Alpha/Beta Ratio for Normal Lung Tissue as Estimated From Lung Cancer Patients Treated With Stereotactic Body and Conventionally Fractionated Radiation Therapy

Purpose: To estimate the alpha/beta ratio for which the dose-dependent lung perfusion reductions for stereotactic body radiation therapy (SBRT) and conventionally fractionated radiation therapy (CFRT) are biologically equivalent. Methods and Materials: The relations between local dose and perfusion reduction 4 months after treatment in lung cancer patients treated with SBRT and CFRT were scaled according to the linear-quadratic model using alpha/beta ratios from 0 Gy to infinity Gy. To test for which alpha/beta ratio both treatments have equal biological effect, a 5-parameter logistic model was optimized for both dose-effect relationships simultaneously. Beside the alpha/beta ratio, the other 4 parameters were d(50), the steepness parameter k, and 2 parameters (M-SBRT and M-CFRT) representing the maximal perfusion reduction at high doses for SBRT and CFRT, respectively. Results: The optimal fitted model resulted in an alpha/beta ratio of 1.3 Gy (0.5-2.1 Gy), M-SBRT = 42.6% (40.4%-44.9%), M-CFRT = 66.9% (61.6%-72.1%), d50 = 35.4Gy (31.5-9.2Gy), and k = 2.0(1.7-2.3). Conclusions: An equal reduction of lung perfusion in lung cancer was observed in SBRT and CFRT if local doses were converted by the linear-quadratic model with an alpha/beta ratio equal to 1.3 Gy (0.5-2.1 Gy). (C) 2014 Elsevier Inc.