Abstract The comparison of different dose-time-fractionation schedules requires the use of an isoeffect formula. In recent years, the NSD isoeffect formula has been heavily critized. In this report, we consider and isoeffect formula which is specifically developed for radiation-induced lung damage. The formula is based on the linear-quadratic model and includes a factor for overall treatment time. The proposed procedures allow for the simultaneous derivation of an α β ratio and a γ β time factor. From animal data in the literature, the derived α β and γ β ratios for acute lung damage are 5.0 ± 1.0 Gy and 2.7 ± 1.4 Gy 2/day respectively, while for late damage the suggested values are 2.0 Gy and 0.0 Gy 2/day. Data from two clinical studies, one prospective and the other retrospective, were also analysed and corresponding α β and γ β ratios were determined. For the prospective clinical study, with a limited range of doses per fraction, the resultant α β and γ β ratios were 0.9 ± 2.6 Gy and 2.6 ± 2.5 Gy 2/day. The combination of the retrospective and prospective data yielded α β and γ β ratios of 3.3 ± 1.5 Gy and 2.4 ± 1.5 Gy and 2.4 ± 1.5 Gy 2/day, respectively. One potential advantage of this isoeffect formalism is that it might possibly be applied to both acute and late lung damage. The results of this formulation for acute lung damage indicate that time-dependent effects such as slow repair or proliferation might be more important in determining isoeffect doses than previously predicted by the estimated single dose (ED) formula. Although we present this an an alternative approach, we would caution against its clinical use until its applicability has been confirmed by additional clinical data.