Is the risk of developing cancer from radiation dependent on previous radiation dose?
In order to answer the question, “Should previous CT dose influence a given exam?” we need to understand if there are cumulative effects of being exposed to radiation that will change the relative risk for subsequent exams.

See our post at: [ Ссылка ]

For each patient that is in need of a diagnosis for a clinical concern the clinician must weigh off the potential risk of inducing cancer due to the radiation against the diagnostic benefit of the scan. The natural inclination may be to try to reduce the radiation dose for patients who have had a number of x-ray or CT exams in the past. However, since the process of cancer induction due to radiation (e.g. radiation carcinogenesis) is stochastic the decision to prescribe each exam should be made independently from prior radiation exposure.

Radiation carcinogenesis can be caused by ionizing radiation such as x-rays and gamma rays. In medical imaging the sources of x-rays include: radiography and fluoroscopy equipment, mammography, DEXA, mobile x-ray, interventional systems and computed tomography. Gamma rays are the same type of radiation as x-rays but come from the decay of unstable ions, and are used in nuclear medicine and PET imaging. Additionally, x-rays and gamma rays are both used to treat cancer in the body and typically significantly higher doses of radiation are used and targeted towards the treatment regions such as tumors.

Radiation at dose high levels has been linked to a number of adverse outcomes including: causing cancer, acute radiation syndromes, fertility , hereditary and gestational risks and inducing cataracts. Since the radiation dose of diagnostic x-ray and CT scans is well below the level of acute radiation syndromes the primary concern when deciding to prescribe an x-ray or CT scan is the possibility that the scan could induce cancer in the patient.

As we described in our post on Radiation biology the effects due to radiation can be separated into those that are stochastic in nature (including cancer induction) and those that are deterministic (including acute radiation syndromes and cataracts induction). The stochastic effects have a random component such that the same result will not occur each time with the same radiation dose exposure.

A simple model for stochastic effects is to think about rolling dice. As you role fair dice you are equally likely to role any given number on the dice. More importantly for this discussion, each role of the dice is independent of previous roles. For instance if you rolled the number 3 multiple times in a row that does not mean that for subsequent rolls that the likelihood that you roll 3 is any more likely or less likely.

The same effect is true for the potential for carcinogenesis (cancer induction) due to x-ray or CT scanning. Each exam that the patient receives can be viewed as a roll of the dice, and the probability that the exam induces cancer is dependent on multiple factors including: the radiation type, the radiation dose, and the tissue exposed.

Should previous x-ray dose influence a given exam?
Each exam is performed separately to answer a diagnostic question and the risk of causing cancer is independent. Therefore, the decision to prescribe each exam should be made independently of prior radiation dose.

What are cumulative dose metrics?
Since the risk of cancer induction is related to the radiation dose level, it is important to understand the radiation dose given to the population from medical imaging including x-ray, CT and nuclear medicine exams. It is also useful for quality control programs to have metrics for assessing the dose.

Radiation dose measurement and tracking is NOT intended offer estimates for any one individual within the population.

We cover the basics of dose units in a separate post where we discuss that in the SI units the absorbed dose is measured in mGy (milliGray). From the absorbed dose the equivalent dose accounts for the radiation type and is measured in mSv (milliSieverts), and the effective dose further accounts for the anatomy that receives the radiation dose.

There have been proposals to track the radiation dose over time. This has been termed longitudinal mSv or cumulative mSv and there have also been proposals to record and display this data for each patient.

However, as discussed above the relative risk of each exam is independent of the previous radiation dose received.

Therefore, we should discontinue the practice of displaying cumulative (mSv) or any such cumulative dose value as it may bias the decisions of the clinicians when determining if prescribing another exam is appropriate.

свернуть