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A calculation of the relative biological effectiveness of 125 I and 103 Pd brachytherapy sources using the concept of proximity function
Author(s) -
Wuu C. S.,
Zaider M.
Publication year - 1998
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.598415
Subject(s) - brachytherapy , relative biological effectiveness , monte carlo method , dosimetry , function (biology) , radiobiology , prostate cancer , nuclear medicine , mathematics , statistics , physics , radiation therapy , medical physics , radiation , medicine , cancer , optics , radiology , biology , evolutionary biology
The clinical application of encapsulated radioactive sources in brachytherapy plays an important role in the treatment of malignancy.125 I and103 Pd sources have been widely used in the permanent implant of prostate cancer. An important consideration for the choice of brachytherapy sources is their relative biological effectiveness (RBE). Previous calculations of this quantity have used the dose‐averaged lineal energy, y D , as a measure of biological effectiveness. In this approach, however, the selection of a relevant site size remains an open question. Here we avoid this problem by using the generalized theory of dual radiation action to calculate the initial slope, α, of the dose‐effect curves using the proximity function, t ( x ) , and the biological response function, γ ( x ) . At low doses and/or low dose rates (e.g., prostate implants) the parameter α determines the RBE. Proximity function, t ( x ) , is the probability distribution function of distances between pairs of sublesions; and the biological function, γ ( x ) , is the probability that two sublesions at a distance x apart results in a lesion. Functions t ( x ) have been calculated for each source using the Monte Carlo transport codes PHOEL and PROTON5 . The function γ ( x ) has been taken from a published analysis. The RBE values thus obtained are: 1.5 for125 I and 1.6 for103 Pd . The question of whether an “effective” site size exists where y Dapproximates best the variation of α with radiation quality is also addressed.