Monte Carlo calculations of absorbed doses in tumours using a modified MOBY mouse phantom for pre-clinical dosimetry studies.

Abstract Background. Clinical treatment with radionuclides is usually preceded by biokinetic and dosimetry studies in small animals. Evaluation of the therapeutic efficacy is essential and must rely on accurate dosimetry, which in turn must be based on a realistic geometrical model that properly describes the transport of radiation. It is also important to include the source distribution in the dosimetry calculations. Tumours are often implanted subcutaneously in animals, constituting an important additional source of radiation that often is not considered in the dosimetry models. The aims of this study were to calculate S values of the mouse, and determine the absorbed dose contribution to and from subcutaneous tumours inoculated at four different locations. Methods. The Moby computer program generates a three dimensional (3D) voxel-based phantom. Tumours were modelled as half-spheres on the body surface, and the radius was varied to study different tumour masses. The phantoms were used as input for Monte Carlo simulations of absorbed fractions and S factors with the radiation transport code MCNPX 2.6f. Calculations were performed for monoenergetic photons and electrons, and the radionuclides (125)I, (131)I, (111)In, (177)Lu and (90)Y. Results. Electron energy and tumour size are important for both self- and cross-doses. If the activity is non-uniformly distributed within the body, the position of the tumour must be considered in order to calculate the tumour absorbed dose accurately. If the uptake in the tumour is high compared with that in adjacent organs the absorbed dose contribution to organs from the tumour cannot be neglected. Conclusions. In order to perform accurate tumour dosimetry in mouse models it is necessary to take the additional contribution from the activity distribution within the body of the mouse into account. This may be of significance in the interpretation of radiobiological tumour response in pre-clinical studies.