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No one-size-fits-all when it comes to CT dosing: BC Technical

November 30, 2015
CT Population Health Risk Management X-Ray
By Jennifer Rioux, Contributing Reporter

A training presentation from BC Technical on radiation dosing for CT technology draws attention to the fact that the benefits of CT are well documented, however cancer risks from radiation exposure continue to be a significant concern. According to the video, although CT is only responsible for 17 percent of imaging exams, it is responsible for nearly half of the collective dose of radiation from medical procedures in U.S.

Risk projection anticipates that 1.5-2 percent of all cancers will be attributable to CT in 2-3 decades. A recent study published in 2012 in the Lancet reported a direct increase in cancer rates related to CT radiation exposure. Industry standards recommend a balance between using a dose “as low as reasonably achievable” (ALARA) for patient safety and a dose “as high as reasonably achievable” (AHARA) to meet diagnostic purposes.
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The video, aimed at radiation technologists, made the point that equipment manufacturers have a role to play in optimizing CT dose, but that technologists and radiologists will be held primarily responsible for minimizing radiation dose from CT. Thus, proper training of radiologists and technicians is critical.

Cancer risk from an equivalent dose depends on the organ receiving the dose. Effective dose is a term referring to a method for comparing the risks of radiation to different organs. Factors that affect radiation dose include CT scanner designs, use parameters, and strategies used by technologists for dose optimization.

Computed Tomography Dose Index (CDTI) is the primary dose measurement concept in CT, involving complex mathematical models. However, CTDI does not account for patient size OR represent peak dose, therefore, CTDI should not be used as an indication of patient dose because it does not reflect the patient’s absorbed dose.

A new method called Size Specific Dose Estimate (SSDE) was introduced in 2011 by the American Association of Physics in Medicine. SSDE is a metric that accounts for patient size, scan length, and tissue attenuation - an indicator of absorbed organ doses. A set of conversion factors were presented to assist clinicians in moving from CTDI to a more accurate estimation of patient dose using SSDE, and between 2011-2012, a dozen studies were published on use of SSDE in clinical populations. However, there is currently no method to calculate SSDE available on CT consoles.

Dose management does not always mean dose reduction. It’s very important to understand direct patient positioning for a CT exam, to ensure that the image will be of the anatomy required for the study, and to carefully define the field of view (FOV) in order to avoid repeat scanning. There is no "one-size-fits all" when it comes to exposure settings, they must be individualized.

Older systems must reference age and weight-based tables to determine exposure settings. Some scanner models made by GE (SmartamA), Philips (DoseRight and DOM) and Siemens (CAREDose4 4D) include automatic exposure control (AEC), which adjusts settings based on the specifics of the patient size and related exposure variables. Optimizing dose in a clinical setting is a balancing act between image quality and precautions against patient overdosing.

CT scans must be appropriate for each individual patient. Responsibility for appropriate justification is shared between the requesting clinician and the radiologist. The imaging community must work with ordering clinicians to direct patients to the most appropriate imaging modality for a specific diagnostic task and to ensure optimization of all technical aspects of the examination, in order to obtain the required level of image quality while keeping dose as low as possible.

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