Can current models explain the lack of liver complications in Y-90 microsphere therapy?

Normal liver complications have not been observed in Y-90 microsphere thera py of hepatic tumors [selective internal radiation (SIR)], despite clinical studies reporting estimated absorbed doses to normal liver between 100 and 150 Gy. The purpose of the study was to see whether predictions of normal tissue co mplication probability (NTCP) models for liver based on clinical data from external beam therapy are consistent with clinical results of SIR. Liver NTCP was calculated using a parallel architecture model and normal li ver dose-volume histograms that have been proposed for SIR. A parallel mode l including internal functional subunit structure is also proposed. Dose ra te effects are incorporated. A criterion for comparing model calculations w ith clinical data is presented. For the parallel architecture model, the predicted NTCP is sensitive to the dose distribution in normal liver and to the model parameters, particularl y the repair time. With reasonable assumptions about the microsphere distri bution, the parallel model with parameters deduced from external beam thera py outcome analysis is consistent with the observed lack of liver complicat ions. Inclusion of FSU structure widens the range of assumptions under whic h consistency is found. The parallel model can be consistent with the clinically observed lack of l iver complications in SIR. More information about the activity distribution and the radiobiology of normal liver under conditions typical of microsphe re therapy should be sought.