The influence of brachytherapy dose heterogeneity on estimates of / for prostate cancer

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Abstract
The sensitivity of estimates of α/β for prostate tumours to dose heterogeneity in 125I brachytherapy implants, as well as to variation in selected radiobiological parameters, is analysed. The tumour control probabilities of brachytherapy and external beam radiotherapy are equated for ranges of α, Tpot, RBE and external beam dose. For each combination of parameters, the equality is used to derive the value of α/β. Different clinical (non-uniform) brachytherapy dose distributions, and three uniform brachytherapy dose distributions (120, 144 and 160 Gy) are used. For 'nominal' input parameter values of Tpot = 45 days, α = 0.2 Gy−1, RBE = 1.4, and an external beam dose of 70 Gy, the values obtained for α/β ranged between 2.1 and 12.3 Gy for all of the clinical DVHs, between 2.1 and 3.8 Gy for the better quality clinical implants and between 1.0 and 1.8 Gy for the uniform brachytherapy doses. When only 2% of the volume receiving the lowest dose is omitted from the clinical DVHs, the estimated α/β values ranged between 1.4 and 2.1 Gy. When ranges of input parameters were also considered, the overall range of α/β values for the clinical brachytherapy dose distributions lay between 1.1 and 12.3 Gy for the three best clinical implants, and between 0.7 and 6.3 Gy for uniform doses. We conclude that estimation of α/β without taking into account dose heterogeneity and inter-patient variation may underestimate the actual value α/β.

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