Differential vaginal wall dosing in high-dose rate brachytherapy for endometrial cancer: The potential for superior coverage of at-risk areas while preserving organs at risk—An analysis of simulated data.

Abstract

e17121 Background: The treatment of curable endometrial cancer (EC)involves surgery with adjuvant radiotherapy. Histology of EC is a major determining factor for intracavitary brachytherapy (ICRT) treatment volume in early stage EC. Many guidelines recommend ICRT treatment of an extended volume up to the whole length of the vaginal stump (VS) for cases of high risk histologies (HRH) wherein a differential dosing of proximal and distal VS has the potential to achieve targeted therapeutic doses to at risk areas without compromise to critical structures.We present a simulated novel way to treat the VS by ICRT in cases of EC with HRH. Analysis of the simulated and approved plans allow us to compare the doses to organs at risk (OAR’s). Methods: Seventy-five cases from July 2017-August 2018 were reviewed and twenty-five (25) met the criteria for inclusion in this study. All these cases received External Beam Radiation Therapy (EBRT) dose of 50.4Gy/28. The ICRT simulation CT data-set was subsequently used to generate the study plan which was then compared to the actual treatment plan. In order to treat the whole VS but spare OARs a club-shaped design was proposed. This technique involved treating the upper 1/3 with a prescription point at 0.5cm lateral to the applicator surface and the lower 2/3 with a prescription point at the applicator surface. The length of the VS was defined from the tip of the applicator to 2cm superior to the introitus. The D2cc values for the contoured bladder and rectum were extracted from the treatment plan and used along with the D2cc values from the EBRT treatment to calculate OAR EQD2. QUANTEC Constraints used were (ICRU 83): EQD2 bladder: 90Gy; EQD2 rectum: 75Gy. Once calculated, the results were analyzed and the percent deviation between the proposed plan and the constraints as well as the approved plan were determined. Results: Twelve (12) of twenty-five (25) cases were HRH cases. The club-shaped design to treat the whole VS didn't further compromise the OAR (EQD2: 22% below constraints) or the rectum (EQD2: 1% below constraints). There was only a 2% difference between the final constraints in both treatment techniques. Conclusions: This technique to treat the full VS length can be used without adversely affecting OAR toxicities with superior coverage to at risk areas while maintaining dose constraints to OAR. Results, however, are limited by the small sample size.