Comparison of two radionuclide doses in melanoma imaging with anti P-97 monoclonal antibody

38 patients were included in phase I study of localization of melanoma with Indium-111 labeled P96.5 monoclonal antibody reactive to melanoma antigen P97. 21 patients were studied using 2.5 mCi of In-111 and 17 patients using 5 mCi of In-111. Results of the two doses of In-111 were compared with results of scans. In 9 pats. receiving doses of 2.5 mCi In-111/20 mg stable antibody, out of 54 evaluable sites of disease, 19 sites were seen on scans performed 48 to 72 h after injection (Sensitivity 35%). In 12 pats. studied with 5.0 mCi of In-111/20 mg stable antibody there were 43 known sites of disease of which 30 were identified giving a sensitivity of 70%. As there was a high percentage of lesions greater than 1 cm in diameter in the group receiving 5 mCi and since tumor size has been previously shown to be related to sensitivity of imaging, the data was reanalyzed after excluding lesions less than 1 cm. When only lesions > 2 cm in diameter were considered there were 17 out of 21 (81%) imaged with the 2.5 mCi tracer dose and 27 out of 29 (93%) with 5.0 mCi dose. These differences are significant by chi square test with P < 0.05. Absorbed radiation dose from a 5 mCi dose of In-111 antibody has been calculated at 1.36 rads for the total body and 6.96 rads for the liver (critical organ). Although these doses could be lowered by using lower tracer dose, the results reported here suggest that the sensitivity would be significantly impaired at the smaller dose of 2.5 mCi. We conclude that the current maximum dose of 5.0 mCi should be used in evaluating In-111 labeled anti-P97 antibody imaging of melanoma