Human Radiation Dosimetry from Imaging: Guidance for Therapy

Internal dosimetry of intrathecal radiopharmaceutical therapy employs quantitative imaging to provide its input data. The MIRD schema is the most widely used approach. It relies upon the estimation of the number of disintegrations that take place in a source and the fraction of the released energy from each disintegration that is absorbed in a target. Both single photon emitters and positron emitters have been used. Whether imaging by planar scintigraphy, SPECT/CT, PET/CT, or PET/MR, a variety of corrections must be employed to make the images quantitative. Some are common, such as attenuation correction, while others are specific to the modality or even to the emission properties of a particular radionuclide. The images are taken at propitious times during the uptake and clearance of the radiopharmaceutical, and regions within the images are analyzed to estimate time-activity curves for the sources. The area under a time-activity curve yields the number of disintegrations that occur within the source region. Those disintegrations can then be related to the dose imparted to a target by standard internal dosimetry models or by specialized calculations.