Dosimetry in nuclear medicine therapy: what are the specifics in image quantification for dosimetry?

where D(target) is the mean absorbed dose in gray (Gy) in the considered target, Ãsource is the total number of decays (the so-called cumulated activity) occurring in a given source, in Bq.s, and S(target ← source) is the mean absorbed dose in the target, per decay from the source (S value). This formalism seemingly splits absorbed dose determination into 2 independent tasks: cumulated activity determination, and S value calculation. The situation is in fact a little more complex: First of all, cumulated activity determination can itself be broken down into 2 independent tasks, namely absolute image quantification in all regions containing radioactivity (space) all along the radiopharmaceutical kinetics (time), and integration of the time-activity curve to derive cumulated activity. If A prerequisite to the calculation of the absorbed dose in targeted radiotherapy (TRT) is the determination of the localization and variation with time of the injected activity. This often requires quantitative scintigraphic imaging. The current quantitative imaging protocols can be divided in three broad categories: 2D, 2.5D and 3D, all used in the context of TRT, and yielding different compromises between accuracy and complexity that are discussed here. The relevance of a quantitative imaging procedure has to be reviewed in the light of the end point of the clinical dosimetry study: As far as radiation safety is concerned, some dosimetric approaches can be carried out using crude imaging protocols – and sometimes no imaging at all, and still were proved useful to derive the activity that can be safely injected to a given patient. Conversely, in some clinical situations, or when the end-point is to assess the efficacy of the treatment on a given cancer target, some sophisticated quantitative imaging approaches may be needed. In all situations, a very careful assessment of the technique used to derive the activity present in the patient must be performed, since activity quantification directly impacts the computation of absorbed dose. Radiopharmaceutical dosimetry requires the determination of absorbed doses delivered to the pa1UMRS_892 INSERM Therapeutic Research Institute University of Nantes, Nantes, France 2UMR 8165 CNRS, Imaging and Modelling in Neurobiology and Cancerology Paris-Sud University, Orsay, France M. BARDIÈS 1, I. BUVAT 2