Why DAP?
Dose-area product is relatively easy to measure. DAP meters have been around for many years, and were actually used in the 1964 and 1970 U.S. X-ray Exposure Studies. Advocates of DAP meters contend that the DAP is a better indicator of risk than entrance dose alone, since DAP incorporates the entrance dose and field size. 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.

Are there problems with DAP?
There are several problems with the use of the DAP value. The configuration of the DAP meter may introduce a bias to the DAP value. For example, if any material is placed between the meter and patient, the patient will receive less than what is implied by the displayed DAP value. For an undertable fluoroscopy system this can be the tabletop and pad. Consequently, the use of DAP to estimate skin entrance exposure or skin dose is complex and should only be attempted by a qualified medical physicist. This is particularly true for fluoroscopic procedures where multiple beam directions, source-skin distances, and field sizes may be used. DAP meters are difficult to calibrate and maintain. Large changes in the DAP meter response can occur over time, particularly if meters are adjusted for couch transmission factors. Calibration should be done in the field after any changes that might alter the DAP and at least annually.

Up to a decade ago, radiological patient safety concerns were focused on stochastic risk. Monitoring and managing stochastic risk requires estimates of the effective dose delivered to the patient. There is no need for real-time feedback. DAP is defined as the integral of dose across the X-ray beam. Therefore DAP includes field non-uniformity effects such as anode-heel-effect, and the use of semi-transparent beam-equalizing shutters (lung shutter). DAP is easy to measure. The simplest method is to place a transmission full-field ionization chamber in the beam between the final collimators and the patient. DAP may also be obtained by calculation. Data is accumulated during fluoroscopy, fluorography, and radiography. Assuming that the incident beam is totally confined to the patient, the recorded value essentially provides an upper limit on the X-ray energy absorbed by the patient (i.e. there is no transmission or scatter). DAP’s ability to estimate stochastic risk is degraded because of the lack of dose distribution information within the patient. The best that one can do is to assume an average weighting factor for all the tissues at risk. This may lead to an over or under estimate of risk in certain cases. As an example, it does not account for the differential risk of breast cancer from an AP or a PA projection. DAP rate and cumulative DAP can easily be displayed in real-time. The primary utility of DAP rate is in a teaching situation. Scattered dose rate at any place in the lab is more or less proportional to DAP rate. The trainee can be shown that reducing DAP rate reduces his or her personal exposure rate. The effect of different control options (e.g., collimation, zoom mode) on DAP rate can be demonstrated. Cumulative DAP does not provide a direct indication of the possibility of skin injury. The same DAP is observed with large fields and low skin doses as with small fields and high skin doses. Exceeding skin tolerance is more likely in the latter case. However, reasonable entrance field size estimates can be made for many procedures. These estimates are dependent on factors such as equipment configuration, patient size, and operator technique. Once known, the nominal field size can be used to obtain an estimate of skin dose. Rules-of-thumb can be established to make this conversion for typical procedures. DAP provides no information regarding the spatial distribution of the entrance beam around the patient’s skin. It produces an overestimate of the possibility of exceeding the deterministic threshold when there is significant beam movement during the procedure.