A simplified approach to alpha dosimetry for small spheres labelled on the surface

The radiation doses to spheres of radii 5-200 microns, labelled on the surface with alpha emitters were studied in order to simulate conditions of radioimmunotherapy for small tumours. The absorbed fraction of emitted energy and the mean absorbed dose per alpha particle in smaller concentric spherical targets were calculated for different source sizes. Calculations were first performed for monoenergetic emissions. We then considered the dosimetry of two common alpha emitters described in the literature (211At and 212Bi). The results agree well with different analytical approaches reported in the literature, but differ from those obtained using a Monte Carlo code. The computing features have been deliberately kept to a low level to allow for simple introduction into hospital work. Comparison of the mean absorbed dose per particle between alpha and beta emitters seems to favour the use of alpha emitters. However, their high toxicity introduces a practical radiation protection problem and our mathematical dosimetric approach is valid only for homogeneous distributions of radionuclides when, in the case of alpha emitters, the total dose delivered would be much too high to be of practical use.