Treatment will be administered on an inpatient basis. An initial cohort of 3 patients will receive a tracer dose of 188Re-PTI-6D2 without any preceding unlabeled antibody. The tracer dose will consist of 10 mCi of rhenium-labeled antibody; the amount of antibody (in mg) will depend on specific activity and is estimated to range from 2 to 10 mg. Additional cohorts of three patients each will receive 10 mg, 20 mg, and 50 mg of unlabeled PTI-6D2 immediately preceding administration of the tracer dose. Patients will undergo serial gamma scans and SPECT/CT imaging at specified time points. Blood samples will be obtained prior to dosing and at specified intervals for PK measurements of the mAb as well as for measurement of serum radioactivity. Urine will also be collected for all patients to measure excreted radioactivity. Patients will be closely monitored for safety throughout the duration of the study.