Dose-dependent differential effects of low and pulsed dose-rate brachytherapy in a radioresistant syngenic rat prostate tumour model

Purpose : To study the response of the Dunning prostate carcinoma (R3327-AT1 subline) to continuous low dose-rate (CLDR) and pulsed dose-rate (PDR) brachytherapy. Materials and methods : After subcutaneous tumour transplantation into the thigh of the Copenhagen rat, doses of 0, 20, 30, 40 and 50 Gy were applied to the tumour surface (tumour diameter 9 ±1mm). Eight animals were irradiated per dose group and exposure condition. Interstitial PDR (192 Ir source, 37 GBq) and CLDR (192 Ir seed, 150 MBq) brachytherapy were carried out with 0.75 Gy/pulse h -1 and a dose-rate of 0.75Gyh -1, respectively. Treatment response was assessed in terms of growth delay expressed as the time (T 5) required for each tumour to reach five times the initial tumour volume. Results : The median T 5 times for the CLDR groups (in the order: control, 20, 30, 40, 50 Gy) were 12 (12), 54.5 (21), 64.5 (31), 85.5 (51), and 65 (47.5) days. Values after PDR brachytherapy are given in parentheses and resulted in a significantly impaired tumour growth delay (log-rank test) in the 20Gy (p =0.006) and 30 Gy (p =0.036) groups. No significant difference was found in the 40-50 Gy dose range. Conclusions : In contrast to previous results and predictions of biological models we observed dose-dependent differential effects of PDR and CLDR brachytherapy with reduced efficacy of PDR in the lower dose range.