The Effects of Bladder Volume during Intracavitary Brachytherapy for Cervical Carcinoma

Abstract

To determine the effects of bladder volume on bladder and rectal doses during high-dose-rate intracavitary brachytherapy for cervical cancer. The effects of bladder volume during high-dose-rate intracavitary brachytherapy for cervical cancer are not known. A large bladder may exert pressure onto the brachytherapy apparatus resulting in closer proximity to the high-dose region. The dose to the adjacent bladder wall may be higher while the dose to the opposing bladder wall may be lower. A drained, smaller bladder may be further away from the brachytherapy apparatus leading to a lower dose to the bladder. But the opposing bladder wall not being distended may receive a relative higher dose leading to a higher average bladder dose. The advance of CT treatment planning has allowed us to analyze the effects of bladder volume. Twenty-eight intracavitary brachytherapy treatments from 7 patients were analyzed in this study. These patients had FIGO stage IIA to IIIB carcinoma of the cervix and underwent CT treatment planning for intracavitary brachytherapy after external beam radiation treatment, 4,500 cGy, to the pelvis. Radiation dose was prescribed to point A in all cases. DVHs for bladder and rectum were generated. Paired CT data sets were obtained with the bladder drained to gravity (empty bladder) and filled and clamped (filled bladder). Initially 50cc of diluted Cysto-Conray contrast were used in the bladder and later 200 cc were used to maximize the effects of bladder volume change. We typically strive to limit the bladder dose to 80% or less of the dose to point A. Therefore, in this study, we selected and analyzed the bladder volume receiving 80% or more of the dose to point A (V80) in relationship to the bladder volume. We also analyzed the effects on the V80 of the rectum by the bladder volume. Scatter plots of bladder volume vs. bladder V80, bladder maximum dose, bladder average dose, and rectal V80 were generated. There were no consistent relationships observed except for a positive trend of V80 change in relationship to bladder volume change (least square, R2 = 0.47, correlation coefficient, R = 0.69) showing a lower V80 with empty bladder (with residual volume). An interpretation and discussion of data will be presented. Our preliminary data suggest that for most patients, increasing bladder volume leads to higher V80, although no consistent bladder volume effects on bladder maximum or average, or rectal doses were observed. While additional data are still being obtained including small bowel dose, it seems that patients should be treated with empty bladder during intracavitary brachytherapy.