People who need repeated CT scans often worry that radiation is building up inside them. The radiation itself does not stay in the body, and a CT scan does not make you radioactive. What adds up is the exposure received across examinations.
CT dose is not one fixed number
A CT scan generally uses more radiation than a standard X-ray, but the dose varies widely. A brain CT, chest CT, and abdomen and pelvis CT do not give the same exposure. Dose also changes with the scan protocol, the patient's size, and whether more than one phase is required.
This is why a single percentage cannot accurately describe the cancer risk from every CT scan. Age, sex, body area, and total exposure all affect risk estimates, and there is uncertainty when very small risks are calculated.
Why doctors still request repeat CT
A repeat scan may answer a new question or show how something has changed. In cancer care, for example, it may show whether treatment is working. In an emergency, it may identify bleeding, a blood clot, or an injury that needs immediate treatment.
The possible long-term risk from radiation has to be weighed against the risk of missing or delaying an important diagnosis now.
How to avoid repetition that does not help
Keep your previous reports and images where possible, especially if you use more than one hospital or imaging centre. A report is useful, but the actual images may be needed for comparison.
You can also ask:
- What question will this CT answer?
- Can a recent scan be used instead?
- Could ultrasound or MRI answer the same question?
- Will the scan use a protocol adjusted for my age and size?
An alternative is only useful when it can answer the clinical question well enough. MRI and ultrasound do not use ionising radiation, but they cannot replace CT in every situation.
Children need extra attention
Children are more sensitive to radiation and have more years ahead in which a radiation-related effect could appear. CT protocols should be adjusted for a child's size, and the examination should be limited to what is needed.
The bottom line
There is no universal number of CT scans that becomes unsafe. Each examination should have a clear reason, use an appropriate protocol, and add information that can affect care.