The impact of rectal/bladder filling and applicator positioning on in vivo rectal dosimetry in vaginal cuff brachytherapy using an enhanced therapy setting

Purpose: The impact of rectal filling and bladder volume on in vivo rectal dosimetry (IVD) in vaginal cuff brachytherapy (VCBT) is unknown. The purpose of this study was to compare rectal doses from IVD with those calculated from treatment planning and to identify influencing factors.

Materials and methods: We collected data of 80 VCBT sessions, four for each of 20 patients. Each was retrospectively compared with doses determined by the treatment planning system. Factors potentially predicting the IVD rectum dose were analyzed.

Results: For a series of 80 brachytherapy applications, the calculated mean dose to the rectum was 2.52 Gy. The mean difference between all calculated and measured doses for the 80 applications with five probe positions each was 0.09 Gy (p = 0.952) proving high overall accordance between IVD and calculated doses at the rectum. The mean volume of the rectum was 119 ± 57 cm³. The rectal volume was not statistically significantly associated with the IVD or the calculated rectum doses. At the third and fourth rectal probe position in craniocaudal ordering, increased filling of the urinary bladder resulted in decreased measured and calculated doses (p < 0.05 for both). A rectum pointing position of the applicator significantly increased the maximum rectum dose compared with a bladder-oriented position (p < 0.05).

Conclusions: IVD provided valuable data for rectal exposure in VCBT. Increased bladder filling and vaginal applicator positioning off the rectum elicited related with less rectal radiation exposure, whereas rectal filling did not. Further confirmation including assessment of IVD in bladder is pending to define optimal dosimetric conditions in VCBT.