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Should previous CT dose influence prescribing an exam? [guide for radiologic technologists, also for x-ray and nuc med]

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.

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.

The AAPM Position Statement on Dose Metric Tracking

The AAPM has released a position statement on dose metric tracking , that we quote directly here for clarity. This statement has also been endorsed by the Radiological Society of North America (RSNA) and the Society of Nuclear Medicine and Molecular Imaging (SNMMI).

“It is the position of the American Association of Physicists in Medicine (AAPM), the American College of Radiology (ACR), and the Health Physics Society (HPS) that the decision to perform a medical imaging exam should be based on clinical grounds, including the information available from prior imaging results, and not on the dose from prior imaging-related radiation exposures.

AAPM has long advised, as recommended by the International Commission on Radiological Protection (ICRP), that justification of potential patient benefit and subsequent optimization of medical imaging exposures are the most appropriate actions to take to protect patients from unnecessary medical exposures. This is consistent with the foundational principles of radiation protection in medicine, namely that patient radiation dose limits are inappropriate for medical imaging exposures. Therefore, the AAPM recommends against using dose values, including effective dose, from a patient’s prior imaging exams for the purposes of medical decision making. Using quantities such as cumulative effective dose may, unintentionally or by institutional or regulatory policy, negatively impact medical decisions and patient care.

This position statement applies to the use of metrics to longitudinally track a patient’s dose from medical radiation exposures and infer potential stochastic risk from them. It does not apply to the use of organ-specific doses for purposes of evaluating the onset of deterministic effects (e.g., absorbed dose to the eye lens or skin) or performing epidemiological research. “

AAPM Frequently Asked Questions

In addition to the statement the AAPM has offered answers to a number of frequently asked questions at AAPM FAQs that are targeted for all healthcare professionals including you (the radiologic technologists). We would like to thank all of the members of the dedicated committee who contributed to the FAQ document including: Rebecca Milman, PhD. , Ioannis Sechopoulos, PhD., (Chair), Kristy Brock, PhD., Jessica Clements, MS., Joel Fletcher, MD., Mahadevappa Mahesh, MS, PhD., Cynthia McCollough, PhD., Michael McNitt-Gray, PhD., Ingrid Reiser, PhD., Aaron Sodickson, MD, PhD., Tim Szczykutowicz, PhD. (aka. Tim Stick), Kevin Wunderle, PhD. and Lifeng Yu, PhD. .

For completeness here we include direct quotations from the AAPM’s FAQs so that you can easily access the information as this is very useful in discussing with referring physicians, radiologists and most importantly the patients.

  • The facility where I work uses a dose management software program to monitor dose metrics
    from imaging exams. Why should we not use this to add up the doses from all of a patient’s
    imaging exams?

    Over the past decade, many medical imaging facilities have incorporated dose monitoring
    software programs to collect and analyze information about radiation doses from medical
    imaging exams. Dose metrics reported by the imaging device at the time of an exam – such as
    CT Dose Index (CTDI), CT Dose Length Product (DLP), Reference Air Kerma, and Kerma Area
    Product (KAP) – are sent to these dose monitoring programs. Some software will also estimate
    values such as absorbed doses to specific organs or tissues or effective dose.
    None of these dose metrics quantifies the biological effect of radiation to an individual patient.
    Further, possible stochastic effects (including potential increases in cancer risk from radiation) due
    to a current exposure are independent of previous exposures. Consequently, while dose
    monitoring software tools have the ability to sum dose metrics and estimate a patient’s cumulative
    imaging radiation dose, it is inappropriate to consider estimated cumulative risks from past
    exposures when evaluating the need for a new medical imaging exam. Rather, only the potential
    risks from the imaging exam under consideration should be compared to the potential benefit
    from that exam.
    Importantly, effective dose is not a measure of radiation dose or individual risk but rather a
    radiation protection quantity that estimates radiation detriment to a population consisting of all
    ages and both sexes. As such, effective dose does not represent the radiation risk to any individual
    patient, even if the dose monitoring software provides a calculated value for a specific patient.
  • Even if cumulative doses are not perfect representatives of total radiation risk, they still give a
    general idea of the total radiation the patient has received. What is wrong with using this
    information when ordering a medical imaging exam?

