Purpose: The aim of the paper is to calculate the dose to bone surface and bone volume using a Monte Carlo particle transport model and to give quantitative arguments for activity prescription.
Methods: This study simulates the dose delivery process to skeletal metastases by bone surface- and bone volume-seeking radiopharmaceuticals. Dose distributions for three radiopharmaceuticals, 186Re-HEDP, 153Sm-EDTMP and 89SrCl2, frequently used for pain palliation therapies, were calculated using the EGSnrc Monte Carlo code. The model simulates a cylindrical geometry with regions of different constant density compositions and radioactivity distribution consistent with known biodistribution features of the three radiopharmaceuticals: superficial for phosphonates (186Re-HEDP and 153Sm-EDTMP) and volumetric for 89SrCl2. After 3D dose distribution calculation, dose-volume histogram reduction was carried out using the "preferred Lyman" method, which yields effective uniform dose (D(eff)) equivalent to the inhomogeneous dose distributions to the reference region (volume and surface).
Results: Our simulations showed that to obtain a delivered dose to bone surface equivalent to that obtained from 89SrCl2, the administered activities of 153Sm-EDTMP and 186Re-HEDP should be increased by 37% and 48%, respectively, in comparison with those usually administered.
Conclusion: These results prove theoretically the empirical results from clinical observations and show that improvement in bone pain palliation by means of radiopharmaceutical therapy should be expected for dose-guided prescription.