There have been many studies on the carcinogenicity of ionizing radiation, mainly from people receiving high doses of radiation. For example, Japan's atomic bombing survivors, Chernobyl nuclear power plant leakage affected people, patients receiving radiotherapy and people who receive large amounts of radiation at work. As to whether low-dose ionizing radiation can increase the risk of cancer, there is no clear evidence; however, professional bodies and scientists tend to agree that low-dose ionizing radiation increases the risk of cancer, only that the risk is very low and related to the amount of radiation received. Ionizing radiation only increases the risk of cancer, it does not encourage the growth and spread of cancer cells that are already present.
Tumors associated with ionizing radiation are mainly leukemias, which appear a few years after receiving radiation; other tumors, such as lung, skin, thyroid, multiple myeloma, breast, and stomach cancers, usually appear 10 to 15 years after receiving radiation. The development of cancer is related to the part of the body that receives the radiation, and children are more sensitive than adults.
A chest radiograph has a radiation dose of 0.02 to 0.1 mSv, equivalent to 2.4 to 10 days of natural radiation; dental X-rays have a radiation dose of only about 0.01 mSv, which has a negligible potential for causing cancer; and molybdenum-targeted exams to screen for breast cancer have a radiation dose of about 0.4 mSv. There are dozens of studies in this area, and a handful of them have found that the radiation dose to healthy women who undergo the MAMMOGRAM annual exams were associated with a 1.5-fold increased risk of breast cancer compared to those at high risk who did not have annual exams, and those at high risk who were screened were associated with a 2.5-fold increased risk of breast cancer compared to those who were not screened. These studies were small and not large enough to make a difference.
The current view is that annual screening in healthy people after age 40 does not lead to an increased risk of breast cancer, which is why annual MAMMOGRAM screening is waited until after age 45. Because its cancer-causing potential comes many years later, if screening is done after age 45-50, a person may not live long enough to get cancer from exposure to this radiation. For those with BRCA1/BRCA2 mutations, extra care should be taken because low-dose radiation has a big effect on people with genetic mutations.
CT is 500 to 1,100 times more radioactive than X-rays, with head CT reaching 2.6 mSv, equivalent to a year's worth of natural radiation. CT of the whole body reaches 10.6mSv, so it can only be done occasionally. With the introduction of radiation as a cancer-causing factor, CT abuse can be found to be serious, and a 2009 study concluded that every 400 to 2,000 routine chest CTs resulted in one case of tumor.
Lung cancer screening uses low-dose CT, with a radiation dose of about 1.5 mSv, which is equivalent to 6 months of natural radiation; although lower than the radiation dose from head and whole-body CT, the agencies involved are still very cautious, and only recently have they recommended annual screening for smokers, and not all smokers. Eligible smokers are those with a 30-year history of smoking 1 pack of cigarettes a day; they either still smoke or have quit less than 15 years ago.
After analyzing the study, the FDA concluded that receiving 10 mSv of ionizing radiation increases the chance of dying from cancer by 0.05%, which is the equivalent of a single abdominal CT. What's the concept of a 0.05% increase? The lifetime chance of dying from cancer in the United States is 20%, after taking an abdominal CT, it may become 20.05%, an increase of 0.25%, which is nothing; however, if you take a CT often, the cumulative total is considered to be a lot. For example, if you take a CT 100 times, the chance of dying from cancer goes from 20% to 25%, an increase of 25%, an increase that is significant enough to offset the effect of some healthy habits to prevent cancer.
Overall, the increased risk of cancer from radiation received from medical checkups is either very low or negligible, but only if the checkups are not too frequent. Medical checkups are an area of great wealth and overuse: many people don't do them, while others do them excessively, doing many tests when they are not necessary and receiving varying degrees of ionizing radiation. If this is done year after year, the cumulative risk of cancer caused is high.