was significantly associated with PFS when stratified by <50% or =50% response rates (p =0.02). Similarly, PFS was associated with post-treatment response when stratified by incomplete or complete response (p=0.01). Specifically, 1y PFS was 85% for those with complete response and 40% for those with incomplete response. No significant effect of treatment response on OS was found. Multivariate analysis showed a significant association between overall treatment response rate and PFS (p=0.003).

Conclusion: In MEIC, radiographic treatment response of =50% after EBRT and complete response after all treatment were significantly associated with improved PFS. Imaging response may serve as an early indicator of treatment efficacy in MIEC, though prospective validation in larger cohorts is needed.