Beta dose-rate distributions in microscopic spherical tumors for intraperitoneal radioimmunotherapy

This work was designed to calculate the radial beta dose-rate profiles through microscopic spherical tumors. Its application is in the treatment of micrometastases in the peritoneal cavity by the intraperitoneal administration of radiolabeled immunoliposomes.

Using previously published data for the dose-rate as a function of distance from a point source of activity, dose-rate profiles through five sizes of tumors (radii: 10 microm, 50 microm, 100 microm, 500 microm, 1 mm) for six different radionuclides ((188)Re, (186)Re, (32)P, (90)Y, (67)Cu, (131)I) were calculated. Dose-rate profiles were calculated for two source geometries: (1) a large bath of radioactivity in which the tumor is submerged, and (2) surface-bound radioactivity that results from tumor targeting.

The bath geometry produced profiles that were uniform for sufficiently small tumors. For high-energy emitters (i.e., (90)Y and (188)Re), uniformity was maintained up to a tumor radius of 100 microm. For lower energy emitters (i.e., (67)Cu and (131)I) deviations from uniformity start to appear at a tumor radius of 50 microm. Surface-bound radioactivity produced a much greater range of dose-rates within tumors of all sizes. Lower energy emitters bound to the surface of tumors produce higher dose-rates for very small micrometastases compared with high-energy emitters. Upon consideration of the simultaneous contributions from both source geometries, we believe that liposome-mediated radioimmunotherapy would benefit from the inclusion of a high-energy beta emitter, possibly as a component of a cocktail of radionuclides.

The calculated dose-rate profiles provide a tool for making tumor control probability estimations for micrometastases and for assessing the potential benefit offered by a targeted approach over a nontargeted approach. These calculations also suggest that the inclusion of a high-energy beta emitter is appropriate for this treatment modality.