Implications of bladder volume on target coverage and dose to critical structures during high dose rate intracavitary brachytherapy for cervical cancer: A dosimetric study.

Background: The distension of the bladder during intracavitary brachytherapy (ICBT) affects the applicator position and in turn the dose to tumour and normal structures.

Aims And Objectives: To quantify the changes in the dose to high risk clinical target volume (HRCTV) and organs at risk (OARs) due to bladder filling.

Methods And Materials: In this prospective study, dosimetric parameters of 40 cervical cancer patients treated with (ICBT) were studied. Two sets of CT images with empty and bladder filled with 100 ml of contrast were used for analysis of doses to HRCTV and organs at risk OARs. The distance between posterior wall of the bladder and reference point on the flange was recorded in X,Y and Z.

Results: Target coverage between bladder distension and empty bladder was 84.15 vs 84.85 % (P=0.003) respectively. Mean dose to bladder was 1.57vs 2Gy (P=0.000), median dose was 6 vs7.25Gy (P=0.000), D5cc was 3.94 vs 4.17 Gy (P=0.103) and D2cc was 4.5 vs 5 Gy (P=0.01) respectively. The doses to rectum was D2cc - 3.76 vs 4.07Gy (P=0.000), Sigmoid was D2cc - 2.18 vs 2.43Gy (P=0.005), bowel D2cc was 2.81 vs 3.43Gy (P=0.000). The mean distance of the posterior wall of the bladder from reference point was 0.042 vs -0.079cm (P=0.391) in the X-axis, -0.618 vs -0.630cm (P=0.603) in Z-axis and 0.19 vs 0.26cm (P=0.009) in the Y-axis between two status of the bladder.

Conclusions: Smaller volume of bladder receive higher dose and larger volumes a lower dose with bladder distension, while dose is same for rectum, sigmoid and bowel. The applicators shift posteriorly when the bladder is distended.