Apparently, many medical practitioners use radioisotopes to detect cancer in patients. From my textbook (McGraw Bio 12, pg. 10);
Using a method called radioisotope tracing doctors can inject radioactive material into a patient and trace its movement in the body. For example, cancerous tissues in the body are characterized by a much higher level of activity than healthy tissues. Consequently, cancerous cells take in more glucose—a common cellular energy source—than healthy cells. Injecting a patient with radioactive glucose and then performing a positron emission tomography (PET) scan, such as the one shown in Figure 1.1, is one method to diagnose a cancerous tumour
Apparently, cancerous tissues break down radioisotopes at a much higher rate than normal tissues. But how?