Use of bladder dose points for assessment of the spatial dose distribution in the posterior bladder wall in cervical cancer brachytherapy and the impact of applicator position.

Purpose: To validate the feasibility and use of dose points to characterize the bladder wall dose distribution and investigate potential impact of the applicator position in cervical cancer brachytherapy.

Methods And Materials: One hundred twenty-eight optimized MRI plans were evaluated. The International Commission of Radiation Units and Measurements (ICRU-38) point doses (B(ICRU)), surrogate for bladder base doses, were compared with D(2cc). Vaginal source to superior-anterior border of the symphysis distances were measured and compared within two groups, namely Group 1-B(ICRU)/D(2cc) ≥1 and Group 2-B(ICRU)/D(2cc) <1. Additionally, points at 1.5 and 2 cm cranial to the B(ICRU), parallel to the tandem and the body axis were analyzed.

Results: Thirty-seven percent of the patients had the ratio B(ICRU)/D(2cc) of 1 or higher, with the 2cc subvolume at the bladder base (Group 1). In 63%, BICRU/D2cc ratio was lower than 1 and the 2cc, cranial to the bladder base (Group 2). Median vaginal source-to-superior-anterior border of the symphysis line distance was -2 cm (range, -3.7 to +1.2 cm) in Group 1 and +1.8 cm (range, -2 to +4.8 cm) in Group 2 (+ cranial/- caudal direction). There was a high correlation between vaginal sources near the symphysis and the 2cc subvolume at the bladder base. The additional points provided no added value.

Conclusions: Location of the 2cc subvolume varies in cervical cancer brachytherapy. Maximum doses are at the bladder base if vaginal sources are also in the vicinity of the bladder base indicated by B(ICRU)/D(2cc) ratio of 1 or higher. Such variation should be considered in dose-effect analyses and intercomparisons, as the same D(2cc) at different bladder locations may correlate with different morbidity profiles and severity Reporting D(2cc) and B(ICRU) doses together therefore remains essential.