Key Safety Terms in Interventional Radiology Explained

Have you ever found yourself confused by the complex terminology in interventional radiology? Is "dose" referring to absorbed dose, effective dose, or peak skin dose? When patients or their families ask about radiation risks, how can you provide clear and accurate answers? This article serves as a quick-reference guide to core concepts in interventional radiology safety, helping you communicate more effectively and ensure patient safety—ultimately achieving a win-win for both medical professionals and patients.

For interventional radiology specialists, understanding these key terms is essential not only for optimizing dose but also for communicating clearly with patients and colleagues to safeguard health.

Kerma, short for "Kinetic Energy Released in Matter," refers to the energy released by an X-ray beam in a small volume of a specific material (such as air or soft tissue). Simply put, it measures the energy produced when X-rays interact with matter. In tissue, kerma is numerically equivalent to absorbed dose. The unit of measurement is the gray (Gy), where 1 Gy equals 1 joule of energy absorbed per kilogram of material.

Note that in air, kerma is slightly higher than absorbed dose because some of the released energy escapes the test volume as electron kinetic energy, reducing its contribution to local dose.

Air kerma is the kerma measured in a small volume of air, typically reported in milligrays (mGy). Interventional devices often report air kerma along with the kerma-area product (KAP). Air kerma describes the intensity of the X-ray beam, replacing the older unit, the roentgen (R). The amount of radiation incident on a patient's skin is now expressed as an air kerma value, measured without the patient present to eliminate backscatter effects. The integrated KAP/air kerma meter reports values at a specified spatial point, usually called the interventional reference point (IRP). Note that the air kerma values reported by machines do not account for beam repositioning or table attenuation, often overestimating the entrance air kerma (EAK) at the patient's skin.

Absorbed dose is the energy absorbed per unit mass of material at a specific point due to ionizing radiation. It is a critical parameter for assessing biological effects and is also measured in grays (Gy). However, absorbed dose does not account for radiation type or the radiosensitivity of exposed tissues. Additionally, it is a point measurement—think of it like temperature, reflecting localized energy. In interventional procedures, absorbed dose values vary across tissues due to factors like beam repositioning, distance from the X-ray source, and changes in patient thickness affecting tube voltage and current.

Different types of radiation (e.g., X-rays, protons, neutrons, alpha particles) cause varying degrees of biological damage per unit of absorbed dose. To address this, a weighting factor based on radiation type is applied. By definition, X-rays have a weighting factor of 1. Since X-rays cause biological damage by releasing high-energy electrons in tissue, electrons also have a radiation weighting factor of 1. The unit of measurement is the sievert (Sv), the same as for effective dose.

Tissues vary in their sensitivity to radiation. For example, breast, bone marrow, and colon are more sensitive than bone surfaces, brain, and skin. To account for this, tissue weighting factors were established. Mathematically, effective dose (ED) is the sum of the equivalent doses to irradiated tissues multiplied by their respective tissue weighting factors. The unit is Sv. For X-rays, a uniform whole-body absorbed dose of 1 Gy would result in an ED of 1 Sv by definition.

Think of ED as a "currency" in sieverts, allowing comparison of the relative stochastic risks of various ionizing radiation procedures. Importantly, tissue weighting factors are based on population averages for age and sex, introducing significant variability in individual risk. Other individual risk factors remain incompletely understood, so ED should not be used retrospectively to determine individual risk.

Entrance skin dose (ESD) is the dose absorbed by the skin. This value is often difficult to report accurately, but it can be estimated if the EAK is known. For greater accuracy, the EAK should be multiplied by a factor accounting for subtle differences in energy absorption between air and soft tissue due to compositional differences. For the beam energies used in interventional machines, this factor is approximately 1.07. A more significant issue is the substantial backscatter generated within the patient, increasing skin dose by a factor of 1.3 to 1.4. In practice, backscatter factors are often omitted from device-reported reference values.

