PO29 Presentation Time: 7:40 AM: Dosimetric Review to Organs at Risk from Short Course Adjuvant Vaginal Cuff Brachytherapy in Early Endometrial Cancer Compared to Standard of Care (SAVE)

Abstract

<h3>Purpose</h3> To evaluate dosimetric characteristics to organs at risk (OARs) from short course adjuvant vaginal cuff brachytherapy (VCB) in early endometrial cancer compared to standard of care in a multi-institutional prospective randomized trial. <h3>Materials and Methods</h3> 108 patients requiring VCB were randomized to an experimental short course arm (11 Gy x 2Fx to surface) and standard of care arms: 7 Gy x 3Fx to 5mm, 5-5.5 Gy x 4Fx to 5mm, and 6 Gy x 5Fx to surface. Rectum, bladder, sigmoid, small bowel, and urethra were contoured retrospectively on the planning CT and doses to OARs were compared by arm. Absolute doses for each OAR (D2cc and D0.1cc), and from each fractionation scheme, were converted to 2 Gy equivalent dose (EQD2; α/β=3 Gy). Each standard of care arm was compared to the experimental arm separately using a one-way analysis of variance, followed by pairwise comparisons using Tukey's HSD test. Additionally, treatment planning system reported point doses (mid cylinder) to the vaginal mucosa was converted to EQD2 (α/β=10 Gy) and evaluated. <h3>Results</h3> The experimental arm had significantly lower doses for rectum, bladder, sigmoid, and urethra when compared to the 7 Gy x3 and 5-5.5 Gy x4 fractionation schemes; however, the experimental arm did not differ statistically from the 6 Gy x5 fractionation scheme. For small bowel doses, none of the standard of care fractionation schemes were statistically different than the experimental arm (see fig. 1f). The highest D2cc EQD2 doses to the examined critical structures were observed to come from the most common dose fractionation scheme of 7 Gy x 3Fx (see fig. 1a-e). The data revealed similar results for the D0.1cc EQD2 doses for the urethra. Furthermore, average reported EQD2 doses to the vaginal mucosa were 40.32 Gy (σ=1.97 Gy) for 11 Gy x2, 56.53 Gy (σ=6.29 Gy) for 7 Gy x3, 47.81 Gy (σ=2.97 Gy) for 5-5.5 Gy x 4, and 41.69 Gy (σ=1.29 Gy) for 6 Gy x 5. With a short median follow up of one year, there have been no isolated vaginal recurrences. <h3>Conclusions</h3> Doses to OARs were generally low; however, in the highest quintile significant doses can be delivered with VCB. Experimental short course VCB of 11 Gy x 2Fx to the surface provides a comparable biologic effective dose to standard of care courses. Experimental short course VCB was found to reduce or be comparable to D2cc and D0.1cc EQD2 doses to rectum, bladder, sigmoid, small bowel, and urethra critical structures. This may translate into a comparable or lower rate of acute and late side effects.