Dosimetric evaluation of ¹⁵³Sm-EDTMP, ¹⁷⁷Lu-EDTMP and ¹⁶⁶Ho-EDTMP for systemic radiation therapy: Influence of type and energy of radiation and half-life of radionuclides

In radiopharmaceutical therapy, delivered doses to critical organs must be below a certain threshold therefore internal radiation dosimetry of radiopharmaceuticals is essential. Advantages and disadvantages of radionuclides with different characteristics were evaluated for selection of appropriate radionuclide. The Monte Carlo MCNPX simulation program was used to obtain radial dose and cumulative dose of ¹⁵³Sm, ¹⁷⁷Lu and ¹⁶⁶Ho used in radiotherapy of bone metastases. A cylindrical geometry with constant density materials was supposed for simulation of femur bone. The radius of bone marrow, bone, and surrounding soft tissue was considered 0.6 cm, 1.3 cm and 4 cm, respectively. It was assumed that the radionuclides were uniformly distributed throughout the tumor. “continuous energy spectrum” of beta particle was used instead of mean beta energy. Our simulations show that absorbed dose in target organ (bone) is greater than other organs and ¹⁶⁶Ho gives a higher dose to the critical organ of bone marrow than either ¹⁵³Sm or ¹⁷⁷Lu. Absorbed dose versus time demonstrate faster dose delivery for the short half-life radionuclides (¹⁵³Sm and ¹⁶⁶Ho). These results are in good agreement with clinical observations which show a pain relief within 1 week after intravenous administration of ¹⁵³Sm-EDTMP, whereas it occurs within 2 week in the case of ¹⁷⁷Lu-EDTMP. According to the results, combination of different radionuclides with different characteristics such as ¹⁵³Sm-EDTMP and ¹⁷⁷Lu-EDTMP could be more advantageous to patients with painful bone metastasis. - Highlights: • The radial dose and cumulative dose of ¹⁵³Sm, ¹⁷⁷Lu and ¹⁶⁶Ho are calculated. • The effect of half-life of the radionuclide on the suitable activity for injection is studied. • Dose delivery is fast for the short half-life radionuclides (¹⁵³Sm and ¹⁶⁶Ho). • The results are in good accordance with clinical observations. • The combination of different radionuclides with different characteristics could be more advantageous