Dose area product (DAP)

Dose Area Product (DAP), is a multiplication of the dose and the area exposed, often expressed in Gy.cm2. 

Modern x-ray systems are fitted with a DAP meter, able to record accumulated DAP during an examination. A schematic of the imaging chain with DAP meter is given below.

How is DAP measured?
An ionization chamber larger than the area of the x-ray beam is placed just beyond the xray collimators. The DAP ionization chamber must intercept the entire x-ray field for an accurate reading. The DAP-value is depending on x-ray technique factors (kVp, mA, filtration) and time of the procedure and the area of the x-ray field.
For example, a 5 x 5 cm x-ray field with an entrance dose of 1 mGy will yield a 25 mGy.cm2 DAP value. If the field is increased to 10 x 10 cm, with the same entrance dose of 1 mGy the DAP increases to 100 mGy-cm2, which is 4 times the DAP for the 5 x 5 cm field.

Why DAP?
Dose-area product is relatively easy to measure. DAP has been shown to correlate well with the total energy imparted to the patient, which is related to the effective dose and therefore to overall cancer risk.
Estimated effective doses from DAP readings, are rough approximations. This is due to the fact that other factors are required for a more accurate estimate of effective dose or skin dose, such as the beam geometry with respect to the patient, kVp, and so on.
Despite these limitations, DAP is currently the most convenient method of monitoring patient doses.