Dose Definitions

Air kerma: The energy released per unit mass of a small volume of air when it is irradiated by an x-ray beam. For diagnostic x-rays, air kerma is the same as the absorbed dose delivered to the volume of air in the absence of scatter. Air kerma is measured in Gy.

Biologic variation: With respect to radiation, the differences among individuals in the threshold dose required to produce a deterministic effect, or the differences in degree of effect produced by a given dose. Biologic variation may be idiopathic or due to underlying disease. Different areas and types of skin also differ in radiosensitivity.

C-arm fluoroscopic system: A fluoroscopic system consisting of a mechanically coupled x-ray tube and image receptor. Such systems typically have two rotational degrees of freedom (left-right and cranial-caudal). Most such systems have an identifiable center of rotation called the isocenter. An object placed at the isocenter remains centered in the beam as the C-arm is rotated.

Cumulative dose (CD): The air kerma accumulated at a specific point in space relative to the fluoroscopic gantry (the interventional reference point) during a procedure. CD does not include tissue backscatter and is measured in Gy. CD is sometimes referred to as cumulative air kerma.

Deterministic effect: A radiation effect characterized by a threshold dose. The effect is not observed unless the threshold dose is exceeded. (The threshold dose is subject to biologic variation.) Once the threshold dose is exceeded in an individual, the severity of injury increases with increasing dose. Examples of deterministic effects include skin injury, hair loss, and cataracts.

Dose: As used in this document, "dose" is the same as the absorbed dose unless specified as "equivalent dose" or "effective dose."

Dose–area–product (DAP): The integral of air kerma (absorbed dose to air) across the entire x-ray beam emitted from the x-ray tube. DAP is a surrogate measurement for the entire amount of energy delivered to the patient by the beam. DAP is measured in Gy·cm².

Effective dose: The sum, over specified tissues, of the products of the equivalent dose in a tissue and the tissue weighting factor for that tissue. Effective dose is measured in Sieverts (Sv). Stochastic risk factors are usually stated relative to effective dose.

Equivalent dose: A quantity used for radiation protection purposes that takes into account the different probability of effects that occur with the same absorbed dose delivered by radiations with different radiation weighting factors. Effective dose is measured in Sv.

Fluorographic image: A single recorded image obtained using an image intensifier or flat digital panel as the image receptor. A digital angiographic "run" consists of a series of fluorographic images.

Fluoroscopy time: The total time that fluoroscopy is used during an imaging or interventional procedure.

Interventional reference point (IRP): For C-arm–type fluoroscopic systems with an isocenter, the IRP is located along the central ray of the x-ray beam at a distance of 15 cm from the isocenter in the direction of the focal spot. The IRP is defined by International Electrotechnical Commission (IEC) standard 60601-2-43

Isocenter: For C-arm–type fluoroscopic systems, the point in space between the focal spot and the image receptor through which the central ray of the x-ray beam passes, regardless of beam orientation.

Kerma: Kinetic Energy Released in Matter; the amount of energy transferred from the x-ray beam to charged particles per unit mass in the medium of interest. For diagnostic x-rays, this is equivalent to absorbed dose in the specified medium (eg, air, soft tissue, bone). Kerma is measured in Gy.

Peak skin dose (PSD): The highest dose at any portion of a patient’s skin during a procedure.

Stochastic effect: A radiation effect whose probability of occurrence increases with increasing dose, but whose severity is independent of total dose. Radiation-induced cancer is an example.

Threshold dose: The minimum radiation dose at which a specified deterministic effect can occur. Threshold doses differ among individuals as a result of biologic variation. The threshold dose for skin injury also differs in different anatomic sites on the same individual.

Although practicing physicians should strive to achieve perfect compliance, in practice, all physicians will fall short of this ideal to a variable extent. Indicator thresholds may be used to assess the efficacy of ongoing quality-improvement programs. For the purposes of these guidelines, a threshold is a specific level of an indicator that should prompt a review. When compliance rates fall below a minimum threshold, a review should be performed to determine causes and implement changes if necessary. If recording patient radiation dose data is one measure of the quality of radiation dose management, compliance rates lower than the defined threshold should trigger a review of policies and procedures within the department to determine the causes and implement changes to improve quality. Thresholds may vary from those listed here; for example, patient referral patterns and selection factors may dictate a different threshold value for a particular indicator at a particular institution. Because institutions and interventional fluoroscopic units vary widely in their ability to measure various metrics of patient dose, radiation dose data may be recorded with use of one or more of four different dose metrics: fluoroscopy time/number of fluorographic images, DAP, CD, and PSD. Therefore, setting universal thresholds is very difficult and each department is urged to alter the thresholds as needed to higher or lower values to meet its own quality-improvement program needs.
 

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