Elements of Danger - Medical Imaging

An article in today's New England Journal of Medicine needs to be considered by everyone with CLL. It is probably true that patients with CLL are more prone to second malignancies than the general population. In fact it is terribly difficult to prove that this is true. It is certainly correct for virally-induced cancers and may well be true for immunologically controled cancers like melanoma, but evidence that the common cancers - breast, lung, colorectal and prostate - are commoner in CLL is hard to come by. However, there is no doubt that exposure to low-dose ionizing radiation can cause both cancer and leukemia.

Workers in radiation oncology units and in the nuclear power industries are monitored for how much radiation they are exposed to. If they exceed a recommended limit they are removed from the high risk area. That limit is 50 mSv in any given year, and a maximum of 100 mSv over 5 years. Patients are not monitored, which probably didn't matter when all they got were chest X-rays which involve minimal exposure (only 0.02 mSv), but modern imaging procedures involve much large dose.

For example a CT of the chest causes 7 mSV, or the pelvis 6 mSV, the head 2 mSv and the abdomen 8 mSV. So a whole body CT involves your yearly allowance of exposure. Myocardial perfusion imaging, an increasingly popular imaging technique used to predict heart attacks, but which has never been shown to be useful in preventing heart attacks, delivers 15.6 mSv. Mammography delivers only 0.4 mSv.

Defensive medicine and the fact that these procedures are a nice little earner for the radiology department, mean that these procedures are increasingly used in medicine, especially in the US.

I have warned before that most CT scans in CLL are unnecessary. The diagnosis and staging of CLL do not require a CT scan. In fact results obtained by CT scanning are dangerously misleading and often lead to unnecessarily early treatment. The perpetrators are usually medical oncologists who treat CLL as if it were just another lymphoma. The one indication for abdominal imaging is when large abdominal nodes are suspected. Outside of clinical trials precise measurement of these is not required and a non-radiological technique such as abdominal ultrasound is quite sufficient. Regular monitoring during the watchful waiting phase does not require CT scanning. In fact the only firm indication for CT scanning that I can think of is in patients who might have a fungal pneumonia, say, after Campath treatment.

There are scare stories around which tell us that CT scanning delivers 400 times as much radiation as a chest X-ray. This may be true, but a chest X-ray delivers such a small dose, it is a silly comparison, and the doctor who is making it is a proponent of the much more expensive MRI technique. I have no axe to grind except to say why pay for a technique that you don't need?