A model for the relative biological effectiveness of protons: The tissue specific parameter α/β of photons is a predictor for the sensitivity to LET changes

Abstract

original article a model for the relative biological effectiveness of protons: the tissue specific parameter a/b of photons is a predictor for the sensitivity to let changes abstract Background. The biological effects of particles are often expressed in relation to that of photons through the concept of relative biological effectiveness, RbE. in proton radiotherapy, a constant RbE of 1.1 is usually assumed. However, there is experimental evidence that RbE depends on various factors. The aim of this study is to develop a model to predict the RbE based on linear energy transfer (LET), dose, and the tissue specific parameter a/b of the linear-quadratic model for the reference radiation. Moreover, the model should capture the basic features of the RbE using a minimum of assumptions, each supported by experimental data. Material and methods. The a and b parameters for protons were studied with respect to their dependence on LET. an RbE model was proposed where the dependence of LET is affected by the (a/b) phot ratio of photons. Published cell survival data with a range of well-defined LETs and cell types were selected for model evaluation rendering a total of 10 cell lines and 24 RbE values. Results and Conclusion. a statistically significant relation was found between a for protons and LET. Moreover, the strength of that relation varied significantly with (a/b) phot . in contrast, no significant relation between b and LET was found. On the whole, the resulting RbE model provided a significantly improved fit (p-value  0.01) to the experimental data compared to the standard constant RbE. by accounting for the a/b ratio of photons, clearer trends between RbE and LET of protons were found, and our results suggest that late responding tissues are more sensitive to LET changes than early responding tissues and most tumors. an advantage with the proposed RbE model in optimization and evaluation of treatment plans is that it only requires dose, LET, and (a/b) phot as input parameters. Hence, no proton specific biological parameters are needed. Oncologica, 2013; 52: 580-588 iSSn 0284-186X print/iSSn 1651-226X online Acta

Radiobiological Intercomparison of the 160 MeV and 230 MeV Proton Therapy Beams at the Harvard Cyclotron Laboratory and at Massachusetts General Hospital

“…The changes are very similar for both 160 and 230 MeV beams. Changes in the value of b are more modest, consistent with other recent studies (41). For each depth region proximal to distal edge, RBE 4Gy values are slightly (but not significantly) lower for the 230 MeV beam, again consistent with the LET data.…”

Relating the proton relative biological effectiveness to tumor control and normal tissue complication probabilities assuming interpatient variability in α/β

“…Phenomenological models are capable of predicting the RBE as a function of the parameters mentioned above (e.g. [7,8]), but input parameters for these models are solely based on cell survival data obtained in vitro. It is unclear if in vitro relationships can be translated to in vivo endpoints even for tumor control.…”

Varying relative biological effectiveness in proton therapy: knowledge gaps versus clinical significance

“…The models used were those of Wedenberg et al [9] and McNamara et al [10], which are both based on in vitro survival curves, LET and the a/b-ratio of the reference radiation. The variable RBE systematically increased the RBE weighted dose to most organs at risk, while the average RBE within the tumor varied from 1.06 to 1.26 for the three sites.…”