Dosimetry is a term used in the area of radiation protection and health physics.  It involves the measurement of the ionizing radiation dose absorbed by the object.  However, when dosimetry is applied to the human body, it is usually concerned with the internal ingestion or inhalation of radioactive substances and externally with the irradiation by sources of radiation. Therefore, dosimetry estimates the radiation of absorbed doses to individual organs and the tumour site. These dosimetry calculations regarding the radiation dose subjected to organs and surrounding tissue help to determine whether a patient can be treated safely.  It also enables the estimation of dose to tumours and contributes to predicting the therapeutic value of radioimmunotherapy.  This can be demonstrated by the initial clinical development of the Zevalin procedure.  During this clinical trial, radiation dosimetry was performed at the investigative site for 205 patients prior to treatment with yttrium-90 radiolabelled Zevalin.  First, the radiation absorbed dose was estimated following imaging with indium-111 (electron capture, half-life of 2.8 days) in order to make a decision for proceeding with yttrium-90 radiolabelled Zevalin treatment. MIRDOSE computer software was used to estimate the radiation dose by applying quantitative imaging and blood sampling data. It was found that all 205 patients met protocol-defined criteria for proceeding with yttrium-90 radiolabelled Zevalin treatment. The estimated radiation absorbed doses were found to be below the allowed maximum of 2000 cGy for normal organs and 300 cGy for red marrow (myeloid tissue).  Therefore, dosimetry is the process of relating the administered amount of radioactivity to the absorbed radiation dose in organs, tumours and the whole body. In some cases, the radiation dose calculated for targeted alpha radiotherapy (TaRT) is less accurate than for external beam radiation therapy (EBRT). This is because of the limited radiation dose input data, inhomogeneous dose distributions and the rationale behind the estimation of the TaRT absorbed doses.  These radiation dose calculations are more complicated for internally distributed radionuclides than for external beam irradiation.  The alpha particle dosimetry component of the calculation adds the complication of the decay cascade due to the daughter products that may have a different distribution of the parent radionuclide.