Assessment of tumor regression by consecutive pelvic magnetic resonance imaging and dose modification during high-dose-rate brachytherapy for carcinoma of the uterine cervix.

PURPOSE To assess tumor regression, as determined by pelvic magnetic resonance imaging (MRI), and evaluate the efficacies and toxicities of the interim brachytherapy (BT) modification method, according to tumor regression during multi-fractionated high-dose-rate (HDR) BT for uterine cervical cancer. MATERIALS AND METHODS Consecutive MRI studies were performed pre-radiotherapy (RT), pre-BT and during interfraction of BT (inter-BT) in 69 patients with cervical cancer. External beam radiotherapy (EBRT) was performed, using a 10 MV X-ray, in daily fraction of 1.8 Gy with 4-fields, 5 d/wk. Radiation was delivered up to 50.4 Gy, with midline shielding at around 30.6 Gy. Of all 69 patients, 50 received modified interim BT after checking the inter-BT MRI. The BT was delivered in two sessions; the first was composed of several 5 Gy fractions to point A, twice weekly, using three channel applicators. According to the three measured orthogonal diameters of the regressed tumor, based on inter-BT MR images, the initial BT plan was modified, with the second session consisting of a few fractions of less than 5 Gy to point A, using a cervical cylinder applicator. RESULTS The numbers of patients in FIGO stages Ib, IIa, IIb and IIIb+IVa were 19 (27.5%), 18 (26.1%), 27 (39.2%) and 5 (7.2%), respectively. Our treatment characteristics were comparable to those from the literatures with respect to the biologically effective dose (BED) to point A, rectum and bladder as reference points. In the regression analysis a significant correlation was observed between tumor regression and the cumulative dose to point A on the follow-up MRI. Nearly 80% regression of the initial tumor volume occurred after 30.6 Gy of EBRT, and this increased to 90% after an additional 25 Gy in 5 fractions of BT, which corresponds to 73.6 Gy of cumulative BED(10) to point A. The median total fraction number, and those at the first and second sessions of BT were 8 (5 approximately 10), 5 (3 approximately 7) and 3 (1 approximately 5), respectively. The median follow-up time was 53 months (range, 9 approximately 66 months). The 4-year disease-free survival rate of all patients was 86.8%. Six (8.7%) patients developed pelvic failures, but major late complications developed in only two (2.9%). CONCLUSION Our study shows that effective tumor control, equivalent survival and low rates of major complications can be achieved by modifying the fraction size during BT according to tumor regression, as determined by consecutive MR images. We recommend checking the follow-up MRI at a cumulative BED(10) of around 65 Gy to point A, with the initial BT modified at a final booster BT session.