Dosimetric Evaluation of Organs at Risk From SAVE Protocol

Purpose: The objective of this work was to evaluate dosimetric characteristics to organs at risk (OARs) from short-course adjuvant vaginal cuff brachytherapy (VCB) in early endometrial cancer compared with standard of care (SOC) in a multi-institutional prospective randomized trial.

Methods and materials: SAVE (Short Course Adjuvant Vaginal Brachytherapy in Early Endometrial Cancer Compared to Standard of Care) is a prospective, phase 3, multisite randomized trial in which 108 patients requiring VCB were randomized to an experimental short-course arm (11 Gy × 2 fractions [fx] to surface) and SOC arm. Those randomized to the SOC arm were subdivided into treatment groups based on treating physician discretion as follows: 7 Gy × 3 fx to 5 mm, 5 to 5.5 Gy × 4 fx to 5 mm, and 6 Gy × 5 fx to surface. To evaluate doses to OARs of each SAVE cohort, the rectum, bladder, sigmoid, small bowel, and urethra were contoured on planning computed tomography, and doses to OARs were compared by treatment arm. Absolute doses for each OAR and from each fractionation scheme were converted to 2 Gy equivalent dose (EQD2₃). Each SOC arm was compared with the experimental arm separately using 1-way analysis of variance, followed by pairwise comparisons using Tukey's honestly significant difference test.

Results: The experimental arm had significantly lower doses for rectum, bladder, sigmoid, and urethra compared with the 7 Gy × 3 and 5 to 5.5 Gy × 4 fractionation schemes; however, the experimental arm did not differ from the 6 Gy × 5 fractionation scheme. For small bowel doses, none of the SOC fractionation schemes were statistically different than the experimental. The highest EQD2₃ doses to the examined OARs were observed to come from the most common dose fractionation scheme of 7 Gy × 3 fx. With a short median follow-up of 1 year, there have been no isolated vaginal recurrences.

Conclusions: Experimental short-course VCB of 11 Gy × 2 fx to the surface provides a comparable biologically effective dose to SOC courses. Experimental short-course VCB was found to reduce or be comparable to D2cc and D0.1cc EQD2₃ doses to rectum, bladder, sigmoid, small bowel, and urethra critical structures. This may translate into a comparable or lower rate of acute and late adverse effects.