124I was either provided in-house by the Memorial Sloan Kettering Radiochemistry & Molecular Imaging Probes Core Facility or purchased commercially (IBA Molecular). Stocks of hu3F8-C825 were labeled with radioactive iodine using pre-coated IODOGEN tubes (Pierce) according to standard protocols (15 , 16 (link)) to final specific activities of 121-152 MBq/mg. The tracer immunoreactivities were evaluated using cell-binding assays (17 (link)) and were 78.8 ± 0.1% (n = 3) for GD2(+)-NB IMR-32 and 4.9 ± 0.4% (n = 3) for GD2(-) SK-N-SH. The chelate p-aminobenzyl-DOTA (DOTA-Bn; Macrocyclics) was radiolabeled with 177LuCl3 (specific activity: 170 MBq/nmol; Perkin Elmer) at a ratio of 37 MBq to 1.1 μmol DOTA-Bn using previously described methods (9 (link)), but without HPLC purification (C825 does not recognize metal-free DOTA-Bn).