Answer to Question #15568 Submitted to "Ask the Experts"
Category: Medical and Dental Equipment and Shielding — Equipment
The following question was answered by an expert in the appropriate field:
I am a radiographer and I am learning about radiation protection. In angiography, the two radiation dose values are shown on the machine. The units of these values are mGy and µGym². I would like to know what these radiation doses are and how can absorbed dose, skin entrance dose, and effective dose for patient be calculated from these values?
The units of mGy and uGym2 are radiation dose units generated by the computer attached to the x-ray unit. Input includes radiation output of the unit, as measured by an air ionization chamber on the unit and the area of the skin irradiated. These units can be used to advise the radiographer of the risk of deterministic effects (e.g., skin erythema) and to calculate effective doses.
Most modern fluoroscopy x-ray units are equipped with an air ionization chamber that measures the radiation output of the machine when it is activated. The signal from this chamber is converted by a computer attached to the unit, using a proprietary algorithm, into dose units (i.e., mGy). A Gy is equal to 100 rads or 1 J of energy deposited in 1 kg of matter (e.g., tissue). The dose reported by the computer is defined at a specific reference point 15 cm from the isocenter of the of x-ray unit towards the tube. It is assumed that the dose reported at the end of the procedure is the skin entrance dose at a single point. However, this reference point moves when the gantry is moved, angulated, or rotated. This dose is often used as a surrogate for the peak skin dose (PSD), which would be the preferred method of reporting skin dose for the purpose of determining the risk of skin damage (e.g., erythema) if it could be measured or calculated definitively.
Gy-cm2 is known as the dose area product (DAP) or, more recently, the kerma area product (KAP). It is a measure of the skin dose received by the patient, with the total irradiated skin area considered. It is independent of the distance from the x-ray tube to the skin surface. It is calculated by multiplying the dose calculated above by the area of skin irradiated at a defined distance from the tube.
These two dose values can be used to track the dose received by the patient during the procedure and to evaluate the deterministic risk of skin damage from excessive doses. However, these doses are only estimates and are directly affected by patient size, gantry angle, and patient location relative to the x-ray tube.
Calculation of effective dose can be accomplished by multiplying the KAP by an appropriate conversion coefficient. The National Cancer Institute has published a comprehensive organ dose calculator. As discussed above, these calculations should be used with caution because the underlying calculations of the skin dose and KAP are affected by several variables.
Calculation of the effective dose is less important than the KAP or PSD because few whole organs are irradiated, and significant skin damage has occurred when fluoroscopy is used extensively. For more information you may wish to visit websites from manufacturer Raysafe and the International Atomic Energy Agency.
Thomas Morgan, III, CHP