Which α/β ratio and half-time of repair are useful for predicting outcomes in prostate cancer?

Abstract 

Purpose

To calculate the α/β of prostate adenocarcinoma.

Materials and methods

From January 1997 to December 2005, 328 patients were treated consecutively with external beam radiotherapy and brachytherapy boost. The patients with at least one of the following adverse prognostic factors were included: PSA>10ng/ml, Gleason score⩾7, T⩾2B. A total EQD2 of 80Gy was delivered uniformly within the same timeframe. Prior to August 2002, the patients were treated to low-dose-rate brachytherapy using ¹⁹²Ir (n=201), and those treated thereafter received a high-dose-rate brachytherapy boost (n=127). The equivalency of dose was established using the incomplete repair model, with generally accepted α/β ratio of 3Gy, and half-time for repair of sublethal damage (HTR) of 1.5h.

Results

In a Cox proportional hazards model, the two groups displayed no difference (HR: 0.99, 95% CI: 0.87–1.1, p=0.98) in biochemical control. Analyzing using the linear quadratic model, the data fit well an α/β ratio of 3.41Gy (95% CI: 2.56–4.26) and the recently published HTR of 1.9h (95% CI: 1.4–2.4), but also an α/β of 5.87Gy (95% CI: 4.67–7.07) and the more widely established HTR of 1.5h.

Conclusions

Unlike the previously published data, calculation of the α/β ratio from consecutive patients and using a uniform treatment duration points to higher values than 2.5Gy.

Keywords: Prostate cancer, Radiotherapy, Radiobiology, α/β ratio, Androgen deprivation therapy