Purpose: The influence of tumor volume, uptake of radioactive compounds in cells of tumors and normal tissues, and characteristics of the emitted ionizing particles on the efficacy of systemic radiation were studied.
Methods and materials: The influence of these variables was assessed using a point kernel approach combined with a distance histogram technique. Simulation calculations were performed to assess dose distributions for three tumor sizes (phi = 200 microns, 2 mm, or 2 cm) and six radionuclides: 67Ga, 125I, 67Cu, 90Y, 131I, and 186Re.
Results: The energy deposition patterns depended on the relation of the tumor size and range of the emitted particles. Selective uptake was especially important in cases where the range was short compared to the dimension of the tumor.
Conclusion: To attain a high dose for treatment of micrometastases, the use of Auger and conversion electron emitters (67Ga and 125I) or beta-emitters with emission spectra including low energetic electrons (67Cu and 131I) was recommended. The results demonstrated the complementary nature of selectivity of energy deposition and crossfire. This implied that for tumor cells or areas with reduced uptake, crossfire from radioactivity in surrounding cells or areas with selective uptake would be provided by intermediate (conversion electrons) or long-range (beta-particles) emissions.