The Effect of Bladder Volume on RT Dosimetry during Multiple Sessions of Intracavitary Brachytherapy for Cervical Cancer

Abstract

Purpose/Objective(s): There is no standardized bladder control strategy in cervix cancer image- guided brachytherapy. This study aims to explore relationship between bladder volume and dosimetry of the bladder and rectum and high risk clinical target volume (HRCTV) for patients receiving intracavitary brachytherapy in cervical cancer. Materials/Methods: This retrospective study included consecutive patients treated in a single institute using departmental guidelines between January 2015 and December 2017. Patients with locally advanced nonmetastatic cervical cancer treated by concurrent chemoradiotherapy and intracavitary brachytherapy were eligible. All patients received weekly cisplatin 40mg/m2 chemotherapy combined with external beam radiotherapy (1.8Gy per daily fraction, 5 fractions per week，45Gy in 25 fractions to whole pelvis, 55-57.5Gy in 25 fractions boosting to pathologically enlarged regional lymph nodes) followed by three-dimensional adaptive intracavitary brachytherapy (treatment aim more than 80Gy cumulative dose to 90% volume, or D90 HRCTV). Patient was catheterized before each brachytherapy applicator insertion and the catheter was kept open during the delivery of the brachytherapy. Patients underwent CT simulation for treatment planning, and HRCTV, rectum, sigmoid, and bladder were contoured on each planning CT for brachytherapy. Results: A total of 66 patients met the eligibility criteria; 250 brachytherapy simulation CT scans were analyzed for the study. The FIGO stage distribution were 4 stage I, 29 stage II, 30 stage III, and 3 stage IV comprising IVA and IVB with para-aortic lymph node metastasis. The median age was 51 years (range 37- 74). The mean bladder volume was 70.6 cc (range 13.5-318.1 cc). The bladder volume correlated significantly and linearly with the D1cc（R 0.336, p<0.001）and D2cc of the bladder (R 0.357, p < 0.001); there was 1 Gy increment of D1cc or D2cc with every 90cc increase in bladder volume. The bladder volume did not significantly change the D1cc（p=0.495） or D2cc （p=0.315）of the rectum, D1cc （p=0.828）or D2cc（p=0.770）of the sigmoid colon, nor HRCTV D90 (p=0.951) and D95 (P=0.711). The bladder volume increased gradually with each consecutive brachytherapy insertion; the mean bladder volumes were not significantly different between the first, second or third brachytherapy insertions, but they differed significantly between first and fourth brachytherapy insertions (P=0.02). Conclusion: Bladder volume had significant impact on the radiation exposure to bladder but not rectum, sigmoid or HRCTV. Restricting bladder volume during brachytherapy may reduce the doses to the bladder. Should this finding be validated, future practice shall adopt bladder volume control strategies to minimize toxicity.