Peak skin dose (PSD) is the highest ESD in the most heavily irradiated local area of the skin. Typically, this is the skin region exposed to the primary beam for the longest duration during a procedure. PSD is challenging to measure because placing film or thermoluminescent detectors directly on the patient is impractical, and angiography devices capable of estimating PSD are not yet widely available.

Fluoroscopy time is the total duration of fluoroscopy use during a procedure. It is the least useful metric for estimating dose or risk because it does not account for fluoroscopy frame rate, collimation, geometry, beam intensity, or fluorographic imaging (e.g., "spot" images and digital subtraction angiography).

KAP, also called dose-area product (DAP), is the product of beam intensity (air kerma) and beam area. It is an appropriate method for measuring the total radiation delivered to the patient. KAP is the most relevant metric for assessing stochastic risk but does not indicate the likelihood of skin reactions. Recent publications may abbreviate KAP as P _KA .

KAP is measured using a KAP meter positioned close to the radiation source. The meter is slightly larger than the beam to capture its entirety. KAP is measured in gray-centimeters squared (Gy·cm²), though it may be reported in variants like µGy·m². Conversion tables for common units are available. Operators should check their equipment to see how KAP is reported and familiarize themselves with converting it to Gy·cm², as this unit is commonly used in literature.

Notably, KAP does not vary along the X-ray beam path because air kerma decreases with the inverse square law while beam area increases proportionally with distance from the source. Thus, the KAP at the beam's origin equals the KAP just before entering the patient.

The air kerma measured at a fixed point in space is called the interventional reference point (IRP). K _a,r is only a rough approximation of skin dose—it does not equal skin dose. The IRP may correspond to skin level, a point inside the patient, or a point outside the patient. Additionally, K _a,r does not account for beam repositioning, backscatter, or table attenuation. K _a,r is also referred to as cumulative dose and reference point air kerma.

For isocentric fluoroscopy systems, the IRP is a point along the central X-ray beam, 15 cm from the isocenter toward the X-ray tube. This is where K _a,r is reported. Due to variability in patient size, operator height, and C-arm angles, the IRP does not always align precisely with the skin surface. Importantly, no meter is placed at the IRP. Instead, air kerma is measured near the source at the beam's center, and the IRP value is calculated using the inverse square law and displayed.

Deterministic effects are harmful outcomes of radiation that only occur above a certain threshold. Once exceeded, the severity of damage increases with dose. Sunburn is a fitting analogy, with skin injury and hair loss being classic examples. Higher doses lead to more severe skin damage.

Stochastic effects become more likely with higher doses, but their severity does not increase. Cancer and genetic effects are inherently stochastic. In other words, the chance of cancer rises with dose, but the cancer's severity does not. This assumes that even very low doses carry some risk—a premise encapsulated by the controversial "linear no-threshold model," which competing theories challenge.

The distance from the radiation source to the patient's skin, SSD partly depends on operator height, which may affect table height. Due to the inverse square law, small changes in this distance significantly impact patient dose. Slightly raising the table height can markedly reduce patient dose.

SID is the distance from the radiation source to the image receptor (e.g., flat-panel detector). Generally, bringing the receptor closer to the patient (reducing the "air gap" and SID) lowers patient dose.

Scatter radiation generated in the patient is the main source of staff exposure. A rule of thumb is that scatter exposure at 1 meter from the beam entrance point is about 0.1% of the entrance exposure.

Radiation shielding materials are designed to attenuate most incident radiation. Their effectiveness is expressed in lead-equivalent thickness—the thickness of lead that would provide equivalent attenuation. Standard shielding is 0.5 mm lead-equivalent, though lighter materials with similar attenuation exist.

The threshold dose is the smallest dose at which a specific deterministic injury may occur. Due to biological variation, this threshold differs among individuals and tissue types. Notable thresholds include 2 Gy (2,000 mGy) for transient skin erythema and 5 Gy (5,000 mGy) as the suggested K _a,r threshold for patient follow-up.