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

Samarium-153 ethylenediaminetetramethylene phosphonic acid (Sm-153-EDTMP) e ffectively palliates painful bony metastases, but the standard recommended administered activity of 38 MBq . kg(-1) may lead to significant myelotoxic ity. Prospective individual dosimetry by urine collection and counting allo w the bone marrow radiation dose to be limited to 2 Gy. Our novel whole-bod y scintigraphic method for prospective dosimetry was compared with the 5 h urine collection technique in 10 patients with bone metastases. Anterior an d posterior whole-body images were obtained using identical acquisition par ameters 10 min and 5 h after the intravenous injection of 740 MBq Sm-153-ED TMP. Total counts in each imaging study were corrected for background activ ity 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, an d these two values were compared. MIRD formulae were applied to calculate t he radiation absorbed dose to the bone marrow from the injected activity. T he total activity delivering a dose of 2 Gy to the bone marrow was then det ermined and constituted the amount given for therapy. Values for bone activ ity determined by imaging and by urine counting were concordant in all pati ents (correlation coefficient = 0.98). The total administered activity of S m-153-EDTMP predicated 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 e xperienced by eight of the ten patients. Administration of Sm-153-EDTMP acc ording to the supplier's recommendations would have delivered bone marrow d oses of 3.27-5.90 Gy in our patients, doses at which myelotoxicity would ha ve been anticipated. ((C) 1999 Lippincott Williams & Wilkins).