A two-component model of an osteoblastic metastatic lesion has been developed to determine the absorbed dose delivered to soft tissue during systemic radiotherapy of osseous metastases. Doses to soft tissue from radioisotopes distributed in bone were calculated using Burlin's general cavity theory. A correction term was used to account for the absence of charged particle equilibrium within the metastatic lesion. Radiation doses for 153Sm, 186Re, 89Sr and 32P were calculated for several physiologically realistic lesion structures. Burlin's cavity weighting factor was greatest for higher energy isotopes and it decreased as the soft tissue cavity size increased. The correction for the absence of charged particle equilibrium also decreased with soft tissue pathlength, but increased with average bone pathlengths. Doses to soft tissue cavities ranged from 0.1 to 0.2 Gy MBq(-1) d(-1) for 153Sm to 0.5 to 0.6 Gy MBq(-1) d(-1) for 32P. Using the factors calculated in this work, the dose to soft tissue cavities within bone metastases can be calculated when the dose to adjacent bone has been determined, perhaps by autoradiography or electron paramagnetic resonance dosimetry. The doses calculated with this more accurate model of bone metastases demonstrate errors of 20% to 50% in previous calculations of the average dose to homogeneous metastatic lesions.