Dosimetry and toxicity of samarium-153-EDTMP administered for bone pain due to skeletal metastases

Palliation of bone pain in patients with cancer metastatic to bone is being evaluated in several cancer centers by the administration of the bone- seeking phosphonate ethylenediaminetetramethylenephosphonic acid (EDTMP) chelated with the beta particle-emitting radionuclide ¹⁵³Sm. Methods: In this study, ¹⁵³Sm-EDTMP was intravenously injected into 19 patients over a 1-min period. Patients received up to four injections of 18.5 MBq (0.5 mCi) or 37 MBq (1.0 mCi) per kilogram of body weight. Skeletal retention was calculated from urinary excretion. Results: No uptake of ¹⁵³Sm-EDTMP in nonskeletal tissues was observed in whole-body gamma camera images. The mean skeletal uptake for all patients was 54% ± 16% of the injected dose (%ID). This resulted in the bone marrow receiving 89 cGy/GBq ± 27 cGy/GBq (3.28 cGy/mCi ± 0.99 cGy/mCi), with calculated marrow doses ranging from 27 cGy to 338 cGy. For each patient, the estimated radiation absorbed dose to the marrow was correlated to the percent decrease in platelet number, ranging from 7.4% to 78.9%. Conclusion: Since the deviation of uptake between the four injections for a given patient (7.6% ID) was less than the deviation for all patients (16% ID), the initial dose may be used to estimate the skeletal uptake for the remaining doses. These radiation dose estimates permit patients at risk to be identified prior to reaching myelotoxicity and develop dose-response models. Thirteen patients (68%) reported significant pain relief from this radionuclide therapy. Bone pain appears to be alleviated by ¹⁵³Sm-EDTMP with limited red marrow doses and no toxic effects in other organs.