Objectives To determine the impact of histological grade on overall survival in patients with clinical stage I endometrioid endometrial adenocarcinoma when radiation therapy is used as primary definitive treatment.

Methods Patients with stage I endometrioid endometrial adenocarcinomas who underwent definitive radiation therapy with brachytherapy ± external beam radiation therapy were identified from the National Cancer Database. Overall survival was estimated using the Kaplan-Meier method. Univariable and multivariable analyses were performed to determine factors affecting overall survival. Inverse probability of treatment weights were also used in multivariable analysis to estimate casual effects of external beam radiation therapy.

Results A total of 947 patients were identified. Median overall survival for grade 1, grade 2, and grade 3 tumors was 62 months (95% CI 53.8 to 70.2), 48.5 months (95% CI 38.2 to 58.8), and 33.5 months (95% CI: 23.1 to 43.8), respectively. Grade, age, and insurance status were associated with overall survival in univariate analysis with only grade and age remaining significant in multivariate analysis. Brachytherapy with external beam radiation therapy was not associated with survival in comparison with brachytherapy alone. Compared with grade 1 tumors, patients with grade 3 (HR 1.4, 95% CI 1.15 to 1.89), but not grade 2 (HR 1.0, 95% CI 0.82 to 1.26), had an increased risk of death, which persisted in an inverse probability of treatment weights-adjusted model (HR 1.56, 95% CI 1.21 to 1.93).

Conclusions Patients with grade 3 stage I endometrioid endometrial adenocarcinoma treated with primary definitive radiation therapy have worse survival than those with lower grade tumors. Addition of external beam radiation therapy to brachytherapy did not affect survival.