JetService

I've heard certain radioisotopes are used for cancer screening. They are injected in the patients and the cancer cells seem to absorb them. Technicians then use a scope that hilights the areas of absorbsion to identify lesions. First, it this accurate? If so I have some questions...
Do radioisotopes themselves cause side-effects to the patient?
Does every single cancer cell in a tumor absorb them or just the outter-most cells?
Finally, if there is excessive radioisotopes injected, what happens to the excess? Is it expelled in body-waste?
Just doing some research. Thanks for any insight!!!