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Multicenter Study
. 2011 Jan 1;79(1):195-201.
doi: 10.1016/j.ijrobp.2009.10.008. Epub 2010 Apr 8.

Confirmation of a low α/β ratio for prostate cancer treated by external beam radiation therapy alone using a post-treatment repeated-measures model for PSA dynamics

Cécile Proust-Lima  1 , Jeremy M G Taylor, Solène Sécher, Howard Sandler, Larry Kestin, Tom Pickles, Kyoungwha Bae, Roger Allison, Scott Williams
Affiliations
Multicenter Study

Confirmation of a low α/β ratio for prostate cancer treated by external beam radiation therapy alone using a post-treatment repeated-measures model for PSA dynamics

Cécile Proust-Lima et al. Int J Radiat Oncol Biol Phys. .

Abstract

Purpose: To estimate the α/β ratio of prostate cancer treated with external beam radiation only by use of a model of long-term prostate-specific antigen (PSA) dynamics.

Methods and materials: Repeated measures of PSA from 5,093 patients from 6 institutions treated for localized prostate cancer by external beam radiation therapy (EBRT) without planned androgen deprivation were analyzed. A biphasic linear mixed model described the post-treatment evolution of PSA, rather than a conventional model of time to biochemical recurrence. The model was adjusted for standard prognostic factors (T stage, initial PSA level, and Gleason score) and cohort-specific effects. The radiation dose fractionation effect was estimated from the long-term rate of rise of PSA level.

Results: Adjusted for other factors, total dose of EBRT and sum of squared doses per fraction were associated with long-term rate of change of PSA level (p = 0.0017 and p = 0.0003, respectively), an increase of each being associated with a lower rate of rise. The α/β ratio was estimated at 1.55 Gy (95% confidence band, 0.46-4.52 Gy). This estimate was robust to adjustment of the linear mixed model.

Conclusions: By analysis of a large EBRT-only cohort along with a method that uses all the repeated measures of PSA after the end of treatment, a low and precise α/β was estimated. These data support the use of hypofractionation at fractional doses up to 2.8 Gy but cannot presently be assumed to accurately represent higher doses per fraction.

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Conflict of interest statement

Conflict of interest notification: no actual or potential conflicts of interest exist.

Figures

Figure 1
Figure 1
Evolution of PSA after end of radiation therapy: post-treatment level of PSA, short term drop of PSA and long term rise of PSA.
See this image and copyright information in PMC

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