Simplified Method for Calculation of Internal Exposure Dose to Bone Metastases During Radionuclide Therapy

Abstract

Purpose: Development of a simplified method for calculating internal doses of bone metastases during radionuclide therapy with radiopharmaceuticals. Material and methods: A comparative analysis of the advantages and disadvantages of 3 existing methods of dosimetry of internal irradiation (MIRD formalism, the method of the dose kernel of a point isotropic source and the Monte Carlo modeling method) was carried out in relation to the task of dosimetry of bone metastases. It has been shown that all of them, having a high accuracy of dose estimation, remain unsuitable for wide clinical practice due to their mathematical complexity, laboriousness, low availability, and overestimated requirements for user qualifications. A simplified method is proposed for determining doses of internal irradiation of tumor foci based on quantitative data from SPECT/CT scanning of an X-ray phantom and a real patient who has been injected with a b-g-radiating therapeutic radiopharmaceutical. Results: On a clinical example of radionuclide therapy with 177Lu-PSMA of a patient with stage 4 prostate cancer, dose estimates were obtained for both internal exposure to b-particles and g-quanta from the radiopharmaceutical accumulated in bone foci, and external exposure to g-quanta of the same radiopharmaceutical contained at the time of measurements in the tissues surrounding the focus and environments of the whole body of the patient. Calculations were made for bone metastases of 7 localizations in dynamics for each of the 5 fractions of radionuclide therapy. It is shown that the total focal doses for 5 fractions of internal exposure vary from 70.6 to 116.8 Gy for different foci, which corresponds to the literature data obtained by more accurate methods of dosimetry of internal exposure. Conclusion: The developed simplified method for obtaining dose estimates for radionuclide therapy of bone metastases is characterized by an accuracy acceptable for clinical purposes while ensuring simplicity and low labor intensity of its practical application by a wide range of medical physicists and radiologists.