Prospective dosimetry with ^99mTc-MDP in metabolic radiotherapy of bone metastases with ¹⁵³Sm-EDTMP

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.