Cavity theory applied to the dosimetry of systemic radiotherapy of bone metastases

A two-component model of an osteoblastic metastatic lesion has been develop ed 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 e quilibrium within the metastatic lesion. Radiation doses for Sm-153,Re-186, Sr-89 and P-32 were calculated for several physiologically realistic lesio n structures. Burlin's cavity weighting factor was greatest for higher ener gy isotopes and it decreased as the soft tissue cavity size increased The c orrection for the absence of charged particle equilibrium also decreased wi th soft tissue pathlength, but increased with average bone pathlengths. Dos es to soft tissue cavities ranged from 0.1 to 0.2 Gy MBq(-1) d(-1) for Sm-1 53 to 0.5 to 0.6 Gy MBq(-1) d(-1) for P-32. 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 a utoradiography or electron paramagnetic resonance dosimetry. The doses calc ulated with this more accurate model of bone metastases demonstrate errors of 20% to 50% in previous calculations of the average dose to homogeneous m etastatic lesions.