DEAR DR. ROACH: My neighbor had to change dentists. Her initial exam with the new doctor resulted in full-face X-rays. She objected, but was told they were low-dosage and would present no problems. Six to eight X-rays were taken. She is concerned about radiation. Can you comment on this? Does she need to be concerned? — V.M.

ANSWER: X-rays increase cancer risk, so it makes sense to avoid any unnecessary exposure. But you can’t completely eliminate exposure to radiation. Every year, we are exposed to natural radiation — in the air, in food and water and from space — of about 3 millisieverts. A typical dental X-ray is about 0.005 millisieverts — about the same you get in one day’s worth of normal living. So you and your neighbor can be reassured that the radiation dose from the dental X-rays has a very small risk.

Other medical X-rays have much larger radiation dosages; for example, a CT scan could be 10-20 millisieverts, which is the equivalent of up to seven years of natural radiation, or about 4,000 dental X-rays. A CT scan may well be worth it, since the risk of cancer is still very low. But it still makes sense for us as physicians to consider the risk of radiation anytime we order an X-ray or CT scan. It may helpful to know that the limit for workers for radiation exposure is 50 millisieverts.

DEAR DR. ROACH: Some time ago, I read somewhere that taking 200 mg sulindac pills removes polyps from the colon. If this is correct, please let me know, what dose one should take, and how often.

ANSWER: Polyps in the colon are small growths that arise from the lining of the large bowel. They can cause bleeding, but more importantly, a small percentage of them can become cancerous. Polyps often are removed during colonoscopy, which is how colonoscopy prevents colon cancer. Sulindac, an anti-inflammatory drug usually used for arthritis, was found in early trials to reduce the number of colon polyps. More-recent studies have shown a range of effect, from no benefit in some trials to 65 percent reduction in others. Unfortunately, even in the best case, 65 percent reduction of polyps isn’t enough to be safe from colon cancer, so currently the best way to prevent harm from colon cancer is regular examination by colonoscopy. A flexible sigmoidoscopy also shows polyps, but colonoscopy is preferred, since it looks at the whole colon.

Everyone between 50 and 70 should have a colonoscopy or other exam for colon cancer, and people over 70 should discuss colon cancer screening with their doctor.

Sulindac, or another medication, ultimately may be shown to reduce colon cancer rates, but since studies have shown no or modest benefit and all medicines have risks of side effects, I don’t recommend sulindac as a way of preventing polyps or cancer.

The booklet on colon cancer provides useful information on the causes and cures of this common malady. Readers can obtain a copy by writing: Dr. Roach —No. 505, Box 536475, Orlando, FL 32853-6475. Enclose a check or money order (no cash) for $4.75 U.S./$6 Can. with the recipient’s printed name and address. Please allow four weeks for delivery.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Health newsletters may be ordered from www.rbmamall.com.

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