A Monte Carlo simulation model for radiation dose to metastatic skeletal tumor from rhenium-186(Sn)-HEDP.

Unlabelled: A Monte Carlo model has been developed for simulation of dose delivery to skeletal metastases by the bone surface-seeking radiopharmaceutical 186Re (Sn)-HEDP.
Methods: The model simulates: (1) the heterogeneous small scale geometry of the soft tissue/bone-spicule structure in the lesions as determined by histomorphometric measurements of histologic specimens, (2) the small scale spatial distribution of the radiopharmaceutical on the lesion bone spicule surface as determined by autoradiography, and (3) the 186Re beta and conversion electron decay spectrum and the associated charged particle transport within the modeled geometries. The results are compared with the commonly employed uniform lesion model, which assumes: (1) homogeneous lesion morphology, (2) uniform distribution of radioactivity within the lesion, and (3) complete energy deposition by charged particles within the lesion due to decay of this activity. Gamma and x-ray photons from the 186Re spectrum were assumed to escape from the lesion volume in both models.
Results: Results show a significant dependence on the bone volume fraction and hence on the histology of the lesion (lytic, blastic or mixed). The uniform lesion model calculations underestimate the radiation dose to blastic lesions by as much as a factor of 1.8. However, for lytic lesions with low bone volume fractions, both models provide similar dose values.
Conclusions: These new model calculations provide a mechanism for optimizing treatment planning and dose response evaluations of therapeutic bone-seeking radiopharmaceuticals.