Several
radionuclides are being studied for use in
radioimmunotherapy. Although 131I has been used most widely, there are several disadvantages to it including its large gamma-ray component, its rather long half-life, and its modest beta-particle energy. However, the beta-particle energy can be an advantage in very small
tumors (less than 1-2 mm). 186Re has several potential advantages over 131I but it has never been directly compared with it experimentally. Dosimetry modeling predicted that 186Re would have a dose advantage over 131I at large
tumor sizes but for
tumors as small as 1 mm diameter, this advantage would be lost. In order to confirm these predictions experimentally, this study compared the relative efficacy of a pancarcinoma
antibody, NR-LU-10 labeled with 186Re or 131I in 0.8-1.0 mm diameter LS174T human
colon adenocarcinoma multicell spheroids. Spheroids were incubated for 90 hr at 37 degrees C and evaluated with clonogenic assay, autoradiography, and histology. When corrected for cumulative activity bound, both
radionuclides were equally effective. Autoradiography demonstrated poor penetration of
radionuclide into the depths of the spheroids. Because 186Re has a theoretical dose advantage in larger
tumors and because it has been shown to be equivalent to 131I in
tumors as small as 1 mm diameter, it may be superior to 131I in most clinical situations. However, in the treatment of
micrometastases of less than 1 mm diameter, theoretical dosimetry modeling predicts that 131I or
radionuclides with similar beta particle energies should be more effective.