Single-cell dosimetry for radioimmunotherapy of B-cell lymphoma patients with special reference to leukemic spread

AIMS: Many lymphoma patients have both macroscopic tumors and single-cell manifestations of their disease. Treatment efficacy could, therefore, depend on the radionuclide used. The aim of this study was to investigate dosimetry at a cellular level for three isotopes of radioiodine. METHODS: Cells were assumed to be spherical with radii of 6.35, 7.7, and 9.05 microm corresponding to the dimensions of the Raji cells. The radius of the nucleus was assumed to be 75% of the cellular radius. The electron energies were 18, 28, and 190 keV, corresponding to the mean electron energy per decay for (125)I, (123)I, and (131)I, respectively. S-values for different activity distributions were simulated using Monte Carlo and dose-volume histograms as well as absorbed doses, and absorbed dose rates were calculated. RESULTS: (125)I gives the highest absorbed dose (approximately 4-40 times that of (131)I), whereas (123)I will give the highest absorbed dose rate (approximately 100 times that of (131)I). Under the given assumptions, the absorbed dose at this level is more dependent on the size of the cells than on whether the radioimmunoconjugate is internalized. CONCLUSIONS: This enquiry showed that both (123)I and (125)I have greater potential than (131)I for the treatment of leukemic spread in patients with lymphoma.