Pharmacokinetic modeling and dosimetric planning of radionuclide therapy of bone metastases

This paper discusses the features of modeling and calculating the pharmacokinetic and dosimetric characteristics of osteotropic radiopharmaceuticals based on a compartment model of their transport in the human body with bone metastases. A software package for pharmacokinetic modeling and dosimetric planning of palliative radionuclide therapy of bone metastases using clinical radiometric data has been developed and tested. Within the framework of the four-compartment model, a method for determining absorbed doses in critical organs and tissues through their masses and through S-factors is proposed. Three approaches to the appointment of the activity of a radiopharmaceutical and the features of individual dosimetric planning of radio-nuclide therapy of bone metastases are considered and analyzed. For 10 patients with bone metastases, individual kinetic parameters of transport (transport constants of the model) of the radi-opharmaceutical "Sm-153-oxabifor" were identified during its intravenous intake into the body and calculations of absorbed doses in bone tissues and metastases, the circulatory system, kidneys and bladder were performed, taking into account its periodic emptying. It is shown that when the standard and specific activities of Sm-153-oxabifor are introduced into the patient's body (the first and second approaches), the absorbed doses in 10 patients differ by 5-6 times, while cases of under- or over-irradiation of bone tissues with metastases are detected, which can significantly reduce the effectiveness of radionuclide therapy or adversely affect the patient's condition later. The individual injectable activity of Sm-153-oxabifor calculated within the framework of the third approach for 10 patients varies widely from 19 to 165 mCi, while there are no cases of under- or over-irradiation of bone tissues. The dose loads on the circulatory system, kidneys and bladder for all patients are tolerant and do not exceed the maximum permissible values.