The contribution of penetrating photon energy to tumor dose is usually ignored because of the difficulty in calculating absorbed fractions and because it is frequently assumed to represent a small proportion of the total energy. The MABDOSE software—written explicitly to simulate photon transport for the calculation of penetrating radiation absorbed fractions—was used to simulate the131I photon spectrum originating from the liver, spleen, and whole body source organs. Specific absorbed fractions were calculated for tumors of radius 1.0, 1.5, and 2.0 cm placed near the liver and spleen in the Reference Man geometry. Cumulated activities were estimated using values reported from the literature. Dosimetry estimates from the combined cumulated activity and specific absorbed fractions indicate that neglecting the photon contribution underestimates the tumor dose by 10%–25%.