Influence of dose rate on local control of breast carcinoma treated by external beam irradiation plus iridium 192 implant.

From 1971 to 1983, 20 T1, 267 T2, and 53 T3 biopsy-proven adenocarcinomas of breast were definitively managed by radiotherapy. The breast and regional nodes received 45 Gy of 60Co irradiation in 25 fractions over 5 weeks (45 Gy/25/5 wks). Electrons were used to deliver a further 15 Gy/7/1.5 weeks to the internal mammary chain and 25 Gy/11/2.5 weeks to the lower axilla. The primary tumor was boosted by Iridium 192 implant for a further 37 Gy prescribed at 85% of the basal dose rate (Paris system). Rigid needles were secured by templates in single plane (58/398) or double plane (340/398) geometry. Results of the 340 two-plane implants have been analyzed to look for a possible influence of dose rate on local control. Follow-up for patients free of local recurrence is 4-204 months (median: 92 months). The 340 tumors were divided into three groups according to dose rate: 0.32-0.49 Gy/hr (n = 95), 0.50-0.59 Gy/hr (n = 120), and 0.60-0.90 Gy/hr (n = 125). The three groups are statistically comparable according to age, tumor size, mean 60Co dose, mean Iridium dose, overall treatment time, and follow-up. The local failure rate in the three groups is 27% (26/95), 20% (24/120), and 13% (16/125) (p less than 0.03, Chi square). At 15 years the estimated local control (Kaplan Meier) is 60%, 72%, and 84% (p less than 0.02, Logrank), respectively. This analysis indicates that there is a significant effect of dose rate on local control for carcinoma of the breast treated by combined external beam (45 Gy) plus Iridium 192 implantation (37 Gy). To maximize local control the authors recommend an implant dose rate of greater than or equal to 0.6 Gy/hr.