Is Constant RBE Accurate Enough for Determination of Biological Effects in Proton Therapy?

Abstract

The relative biological effectiveness (RBE) of protons currently used in clinical practice is assigned a constant value of 1.1, despite the evidence that the actual RBE of protons varies as a function of proton energy, linear energy transfer (LET), dose per treatment fraction, tissue type, and clinical endpoint. The purpose of this study is to compare the performance of the traditional, constant RBE model, and two variable RBE models in explaining the radiation induced brain necrosis indicated on follow-up imaging studies for a patient treated with postoperative proton radiation therapy for malignant meningioma. In this study, treatment plan information is extracted from a clinical treatment planning system (TPS) and used to recalculate physical proton dose and linear energy transfer (LET) distributions using Monte Carlo simulations. One constant and two variable RBE models which account for tissue specific parameters, LET, biological endpoint, and dose per treatment fraction are applied on a voxel-by-voxel basis to compute three biologically effective dose distributions. These dose distributions are subsequently correlated to follow-up imaging studies which have indicated the presence of radiation induced necrosis in the treated region of the patient being analyzed. Corresponding areas of toxicity and dose/RBE computed with three different models are compared to determine which model performs the best in explaining the radiation induced toxicity. Areas of increased biologically effective dose computed using variable RBE models compared to the original dose distribution calculated by the clinical TPS using a constant RBE model are found to correspond to areas of radiation induced brain necrosis. Variable RBE doses were up to 8% higher in the region of toxicity. Variable RBE models perform better in explaining the radiation induced brain necrosis.