Dose area product

Last revised by Sarah Catchpoole on 15 Apr 2023

The dose area product (DAP) or kerma area product (KAP) is a method of radiation dose monitoring used in radiographic and fluoroscopic studies. It provides one indication of the radiation dose received by a patient and is the measurement used in dose audits (such as comparing Diagnostic reference levels).

It is calculated as the product of dose and beam area (Gy.cm2), and is measured using an ionisation chamber placed between the x-ray tube/collimator set up and the patient (which in theory is independent of its position in the beam). The dose area product can be divided by the area of exposure (in cm2) to give the incident accumulated exposure (air kerma) at that site. The dose area product is linked to a patients stochastic risk.

The cumulative air kerma (CAK) can be used as an indication of the dose received by the skin. It can be derived from the DAP and is the dose measured at the interventional reference point (IRP); 15cm from the isocentre towards the x-ray source, assumed to be the patient skin. It is therefore also known as reference air kerma (RAK), interventional reference point air kerma, or sometimes simply reference kerma.

The peak skin dose (PSD) aims to estimate the highest dose to a single area of the skin (Gy). It gives a better estimation of deterministic risk (i.e. tissue reactions) than CAK, but the calculation is more complex; using the DAP or CAK as a starting point and taking into account further factors such as backscatter and overlapping projections.

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