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

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/β ratios from 0 Gy to ∞ Gy. To test for which α/β ratio both treatments have equal biological effect, a 5-parameter logistic model was optimized for both dose-effect relationships simultaneously. Beside the α/β 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.

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/β ratio equal to 1.3 Gy (0.5-2.1 Gy).

The optimal fitted model resulted in an α/β 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).

To estimate the α/β ratio for which the dose-dependent lung perfusion reductions for stereotactic body radiation therapy (SBRT) and conventionally fractionated radiation therapy (CFRT) are biologically equivalent.