Prospective 153Sm-EDTMP therapy dosimetry by whole-body scintigraphy

Samarium-153 ethylenediaminetetramethylene phosphonic acid (153Sm-EDTMP) effectively palliates painful bony metastases, but the standard recommended administered activity of 38 MBq.kg-1 may lead to significant myelotoxicity. Prospective individual dosimetry by urine collection and counting allow the bone marrow radiation dose to be limited to 2 Gy. Our novel whole-body scintigraphic method for prospective dosimetry was compared with the 5 h urine collection technique in 10 patients with bone metastases. Anterior and posterior whole-body images were obtained using identical acquisition parameters 10 min and 5 h after the intravenous injection of 740 MBq 153Sm-EDTMP. Total counts in each imaging study were corrected for background activity and time of injection and the bone activity at 5 h was determined. Bone activity was also calculated from a complete urine collection over 5 h, and these two values were compared. MIRD formulae were applied to calculate the radiation absorbed dose to the bone marrow from the injected activity. The total activity delivering a dose of 2 Gy to the bone marrow was then determined and constituted the amount given for therapy. Values for bone activity determined by imaging and by urine counting were concordant in all patients (correlation coefficient = 0.98). The total administered activity of 153Sm-EDTMP predicted on a 2 Gy bone marrow dose varied between 35 and 63% of the standard recommended regimen of 37 MBq.kg-1 and pain relief was experienced by eight of the ten patients. Administration of 153Sm-EDTMP according to the supplier's recommendations would have delivered bone marrow doses of 3.27-5.90 Gy in our patients, doses at which myelotoxicity would have been anticipated.