¹⁵³Sm-EDTMP for bone pain palliation in skeletal metastases

¹⁵³Sm-ethylene diamine tetramethylene phosphonate (EDTMP) is a widely available and extensively tested radiopharmaceutical for systemic radionuclide therapy in patients with symptomatic multiple skeletal metastases. Its use is approved for any secondary bone lesion which has been shown to accumulate ^99mTc-methylene diphosphonate, including breast carcinoma. The molecule is stable in vitro and upon injection more than 50% of the dose is avidly fixed by lesional and non-lesional bone, with the rest being rapidly eliminated unchanged via the urine. The short half-life (46.3 h), the relatively low-energy beta emissions (E _ave=233 keV) and the gamma emission (103 keV) make ¹⁵³Sm a very attractive radionuclide, allowing therapeutic delivery of short-range electrons at relatively high dose rates with external imaging to corroborate biodistribution and possible dosimetric estimates. For a standard dose of 2,590 MBq/70 kg, the estimated radiation dose to metastases is 86.5 Gy. Critical organs are the bladder wall (2.5 Gy/2,590 MBq) and red marrow (4 Gy/2,590 MBq), with the latter being the critical factor in clinical practice as the dose-limiting factor is marrow radiotoxicity. The therapy has, however, proved safe provided that the platelet count exceeds 100×10⁹/l and the white blood cell count exceeds 3.5×10⁹/l. Clinical data obtained in fewer than 250 patients, within several studies, lead to the following conclusions: a dose of 37 MBq/kg has a better therapeutic ratio than a dose of 18.5 MBq/kg; the mean pain palliation rate after a single treatment in breast cancer is about 80%; toxicity is generally mild and transitory; and re-treatments are effective and safe provided that haematological values have fully recovered.