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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?

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    $\begingroup$ In addition to the detailed info in the answers, it's important to understand that nothing in biology breaks down radioisotopes. They decay at a rate determined by nuclear physics (half life), regardless of whether they're in the body, or in a bottle sitting on a shelf. $\endgroup$
    – jamesqf
    Feb 3, 2019 at 17:41

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Cancerous tissues break down glucose faster, not radioisotopes. Doctors use PET technique to detect breakdown of glucose, and "hot spots" are investigated as potential tumors.

Glucose can be labeled with radioisotope to show up on PET scan. Normal glucose is "invisible".

Hence, candidate for tumor will breakdown any glucose, but radioactive glucose will show up on scan.

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  • $\begingroup$ In other words, radioactive glucose, as a product of being coated in radioisotopes, will show up on our diagnostic machines, correct? This is why it's useful? $\endgroup$
    – arara
    Feb 3, 2019 at 2:01
  • $\begingroup$ @Korvexius it is not covered with anything, but contently bound with radioisotope of fluoride. See first link $\endgroup$
    – aaaaa says reinstate Monica
    Feb 3, 2019 at 2:09
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Cancerous tissue is growing faster than most other tissue in the body. That is the nature of the disease. This growth will must assimilate nutrients, so any traceable nutrient will be seen to concentrate where the cancer is.

Radioactive isotopes are a useful tracer because they can be composed into chemically normal nutrients, and concentrated accumulations can be imaged noninvasively to infer where the cancer is. A possibly bonus is that the cancer gets preferentially irradiated.

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