Prospective dosimetry with 99mTc-MDP in metabolic radiotherapy of bone metastases with 153Sm-EDTMP
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On the basis of the encouraging results achieved in several clinical trials and its proven therapeutic efficacy, 153Sm-ethylene diamine tetramethylene phosphonic acid (EDTMP) has become widely used to palliate pain from bone metastases. The results reported in the literature have led the product suppliers (QUADRAMET®, Schering) to suggest administering a fixed activity per kilogram (37 MBq/kg). However, considering the observed extreme inter-patient variability of skeletal uptake of 153Sm-EDTMP, a real therapy optimization would require the individualization of the activity to be administered on a dosimetric basis. This should be planned taking into account the generally accepted 2-Gy dose constraint to the haematopoietic red marrow, the critical organ in palliative treatments with beta-emitting, bone-seeking radiopharmaceuticals.
Seven to 14 days before treatment with 153Sm-EDTMP, 44 patients underwent 99mTc-methylene diphosphonate (MDP) total-body bone scan with two scans (the first within 10 min of injection, the second after 6 h). The percentage bone uptake (Tc%) was evaluated as the ratio between total counts at 6 h, adjusted for decay, and total counts at the first scan. Tc% was then compared to Sm% similarly derived from 10-min and 24-h whole-body scans. Tc% and Sm% were compared both with and without Brenner’s method for soft tissue uptake.
The correlation between Tc% and Sm% was R 2 = 0.81 and R 2 = 0.88 with and without soft tissue correction, respectively. The difference between their average values was statistically significant (Sm% = 64.3 ± 15.2, Tc% = 56.2 ± 16.0; p = 0.017) with soft tissue correction, while was not statistically significant (Sm% = 68.2 ± 15.5, Tc% = 66.9 ± 14.0; p = 0.670) without soft tissue correction.
The rate of retention of 99mTc-MDP in bone provides a reliable estimate of the 153Sm-EDTMP rate of retention. The proposed method can be usefully adopted for prospective dosimetry seeing its extreme simplicity, and it requires no special investment in terms of human or instrumental resources. This allows an optimization of administered 153Sm-EDTMP activity.
KeywordsBone metastases Radionuclide therapy Internal dosimetry 153Sm-EDTMP 99m Tc-MDP
The authors thank Rita Lorusso, Ph.D., Daniela Corletto, Ph.D., for their cooperation in data collection and analysis, and Luca Pedrazzini, M.D., Giuseppe Lomuscio, M.D., for the help in patient recruitment. We declare that the experiments comply with the current law of the country (Italy) where they were performed inclusive of ethical approvals.
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