Radiopharmaceutical Dosimetry

Abstract

In nuclear medicine, radiopharmaceuticals are administered to patients for diagnosis and therapeutic purposes. In diagnostic nuclear medicine, patient dosimetry aims predominantly to assess the risk for late effects of ionizing radiation (e.g., cancer and leukemia). To this end, the medical internal radiation dose (MIRD) dosimetry protocol is applied adopting standard biokinetics of the radiopharmaceutical and a standard anatomy of the reference male or female for the patient. For treatment purposes high activities of disease‐specific pharmaceuticals labeled with β‐emitters are given, necessitating an accurate patient‐specific dosimetry. In general patient‐specific biokinetics are derived from sequential images after administration of a tracer activity and combined with the MIRD methodology to calculate absorbed doses to target and critical tissues. In a complete patient‐specific dosimetry approach the individual patient anatomy is also taken into account and derived from computed tomography (CT) or magnetic resonance imaging (MRI) imaging. Three‐dimensional (3D) absorbed dose estimates are then determined from single single photon emission computed tomography (SPECT) or positron emission tomography (PET) activity data using dose‐point kernel convolution methods or by direct Monte Carlo calculation. A first part of the paper deals with the principles of the MIRD protocol and microdosimetric considerations, especially applicable to diagnostic nuclear medicine. The second part describes the methodology of dosimetry in metabolic radiotherapy with emphasis on the radioiodine therapy for treatment of benign and malignant thyroid diseases, meta‐iodo benzyl guanidine( 131 I‐MIBG) therapy for neuroblastoma and radioimmunotherapy. Finally, the dosimetric principles in the risk estimation of volunteers after administration of radiolabeled compounds in the framework of the development of new drugs are discussed.