124-I was developed at Memorial Sloan-Kettering in the 1950s and has been used here and at many other medical centers around the world for diagnostic studies. It has been found to be very safe and effective at finding metastatic lesions. The high resolution of newer PET scanners now allows us to carefully determine how much radiation each metastatic lesion will receive. If 124-I can accurately predict which patients will not respond to 131-I treatments we can then avoid exposing those patients to unnecessary radiation. For the rest of the patients we can custom tailor the 131-I dose to destroy the metastatic lesions.