[en] Purpose: Several studies have documented that patients with high risk prostate cancer benefit from androgen ablation (AA) in conjunction with radiotherapy, as compared to those treated with androgen ablation or radiotherapy alone. The hypothesis is that a supra-additive effect is manifested when the treatments are given concomitantly as opposed to sequentially. However, the supra-additivity of this approach is difficult to prove in clinical trials due to tumor heterogeneity and because it takes over 6 years to obtain meaningful data on survival differences. Moreover, under certain conditions androgen ablation might induce quiescence (a more radioresistant state), resulting in a sub-additive interaction. For these reasons, we investigated the effects of androgen ablation and radiation using the androgen sensitive R3327-G Dunning rat prostate model. Materials and Methods: The R3327-G tumor line was used in the 23rd-24th in vivo transplant generations. The tumors were grown in the flanks of 250-300g male Copenhagen rats and were used when they reached approximately 1 cc. The growth fraction was determined by continuously labelling the tumors in vivo with chlorodeoxyuridine (CldUrd) via Alzet minipumps implanted in the opposite flank and measuring the incorporated CldUrd and DNA by flow cytometry. Pulse labelling with iododeoxyuridine (IdUrd) to determine the cell kinetic parameters of labelling index (LI), length of S-phase (Ts), and potential doubling time (Tpot) was accomplished by intraperitoneal injection; these parameters were also calculated from flow cytometric data. Apoptotic index was quantified using an immunohistochemical deoxynucleotidyl transferase (TUNEL) assay on formalin-fixed paraffin-embedded tissue; 2000 cells (20 or more high powered fields) were counted per tumor. Results: Tumor volume measurements revealed that the doubling time (Td) increased from an average of 10 d in intact rats to 37 d in castrates. The pulse labelling of tumors with IdUrd at different times after castration revealed that the LI dropped from a pretreatment level of 9.8 ± 0.4% (±SE) to 1.6 ± 0.2% at 3 days. Measurements taken after 3 d post-castration were similar with LIs leveling off at 1-2%, indicating that a new cell kinetic equilibrium had been reached. Whereas the LI dropped significantly in response to androgen ablation, Ts changed minimally from 19.3 ± 0.6 hr to 22.6 ± 0.7 hr. The dramatic change in LI, and consequently Tpot, in response to androgen ablation occurred with minimal cell loss by apoptosis, which remained at ∼1% after castration. The drop in LI in the absence of a major change in Ts or apoptosis suggests that the principal effect of androgen ablation was to reduce the proportion of tumor cells in the cell cycle. In fact, the growth fraction under equilibrium conditions was 70 % in intact rats and <15% in castrates. These results suggest that irradiating the tumors at 3 d post-castration might be less effective because over 85% of the tumor cells are in a resting state at this point. To examine this further, we measured apoptosis levels after radiotherapy alone (7 Gy, single fraction, cobalt-60) compared to radiotherapy administered 3 d after castration. Peak apoptotic indices, seen at 6 hr following irradiation, were 2% with radiotherapy alone and 10% with the combination treatment. Hence the enhancement in apoptotic index was supra-additive when the combination was used. Conclusion: The loss of the apoptotic response to androgen ablation may be a fairly early occurrence in humans and this was reflected in the R3327-G tumor line. Although androgen ablation induced a tremendous shift of tumor cells into a quiescent state with very little apoptosis, the addition of radiation resulted in a 5-10 fold increase in apoptosis levels over radiation or androgen ablation alone. These findings document an enhancement in cell killing when androgen ablation and radiation treatments are combined in this prostate cancer model and suggest that this interaction contributes to the improved clinical out come of prostate cancer patients so treated