Cancer divides faster than most other cells due to lack of cell cycle regulation, so it uses far more resources than other cells. These means the likelihood of a cancerous tumor incorporating an administered isotope (e.g., radioactive glucose) is more likely than noncancerous cells, which can be exploited by imaging techniques.

Cancer divides faster than most other cells due to lack of cell cycle regulation, so it uses far more resources than other cells. These means the likelihood of a cancerous tumor incorporating an administered isotope (e.g., radioactive glucose) is more likely than noncancerous cells, which can be exploited by imaging techniques.

Cancer cells take up those isotopes that are required for some function like nourishment. Like glucose as said above. It is not because it is an isotope. It is because glucose uptake is 10 to 20 times higher in cancer than in normal cells. The fact that we use an isotope labeled glucose makes no difference in the uptake.