Answer to Question #11506 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
I have questions regarding the effects of radiation on the brain. I understand that the small possibility of cancer from radiation comes from the possibility of double-strand breaks of DNA. However, I also understand that neurons are not very sensitive to radiation and stop dividing at an early age. How are neurons affected by double-strand breaks if they don't divide? If they don't divide, how can they cause cancer? Is there a threshold of radiation to the brain before double-strand breaks occur and cause cancer?
The cause of cancer is closely associated with chromosome and DNA aberrations. There is a small number of chromosome aberrations in our bodies when we are born and this number increases as we age. In addition, chromosome aberrations can be increased by chemicals (smoking is an example), radiation, and other biological stresses.
The tissues most sensitive to radiation damage are those with rapidly dividing cells, like:
- Bone marrow and lymphatic system (which are constantly producing new blood cells).
- Hair (hair loss is expected when receiving chemotherapy).
- Lining of the gastrointestinal (GI) system (GI cells replace themselves every seven days).
- Cancer cells (that's why radiation therapy is so effective in killing cancer cells).
The reason that rapidly growing cells are sensitive (in the sense that they are easily damaged or killed), is that during cell division, one double-stranded DNA becomes two single-stranded DNA chains. The single-stranded DNA chains can be broken much more easily by radiation, and after they break, they are less likely to repair themselves properly. This most likely leads to cell death, because the cell reproduction process is disrupted. Since neurons are not rapidly dividing cells, they are not as sensitive to cell death caused by radiation.
When dormant cells are irradiated and there is a single-strand break in the DNA, often the DNA will repair itself and be as good as it was before the damage. However, with more intense radiation, a DNA chain may have two simultaneous strand breaks on opposite sides of the DNA chain. This can result in poorly repaired DNA, called a chromosome aberration. A cell with a chromosome aberration may lie dormant for years before becoming cancerous or may never become cancerous.
The mechanisms for making a chromosome aberration and then the probability of that leading to cancer are all random, chance-type events. So the bottom line is that there is no threshold amount of radiation (measured in millisieverts [mSv]) that will cause double-strand breaks in the brain cells that may or may not lead to cancer.
John P. Hageman, MS, CHP