Re: If radiation causes cancer, why is it used to treat cancer?

Pam:

One of the characteristics of cancer cells is that they divide rapidly and with little control. That's one of the things that makes cancer cells dangerous.

Many methods for treating cancer are aimed at crippling the ability of cancer cells to divide by damaging their genetic material, the DNA. Ionizing radition works this way, as do many anti-cancer drugs. The problem is that if you damage the DNA just a little bit, you can cause mutations without killing the cell. If this occurs in normal cells, then there is some probability that the mutation will push the cell along the path to carcinogenesis.

Fortunatley, radiation-induced cancer is not a huge problem for radiation therapy because:

• By itself, ionizing radiation is not a very good carcinogen; although the combination of radiation and some anti-cancer drugs can be rather carcinogenic.
• Radiation-induced cancer takes a long time to develop, typically more than 20 years; and most cancer occurs in people who are 50+ years old. This means that most cancer patients will die of something else before there is much chance of a radiation-induced cancer appearing.
• The radiation dose within the treatment field is sufficiently high that the majority of the cells that are irradiated are dead, and dead cells don't form cancer.

The above also means that radiation-induced second cancers are most common:

• When patients irradiated at a young age and cured of their original cancer,
• When radiation is combined with anti-cancer drugs,
• At the the edge of treatment fields, where the dose is high enough to cause mutations, but not high enough to kill most cells

John Moulder
Radiation Biologist
Medical College of Wisconsin