The objectives of this study were to model and calculate the absorbed fraction ϕ of energy emitted from yttrium-90 ( 90 Y) microsphere treatment of necrotic liver tumors.The tumor necrosis model was proposed for the calculation of ϕ over the spherical shell region. Two approaches, the semianalytic method and the probabilistic method, were adopted. In the former method, the range--energy relationship and the sampling of electron paths were applied to calculate the energy deposition within the target region, using the straight-ahead and continuous-slowing-down approximation (CSDA) method. In the latter method, the Monte Carlo PENELOPE code was used to verify results from the first method.The fraction of energy, ϕ, absorbed from 90 Y by 1-cm thickness of tumor shell from microsphere distribution by CSDA with complete beta spectrum was 0.832 ± 0.001 and 0.833 ± 0.001 for smaller (r T = 5 cm) and larger (r T = 10 cm) tumors (where r is the radii of the tumor [T] and necrosis [N]). The fraction absorbed depended mainly on the thickness of the tumor necrosis configuration, rather than on tumor necrosis size. The maximal absorbed fraction φ that occurred in tumors without central necrosis for each size of tumor was different: 0.950 ± 0.000, and 0.975 ± 0.000 for smaller (r T = 5 cm) and larger (r T = 10 cm) tumors, respectively (p < 0.0001).The tumor necrosis model was developed for dose calculation of 90 Y microsphere treatment of hepatic tumors with central necrosis. With this model, important information is provided regarding the absorbed fraction applicable to clinical 90 Y microsphere treatment.