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 determined if individual variations in organ at risk (OAR) volume and cylinder insertion geometry (CIG) impacted dose and whether planned minus fractional (P - F) 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 D_2ccs 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 = 2.433, p = 0.017). The cumulative mean planned OAR D_2cc vs. delivered D_2cc was only significantly different for the bladder (planned dose lower, t = -2.025, p = 0.053). Regression analysis revealed planned rectal D_2cc (p < 0.0003) and a positive (posterior) y insertion angle (p = 0.015) to significantly impact delivered rectal D_2cc. Additionally, P - F rectal volume (p = 0.037) was significant in determining rectal delivered dose. Conclusions: A more posterior y angle of insertion was found to increase rectal D_2cc leading us to believe that angling the vaginal cylinder anteriorly may reduce rectal dose without significantly increasing bladder dose. Although attention should be paid to OAR volume and CIG to minimize OAR dose, the clinical significance of P - F changes remains yet to be shown.