When tumor repopulation starts? The onset time of prostate cancer during radiation therapy.

Purpose: To analyze published clinical data and provide a preliminary estimate of tumor repopulation rate and its onset time during radiation therapy for prostate cancer.

Methods: Data on prostate cancer treated with external beam radiotherapy (EBRT) by Perez et al. (2004), Amdur et al. (1990) and Lai et al. (1991) were analyzed in this study. The stage-combined pelvic control rate from Perez et al. was calculated to be 0.95±0.01, 0.87±0.02, and 0.72±0.04 for patients treated ≤7 weeks, 7.1-9 weeks, and >9 weeks respectively. Based on the Linear-Quadratic model, extended to account for tumor repopulation, the least χ² method was used to fit the clinical data and derive the onset time (T(k)) and effective doubling time (T(d)) for prostate cancer. Similar analysis was performed for the other two datasets.

Results: Best fit was achieved with onset time T(k)=34±7 days and doubling time T(d)=12±2 days. These parameters were independent of the choice of the α/β values currently published in the literature. Analyses of the other two datasets showed T(k)=42±7 days with T(d)=9 ± 3 days, and T(k)=34±6 days with T(d)=34±5 days, respectively. T(k) was found to be dependent on tumor stage.

Conclusions: Consistent values for onset time T(k) were obtained from different datasets, while the range of doubling time T(d) was large. Tumor repopulation starts no later than 58 days (at 90% confidence level) in the course of EBRT for prostate cancer.