ORIGINAL ARTICLE Radiopharmaceutical therapy of bone metastases with 89 SrCl 2 , 186 Re-HEDP and 153 Sm-EDTMP: a dosimetric study using Monte Carlo simulation L. Strigari & R. Sciuto & M. D’Andrea & R. Pasqualoni & M. Benassi & C. L. Maini Received: 28 July 2006 / Accepted: 14 October 2006 / Published online: 20 January 2007 # Springer-Verlag 2007 Abstract 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, 186 Re-HEDP, 153 Sm-EDTMP and 89 SrCl 2 , 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 ( 186 Re- HEDP and 153 Sm-EDTMP) and volumetric for 89 SrCl 2 . After 3D dose distribution calculation, dose-volume histo- gram 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 89 SrCl 2 , the administered activities of 153 Sm-EDTMP and 186 Re- 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 improve- ment in bone pain palliation by means of radiopharmaceu- tical therapy should be expected for dose-guided prescription. Keywords Dose distribution . Monte Carlo simulation . Pain palliation Introduction Symptomatic bone metastases are a very common problem in clinical oncology, affecting a large number of patients with different primary tumours, over 80% of them being prostate, breast or lung carcinomas [1, 2]. Management of these patients is a challenging clinical problem, with conventional therapeutic strategies (chemotherapy, hor- mone therapy, external beam radiation and analgesics) usually effective in controlling bone pain only early in the disease [3]. Systemic radiopharmaceutical therapy, either as mono- therapy or combined with radiosensitisers or chemother- apeutics, is usually reserved for advanced disease and has proved safe and effective [4, 5]. Commercially available radiopharmaceuticals include 89 Sr chloride ( 89 SrCl 2 ), 186 Re-1-1-hydroethylidene diphosphate ( 186 Re-HEDP) and 153 Sm-ethylene diamine tetramethylene phosphonate ( 153 Sm-EDTMP), the actual choice being dictated more by local logistics and personal experiences, as clinical results show only minor differences with common administration schedules [4]. These schedules, however, and specifically radioactivity administered dose, derive from quite limited series of clinical studies [6–12], and therefore may be considered at best empirical by common clinical oncology Eur J Nucl Med Mol Imaging (2007) 34:1031–1038 DOI 10.1007/s00259-006-0302-4 L. Strigari (*) : M. D’Andrea : M. Benassi Laboratory of Medical Physics and Expert Systems, Regina Elena Cancer Institute, Rome, Italy e-mail: strigari@ifo.it R. Sciuto : R. Pasqualoni : C. L. Maini Nuclear Medicine Department, Regina Elena Cancer Institute, Rome, Italy