Geometric dose summation in adjuvant EBRT and VCBT for endometrial cancer: A retrospective dosimetric analysis

In patients with gynecologic malignancies treated with external beam radiotherapy (EBRT) and vaginal cuff brachytherapy (VCBT), accurate dose summation is essential for assessing organ-at-risk (OAR) exposure. This study aimed to compare dosimetric differences between rigid image-based plan summation and conventional linear equivalent dose in 2 Gy fractions (EQD2) summation in adjuvant endometrial cancer radiotherapy. Fourteen patients who received adjuvant EBRT and VCBT following hysterectomy were retrospectively analyzed. VCBT plans were converted to EQD2 (α/β = 10) and combined with EBRT plans using rigid image fusion to create a composite "plan sum." Rectum, bladder, and sigmoid D2cc values were extracted from this summed geometry and compared with D2cc values calculated using the simple-sum method. The "dog-ear" region, representing residual vaginal tissue, was also evaluated. The plan-sum method consistently yielded lower D2cc values for all OARs compared to the simple-sum technique (p = 0.01). Median D2cc values for the rectum, bladder, and sigmoid were 2.12 Gy, 4.06 Gy, and 1.84 Gy lower, respectively. No statistically significant difference was found in mean or minimum dog-ear doses between methods. Rigid image-based plan summation provides more anatomically realistic dose estimates than conventional EQD2 summation. This technique may improve cumulative dose assessment and planning precision in adjuvant VCBT for endometrial cancer, particularly in anatomically stable high-risk regions.