After three evaluable patients have been followed for a minimum of 2-6 weeks after 188Re-PTI-6D2 infusion, the Principal Investigator and the PTI Medical Monitor will review safety data for patients at the current dose level. If there is no evidence of a dose-limiting toxicity, the dose of 188Re-PTI-6D2 will be escalated for the next cohort of patients. Dose escalation of 188Re-PTI-6D2 will occur according to the following titration scheme: