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CT radiation risks

 

The probability for absorbed x rays to induce cancer or heritable mutations leading to genetically associated diseases in offspring is thought to be very small for radiation doses of the magnitude that are associated with CT procedures.

Estimates of cancer and genetically heritable risk from x-ray exposure have a broad range of statistical uncertainty, and there is controversy regarding the effects from very low doses and dose rates.

Under some rare circumstances of prolonged, high-dose exposure, x rays can cause other adverse health effects, such as skin erythema,skin tissue injury, and birth defects following in-utero exposure.

But at the exposure levels associated with most medical imaging procedures, including most CT procedures, these other adverse effects would not occur.

Doses from a single pediatric CT scan can range from about 5 mSv to 60 mSv.

Among children who have undergone CT scans, approximately one-third have had at least three scans.

Commonly assumed that the risk for adverse health effects from cancer is proportional to the amount of radiation dose absorbed and the amount of dose depends on the type of x-ray examination.

A CT examination with an effective dose of 10 millisieverts (abbreviated mSv; 1 mSv = 1 mGy in the case of x rays.) may be associated with an increase in the possibility of fatal cancer of approximately 1 chance in 2000.

This increase in the possibility of a fatal cancer from radiation can be compared to the natural incidence of fatal cancer in the U.S. population, about 1 chance in 5.

In other words, for any one person the risk of radiation-induced cancer is much smaller than the natural risk of cancer.

The effective doses from diagnostic CT procedures are typically estimated to be in the range of 1 to 10 mSv.

This range is not much less than the lowest doses of 5 to 20 mSv received by some of the Japanese survivors of the atomic bombs, who are estimated to have experienced doses only slightly larger than those encountered in CT, have demonstrated a small but increased radiation-related excess relative risk for cancer mortality.

A particular radiation dose will depend on the size of the body part examined, the type of procedure, and the type of CT equipment and its operation.

Typical values cited for radiation dose should be considered as estimates that cannot be precisely associated with any individual patient, examination, or type of CT system.

The actual dose from a procedure could be two or three times larger or smaller than the estimates.

Screening procedures may adjust the radiation dose used to levels less than those typically used for diagnostic CT procedures.

In low dose CT scans radiation doses may be reduced by factors such as 1/2 to 1/5.

Reducing the dose can have an adverse impact on the image quality produced.

The risk of cancer detriment from a CT procedure is referred  to as the effective dose.

Effective dose is evaluated in units of millisieverts, and it allows comparison of the risk estimates associated with partial or whole-body radiation exposures and the different radiation sensitivities of the various organs in the body.

Estimates of the effective dose from a CT procedure can vary by a factor of 10 or more depending on the type of CT procedure, patient size and the CT system and its operating technique.

Diagnostic Procedure Typical Effective Dose (mSv)1 Number of Chest X rays (PA film) for Equivalent Effective Dose Time Period for Equivalent Effective Dose from Natural Background Radiation

Chest x ray (PA film) 0.02mSv 12.4 days

A chest X-ray carries a dose of approximately 0.02-0.2 mSv and natural background radiation exposure is around 2.3 mSv/year.

Skull x ray 0.1mSV 5 12 days

Lumbar spine 1.5 mSv 75 182 days

I.V. urogram 3 mSv 150 1.0 year

Upper G.I. exam 6mSv 300 2.0 years

Barium enema 8mSv 400 2.7 years

CT head 2mSv 100. <243 days

CT abdomen 8mSV 400 2.7 years

Cardiac CT angiography can result in a relatively high radiation exposure (around 12 millisievert), but newer protocols, have recently been developed which drastically reduce this exposure to around 1 mSv.

Each cardiac CT scan carried out with current protocols (dose approximately 1 mSv) is equivalent to approximately 5-50 chest X-rays or less than 1 year of background radiation. 

 

 

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