Alpha/beta ratio for normal lung tissue as estimated from lung cancer patients treated with stereotactic body and conventionally fractionated radiation therapy.

ABSTRACT

PURPOSE: To estimate the α/ß 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 α/ß ratios from 0 Gy to ∞ Gy. To test for which α/ß ratio both treatments have equal biological effect, a 5-parameter logistic model was optimized for both dose-effect relationships simultaneously. Beside the α/ß ratio, the other 4 parameters were d50, the steepness parameter k, and 2 parameters (MSBRT and MCFRT) representing the maximal perfusion reduction at high doses for SBRT and CFRT, respectively. RESULTS: The optimal fitted model resulted in an α/ß ratio of 1.3 Gy (0.5-2.1 Gy), MSBRT=42.6% (40.4%-44.9%), MCFRT=66.9% (61.6%-72.1%), d50=35.4 Gy (31.5-9.2 Gy), 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 α/ß ratio equal to 1.3 Gy (0.5-2.1 Gy).