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

Disregarding RBE variations might lead to suboptimal proton plans giving lower effect in the tumor and higher effect in normal tissues than expected. For cases where the target is situated close to structures sensitive to hot spot doses, this trend may lead to bias in favor of proton plans in treatment plan comparisons.

This study constitutes a feasibility assessment of dynamic conformal arc (DCA) therapy as an alternative to volumetric-modulated arc therapy (VMAT) for stereotactic body radiation therapy (SBRT) of lung cancer. The rationale for DCA is lower geometric complexity and hence reduced risk for interplay errors induced by respiratory motion. Forward planned DCA and inverse planned DCA based on segment-weight optimization were compared to VMAT for single arc treatments of five lung patients. Analysis of dose-volume histograms and clinical goal fulfillment revealed that DCA can generate satisfactory and near equivalent dosimetric quality to VMAT, except for complex tumor geometries. Segment-weight optimized DCA provided spatial dose distributions qualitatively similar to those for VMAT. Our results show that DCA, and particularly segment-weight optimized DCA, may be an attractive alternative to VMAT for lung SBRT treatments if the patient anatomy is favorable. K E Y W O R D S conformal arcs, DCA, lung SBRT, VMAT This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.