    Considering cumulative dose or effective dose values when deciding which medical imaging
    exam to order may be detrimental to a patient’s care. For example, using a patient’s cumulative
    dose as a reason to not perform a medically indicated imaging exam that uses ionizing radiation,
    or as a reason to substitute an imaging technique that uses ionizing radiation with one that does
    not, ignores many other important considerations. These include overall diagnostic performance,
    variability in diagnostic performance based on operator or reader, equipment availability, need
    for sedation, exam time, contraindications based on renal function or presence of metal or
    implanted devices, and cost.
  • If tracking a patient’s total radiation dose from imaging exams is not valuable in medical
    decision making, what is dose monitoring software good for?

    Monitoring dose metrics is helpful for quality assurance, protocol optimization, and compliance
    with accreditation and regulatory programs. Analyzing dose data in aggregate across patients
    can identify exam protocols that may benefit from further optimization, perhaps by changing preset exposure parameters. Outliers among the aggregate data can be scrutinized for opportunities
    to improve consistency of image quality or patient positioning. Such analyses can also be used to
    compare protocols across scanners and institutions. On a large scale, these data can be pooled
    to help establish updated Diagnostic Reference Levels (DRLs) for specific exam types and by
    patient size. In the future, these data, considered together with patients’ health outcomes, may
    aid in epidemiological studies of radiation risk from medical imaging.
  • My facility’s dose management software adds up the dose metrics it has for a patient’s
    previous imaging exams. If this information is not appropriate for medical decision making,
    how can we avoid making this information available?

    You can contact the manufacturer of your dose monitoring software and ask if this functionality
    can be turned off. Another important task is to have your Radiology Information System stop
    sending the DICOM modality worklist to the dose monitoring software. This is because access to
    the modality worklist is what allows dose monitoring programs to trigger prospective alerts or
    cumulative dose alarms.
  • Should we track a patient’s total organ doses, especially for parts of the body that we know
    are more sensitive to radiation, such as the thyroid?

    No, this information should not be used for clinical decision-making. Organ dose tracking is not
    currently helpful for guiding clinical care because there is no meaningful way to incorporate
    cumulative doses into actionable risk-benefit decisions. Specifically, there is no scientific or
    medical consensus on what to do when a specific cumulative organ dose is reached. From a
    research perspective, however, organ dose tracking in large populations of patients, when
    matched to a patient’s development or absence of cancer in those organs, may be valuable for
    improving radiation risk models in the future.
  • Are there any standards that ensure that the cumulative dose or effective dose values
    provided by dose monitoring software are accurate?

    No. There is no standard for calculating organ doses or effective dose from device-reported dose
    metrics. Tolerances for the accuracy and precision of such calculations also do not exist. Further,
    dose monitoring software may not include imaging exposures from independent facilities,
    meaning that any reported cumulative dose metrics may represent only a portion of a patient’s
    imaging exposures.
  • Is there a lifetime limit to the amount of radiation that a patient can receive from medical

    No. The foundational principles of radiation protection in medicine are justification and
    optimization. The International Commission on Radiological Protection2 and others have
    consistently emphasized that, unlike for occupational exposures, exposure limits to patients are
    not appropriate in medicine. This same philosophy is applied to all other medical interventions,
    such as pharmaceuticals or surgeries. Every medical imaging exam should be clinically justified –
    the potential benefit to the patient should be higher than the potential risk. Since the stochastic
    risk associated with a medical exposure to ionizing radiation – be it the patient’s 1st or 50th – does
    not change based on past exposures, then the radiation exposure from the 50th exam is justified
    as long as that exam is reasonably expected to provide a clinical benefit.
  • Shouldn’t we at least track the total dose for pediatric patients, who we know are more
    sensitive to radiation?

