Answer to Question #14371 Submitted to "Ask the Experts"

With regard to intraoperative fluoroscopy, as an orthopedic surgeon, I have read many articles. The air kerma rate (AKR), dose area product (DAP), and dose length product (DLP) are used. Can you help me with a minimal math high level concept distinction regarding the difference between these three? What units of measure are used for them? I understand the difference between gray (Gy), sievert (Sv), rad and rem as absorbed and effective doses. Thank you for your time.

These are really great questions and I'll address each question separately.

Air kerma rate (AKR) and dose area product (DAP) are somewhat related. The term "kerma" actually refers to "kinetic energy transferred per unit mass." It's a term that only applies to indirectly ionizing radiations that don't have a positive or negative charge (e.g., x rays, gamma rays, or neutrons). These radiations initially interact with orbital electrons in a material and transfer their kinetic energy to those electrons. Those electrons go on to ionize other atoms in the material which results in a certain amount of that energy that is absorbed by the material (i.e., the absorbed dose). Thus, the numerical value for the energy transferred (i.e., the kerma) does not always equal the energy absorbed (i.e., the absorbed dose) which is what we"re normally interested in as it determines the potential for a biological effect. Regardless, kerma and absorbed doses are expressed in the same physical units—gray (Gy) or milligray (mGy) where 1 Gy equals 1,000 mGy. Kerma and absorbed doses vary from one material to another. Thus, in fluoroscopy, when we talk about "air kerma" we're referring to the kinetic energy transferred to the electrons in a defined volume of air and the absorbed dose in that volume of air. Adding the word "rate" simply means that we're transferring that energy over a certain time period. The AKR is generally displayed in the unit of milligray per minute (mGy min^-1).

So, how does this relate to the DAP and the dose to the patient? Dose to the skin surface is usually the primary interest in fluoroscopy as it is possible to damage skin with high fluoroscopy doses (usually about 2.5 to 3.0 Gy to cause skin erythema). The AKR displayed by a fluoroscopy unit is typically defined at a point in space, the interventional reference point (IRP) which can be measured. Fluoroscopes that meet current Food and Drug Administration (FDA) standards also provide the "cumulative air kerma" (usually in mGy) which is basically the AKR multiplied by the time of exposure during the fluoroscopy procedure. That cumulative value would also include any higher-level exposures such as those from digital subtraction angiography—something you probably wouldn't use for orthopedic procedures. Many times the IRP is at or near the surface of the skin where the x-ray beam enters the patient. If that is the case, the IRP provides a rough approximation of the dose rate and cumulative dose to the surface of the skin. There are a lot of other factors that affect what the peak skin dose is such as backscatter, the actual distance from the x-ray source to the skin, movement of the x-ray tube, et.al., which is why I used the term "rough approximation" but at least the AKR and cumulative dose can get you in the ballpark with respect to the skin dose.

The term DAP has largely been replaced by the term "kerma area product (KAP)," but the two terms are basically the same. Essentially, the KAP is the cumulative air kerma multiplied by the area being irradiated (i.e., the area of the fluoroscopy field on the surface of the patient). KAP is basically the total amount of radiation energy deposited to that area of the patient and has the unit of gray-square-centimeter (Gy-cm²) or mGy-cm². KAP can be used to determine various organ doses in the x-ray field and ultimately a calculation of the "effective dose (E)"; however, it can be misleading when applied to the skin dose, which again, is usually the dose of interest in fluoroscopy procedures. Here's why. Using a skin dose of 100 mGy and a field size of 10 cm x 10 cm, the KAP would be 100 mGy x (10 cm x 10 cm) or 10,000 mGy-cm². If one increases the field size to 15 cm x 15 cm, the KAP increases to 100 mGy x (15 cm x 15 cm) or 22,500 mGy-cm². Note that the actual dose to the skin surface is the same for both field sizes, but the KAP is considerably higher simply because the area of exposed skin is larger. If the field size stays constant throughout a fluoroscopic procedure, one could use it to estimate the skin dose; however, if the field size changes during the procedure, estimating the skin dose would be more difficult. As such, the AKR and the cumulative air kerma are typically the better indicators of skin dose rate and total skin dose from a fluoroscopic procedure.

DLP is a dosimetric quantity associated with computed tomography (CT) scanners. Many, if not most CT scanners now display a couple of dose parameters—the "computed tomography dose index" to the volume of tissue being irradiated (CTDI_vol) and the DLP. One can think of the CTDI_vol as the average dose to the area of the body being scanned. The DLP is simply the CTDI_vol usually expressed in mGy, multiplied by the length of the CT scan in cm. Thus, the DLP value has the unit of mGy-cm. DLP can be used as a comparative value between different CT scanners or scans and also used to calculate the "effective dose" associated with a given CT scan. Depending upon what portion of the body is being scanned, the American Association of Physicists in Medicine (AAPM) developed conversion factors that can be multiplied by the DLP to calculate the effective dose.

You mentioned "effective dose" in your question. Both KAP and DLP are sometimes used to calculate effective dose. Effective doses are essentially calculated values that equate the risk from a radiation dose to a portion of the body (i.e., from a CT scan or fluoroscopy procedure) to a uniform dose to the whole body. They can be used to compare effective doses from different types of x-ray procedures and also to effective doses from other radiation sources such as natural background radiation. When dealing with fluoroscopy, effective dose is usually of less interest than skin dose due to that fact that few organs are generally exposed due to the limited size of the fluoroscopic field and potential skin injury can occur with extensive fluoroscopy use as mentioned above.

I hope this answers your questions adequately regarding these various dosimetric terms and their application in the medical environment.

Mack L. Richard, MS, CHP