Local control related to Kerma rate in T2/T3 breast cancers treated by external irradiation and brachytherapy

Introduction: This study was conducted to determine the impact of the brachytherapy parameters on local control in patients treated with irradiation alone for breast cancer. Material and Methods: From 1985 to 1991, 241 patients with T2 (187pts) T3 (54 pts) N0 (196 pts) N1 (45 pts) M0 breast cancer have been treated with the combination of external irradiation and brachytherapy boost without surgery. Doses delivered to the whole breast and the regional nodes were 54 Gy and 50 Gy respectively (1.8 or 2 Gy/fraction, 5 fr/wk). Twenty Gy were then boosted to the primary tumor, according to the tumor shrinkage, by Ir 192 implants. Plastic tubes and rigid needles were used in 85 % and 15 % of the cases respectively. The brachytherapy boost was delivered at 85% of the basal dose rate (Reference dose rate) according to the Paris system. In some instances, dose distribution heterogeneities were observed leading to the choice of another reference isodose. Results: Median follow-up for the entire population was 75 months (9-122). Local control at 6 years follow-up for T2 and T3 was 76% (+/- 6%). Age, SBR grading and steroid receptors were found to be significantly correlated to local control. The role of the brachytherapy boost was analysed. Reference dose rate was not found to have any impact on local control, (≤ 0.6 vs > 0.6 Gy/h). This result was perhaps linked to the particular choice of the reference isodose. To avoid this bias we have evaluated the impact of the Kerma rate (Activity x Time x total length) on local control. Median kerma rate was 8600 μGy h-1 cm-1 m2 (2200-16600). Local control rate for T2 at 6 years was 77% +/- 8% (Kerma >8600) and 73 %+/- 10% (Kerma ≤ 8600). For T3 local control rate was 82% +/-12% (Kerma >8600) and 37 % +/- 34 % (p<0.0002). Conclusion: This study demonstrated that the impact of brachytherapy on local control was better evaluated by the Kerma rate than by the reference dose rate. Whereas the basal dose rate cannot be changed after the application, it may be possible to adapt the kerma parameters (activity, length, treatment duration) in order to achieve a more efficient treatment