INFLUENCE OF DOSE-RATE AND DOSE PER FRACTION ON CLINICAL OUTCOME OF BREAST-CANCER TREATED BY EXTERNAL-BEAM IRRADIATION PLUS IR-192 IMPLANTS - ANALYSIS OF 289 CASES

Purpose: To study the influence of Iridium 192 implant dose-rate and dose per fraction of external irradiation on clinical outcome, the results in 289 patients with early breast cancer were analyzed retrospectively. Methods and Materials: From 1980 to 1990, 118 T1 and 171 T2 lesions of breast were treated definitively by radiotherapy, following conservative surgery. External irradiation dose of 45 Gy was delivered either with 2.5 Gy or 1.8 Gy per fraction to the entire target volume, plus boost to the primary tumor. Boost dose of 15 to 30 Gy was given to the primary tumor either with iridium-192 implants or electrons. The implant dose-rate varied between 20 cGy/hr to 160 cGy/hr. Results: The minimum follow-up was of 12 months and maximum of 11 years (median: 56 months). Out of 273 tumors boosted with implants, the 270 patients were divided into five groups according to dose-rate as, groups 1 (20-29 cGy/hr, n = 17), group 2 (30-49 cGy/hr, n = 144), group 3 (50-69 cGy/hr, n = 69), group 4 (70-99 cGy/hr, n = 27) and group 5 (100-160 cGy/hr, n = 13). The local failure rate was significantly increased in the group of patients treated with implant dose-rate < 30 cGy/hr (p < 0.05). While the incidence of late normal tissue complications and poor cosmetic outcome was significantly higher in the group of patients treated with implant dose-rate > 100 cGy/hr (p < 0.05). Conclusion: The present analysis indicate that the implant dose-rate should be maintained between 30-70 cGy/hr to maximize local control and reduce the late normal tissue injury. Also the increase in dose per fraction of external irradiation while not influencing local control rate was crucial for incidence of late complications and cosmetic outcome.