The Relevance of Uptake and Intracellular Distribution for the Dosimetry of Auger and Beta-Emitting Radionuclides in Micrometastases

The microdosimetry of the ß-emitters 90Y and 131I, and the Auger emitters 125I and 67Ga was comparatively assessed. The presence of a micrometastasis (ø = 200 µm) in a normal tissue volume (ø = 5 cm), considering various degrees of uptake (k) and different intracellular distributions in the micrometastasis was simulated. Using a distance histogram technique, the absorbed dose rates in cell nuclei and a were calculated: the fraction of the dose rate in the centre due to the tumour activity at k = 1. Therefore, a represented the selectivity of energy deposition, and 1-a represented the cross fire from the surroundings. The efficacy was determined by selectivity and cross fire. The results show that the long range cross fire makes ß emitters less efficient in microscopic disease. In this situation, the use of short range emitters might be preferable, because of the more selective energy deposition and cross fire ranging several cell diameters.