AbstractID: 7727 Title: How Low is the alpha/beta Ratio for Prostate Cancer?
It has been suggested recently that the α/β ratio for prostate cancer is low (around 1.5 Gy) and much debate on the evidence for such a low value is
ongoing. Analyses reported so far ignored the contribution of tumor repopulation. Extremely low α values and unrealistic cell numbers of tumor
clonogens are found in these studies. In this work, the LQ model extended to include the effect of repopulation is used to analyze the recently
reported clinical data for prostate cancer using either external-beam radiotherapy (EBRT) or brachytherapy. Based on the clinical finding that the
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EBRT and the I and Pd permanent implants are biologically equivalent, we have derived a unique set of LQ parameters for prostate tumors:
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α=0.15±0.04 Gy , α/β= 3.1±0.5 Gy and the best estimate of repair half-time of sub-lethal damage is 16 minutes. The clonogenic cell number in
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prostate tumors is found to range from 10 to 10 according to the patient PSA level, which is more realistic than those reported previously (only 10–
100). In conclusion, we have shown that the effect of tumor repopulation is not negligible in describing the radiation therapy for prostate cancer,
especially for the low-dose-rate permanent implants. We have confirmed, with a much more self-consistent analysis that, the α/β ratio for prostate is
low and it is around 3.1 Gy. The newly-derived set of LQ parameters is found to agree with in-vitro experimental data. The puzzle associated with
unrealistic clonogenic numbers and extremely low radiosensitivity is naturally solved.