Gynecologic Oncology Do changes in interfraction organ at risk volume and cylinder insertion geometry impact delivered dose in high-dose-rate vaginal cuff brachytherapy?

PURPOSE: Within a multifraction high-dose-rate vaginal cuff brachytherapy course, we deter- mined if individual variations in organ at risk (OAR) volume and cylinder insertion geometry (CIG) impacted dose and whether planned minus fractional (PF) differences led to a discrepancy between planned dose and delivered dose. METHODS AND MATERIALS: We analyzed vaginal cuff brachytherapy applications from consecutive patients treated with three fractions of 5 Gy after each undergoing a planning CT and three repeat fractional CTs (fCTs). Rectal and bladder D2ccs and volumes were recorded in addition to the x (in relationship to midplane) and y (in relationship to the table) angles of CIG. Paired t-tests and multiple regression analyses were performed. RESULTS: Twenty-seven patients were identified. In comparing the planning CT vs. mean fCT rectal volumes, bladder volumes, x angles, and y angles, only bladder volume was significantly different (planned volume higher, t 5 2.433, p 5 0.017). The cumulative mean planned OAR D2cc vs. delivered D2cc was only significantly different for the bladder (planned dose lower, t 5 � 2.025, p 5 0.053). Regression analysis revealed planned rectal D2cc (p!0.0003) and a pos- itive (posterior) y insertion angle (p 5 0.015) to significantly impact delivered rectal D2cc. Addi- tionally, PF rectal volume (p 5 0.037) was significant in determining rectal delivered dose. CONCLUSIONS: A more posterior y angle of insertion was found to increase rectal D2cc leading us to believe that angling the vaginal cylinder anteriorly may reduce rectal dose without signifi- cantly increasing bladder dose. Although attention should be paid to OAR volume and CIG to mini- mize OAR dose, the clinical significance of PF changes remains yet to be shown. 2016