Challenges in Nuclear Medicine Radiation Dosimetry

In spite of considerable progress, much remains to be done in the estimation of absorbed doses to organs and tissues in the body and in the prediction of biological effects from radiopharmaceuticals. In patients undergoing diagnostic procedures, the biokinetics of the radionuclide has to be determined for a number of representative patients. When radiopharmaceuticals are used for therapy, it is essential to determine the individual kinetics to be able to calculate the absorbed doses to critical normal organs/tissues and to the target volume(s) with high accuracy. There is still a lack of quantitative determinations of the organ/tissue contents of radionuclides and their variation by time. Planar gamma camera imaging is the main method for such studies. To get acceptable statistics in SPECT-images, very long acquisition times are needed. New more sensitive SPECT cameras may help. In a similar way as SPECT/CT, PET/CT is used for patient specific 3D image based internal dosimetry using the patient ́s own anatomy and spatial distribution of activity as a function of time. The transition from stylized reference phantoms to voxel phantoms, representing a broad population of patients will lead to improved dose estimates. The real challenge – at least for the therapeutic situation is to describe the individual patient by imaging (CT, MRI) and then make individual calculations. There is an increasing interest to combine targeting substances (antibodies, peptides, etc.) with alpha particle or Auger electron emitters. It is a challenge to develop a dosimetry that predicts the biological effects of these short-range particle emitters. For staff exposure, it is important to take into account the increasing use of PET agents for imaging and to investigate the occupational exposure from a number of radiopharmaceuticals for therapy and how these affect the effective dose and the dose to fingers, hands and the lens of the eye. Key-words: Dosimetry, nuclear medicine, radiopharmaceuticals, biokinetics