PURPOSE: To studynthe interaction between radiation dose distribution andnhypofractionated radiotherapy with respect to the risk of radiationnpneumonitis (RP) estimated from normal tissue complicationnprobability (NTCP) models.
MATERIAL AND METHODS:Eighteen non-small cell lung cancer patients previouslyntreated with helical tomotherapy were selected. For each patient an3D-conformal plan (3D-CRT) plan was produced in addition to thendelivered plan. The standard fractionation schedule was set to 60nGy in 30 fractions. Iso-efficacy comparisons with hypofractionationnwere performed by changing the fractionation and the physicalnprescription dose while keeping the equivalent tumor dose in 2 Gynfractions constant. The risk of developing RP after radiotherapynwas estimated using the Mean Equivalent Lung Dose in 2-Gy fractionsn(MELD(2)) NTCP model with α/β=4 Gy for the residual lung. Overallntreatment time was kept constant.
RESULTS: The meannrisk of clinical RP after standard fractionation was 7.6% fornTomotherapy (range: 2.8-15.9%) and 9.2% for 3D-CRT (rangen3.2-20.2%). Changing to 20 fractions, the Tomotherapy plans becamenslightly less toxic if the tumor α/β ratio, (α/β)(T), was 7 Gyn(mean RP risk 7.5%, range 2.8-16%) while the 3D-CRT plans becamenmarginally more toxic (mean RP risk 9.8%, range 3.2-21%). Ifn(α/β)(T) was 13 Gy, the mean estimated risk of RP is 7.9% fornTomotherapy (range: 2.8-17%) and 10% for 3D-CRT (rangen3.2-22%).
CONCLUSION: Modernnhighly conformal dose distributions are radiobiologically morenforgiving with respect to hypofractionation, even for a normalntissue endpoint where α/β is lower than for the tumor innquestion.