In nuclear medicine imaging tests, injected radioactive substances do not harm the body. The radioisotopes used in nuclear medicine decay quickly, in minutes to hours, have lower radiation levels than a typical X-ray or CT scan, and are eliminated in the urine or bowel movement.
But some cells are severely affected by ionizing radiation -- alpha, beta, gamma and X-rays. Cells multiply at different rates, and the quickly multiplying cells are affected more strongly than standard cells because of two properties:
- Cells have a mechanism that is able to repair damaged DNA.
- If a cell detects that its DNA is damaged while it is dividing, it will self-destruct.
Quickly multiplying cells have less time for the repair mechanism to detect and fix DNA errors before they divide, so they are more likely to self-destruct when corrupted by nuclear radiation.
Since many forms of cancer are characterized by rapidly dividing cells, they can sometimes be treated with radiation therapy. Typically, radioactive wires or vials are placed near or around the tumor. For deep tumors, or tumors in inoperable places, high-intensity X-rays are focused on the tumor.
The problem with this sort of treatment is that normal cells that happen to reproduce quickly can be affected along with the abnormal cells. Hair cells, cells lining the stomach and intestines, skin cells and blood cells all happen to reproduce quickly, so they are strongly affected by radiation. This helps explain why people undergoing treatment for cancer frequently suffer hair loss and nausea.
Nuclear materials are also used to create radioactive tracers that can be injected into the bloodstream. One form of tracer flows in the blood, and allows the structure of the blood vessels to be viewed. This form of observation allows clots and other blood vessel abnormalities to be easily detected. Also, certain organs in the body concentrate certain types of chemicals -- the thyroid gland concentrates iodine, so by injecting radioactive iodine into the bloodstream, certain thyroid tumors can be detected. Similarly, cancerous tumors concentrate phosphates. By injecting the radioactive phosphorus-32 isotope into the bloodstream, tumors can be detected by their increased radioactivity.