    Even though children are generally more sensitive to radiation, all of the points made here also
    apply to pediatric patients – each imaging exam should be justified on its own merit, regardless
    of previous exposures to medical radiation.
  • Is there any benefit to knowing a patient’s medical imaging history, even if the cumulative
    dose or effective dose values are not known?

    Yes. Knowing a patient’s imaging history can help determine whether an additional imaging study
    is likely to be beneficial. If a previous study exists that already answers the clinical question, there
    may be no benefit from further imaging. For example, if a patient had a CT of the head in Hospital
    A and then travels to Hospital B on the same day, the benefit to repeating the same study might
    be small or non-existent, presuming the exam at Hospital A was technically adequate. If the
    physicians at Hospital B have access to the previous study, additional imaging to answer the same
    clinical question could be avoided.
  • Employees exposed to ionizing radiation as part of their job responsibilities wear personal
    dosimeters to monitor their job-related radiation exposure. These exposures are then summed
    over time. If a radiation worker’s cumulative dose is above a certain limit, the employee must
    stop working (in the radiation environment) for a period of time. Why is it appropriate to use
    cumulative dose in that scenario but not with medical exposures to patients?
    Unlike patients, radiation workers do not derive direct benefit from occupational exposures. The
    International Commission on Radiological Protection and numerous other radiation protection
    organizations have consistently endorsed exposure limits for occupational exposures to protect
    workers from unnecessary risk in the performance of their job. Current regulatory occupational
    exposure limits are selected to ensure that jobs involving exposure to ionizing radiation pose no
    higher risks than other professions.
  • Does this position statement apply to interventional radiology procedures? Aren’t we
    encouraged to track radiation doses from these interventional procedures?

    Radiation doses from interventional procedures should be tracked, but only for the purposes of
    determining a patient’s risk of developing radiation-induced tissue reactions. This position
    statement specifically addresses stochastic risks from radiation exposure – i.e., the potential for
    increased risk of developing cancer. It does not address direct tissue reactions (i.e., deterministic
    effects) such as radiation-induced skin damage, where it is known that radiation effects are
    cumulative over a short period (hours to a few months). In medical imaging, radiation-induced
    tissue reactions are very rare and are typically only seen in prolonged fluoroscopy-guided
    procedures where the lengthy radiation exposure is necessary to address a serious medical
    situation. For patients undergoing fluoroscopy-guided procedures, it is reasonable to track and
    consider their recent dose history (e.g., the most recent 2 to 3 months) so the risks and benefits of
    the procedure can be appropriately considered and managed.
  • Does this position statement apply only to stochastic risks? Why doesn’t it apply to deterministic
    risks, such as dose to the skin or dose to the lens of the eye?

    Yes, this position statement only applies to stochastic risks. The biological mechanisms for
    stochastic effects (i.e., the potential increased risk of developing cancer) are fundamentally
    different from those involved in deterministic effects (i.e., damage to the skin, hair follicles, or lens
    of the eye.) Deterministic tissue effects are known to develop as the tissues undergo incremental
    damage that is not repaired prior to a subsequent exposure. For example, radiation exposure to
    the skin can cause small amounts of damage to outer layers of the skin. If there is additional
    radiation exposure to the same area before the outer layers of the skin have had time to undergo
    repair processes, the protection provided by these outer skin layers is compromised and can lead
    to additional damage to underlying tissues. In contrast, there is no known biological mechanism
    for previous radiation exposure to increase stochastic effects in subsequent imaging exams.
  • Does this mean that it doesn’t matter how much radiation we use during imaging exams?
    No. It is important that for any imaging exam, the use of radiation is both justified and optimized.
    It is essential that the amount of radiation used for an imaging exam is optimized, not minimized. “

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