Despite the progressive development of radiology and the associated reduction of radiation dose involved in any radiographic examination today, the main artificial source of radiation for people comes from medical applications, mainly x-ray diagnostics. Therefore, it’s imperative to minimize the exposure risk during medical applications. As such, it’s necessary that:
• Every exposure of patients as part of their diagnosis or treatment has to be justified
• Patient exposures have to be optimized by keeping doses to a minimum without loss of diagnostic information
• ALARA (As Low As Reasonably Achievable) principle
In Publication 60 of the International Commission of Radiological Protection, the diagnostic reference levels (DRL) were introduced in order to establish a reference of values of measured quantities in diagnostic radiology at a local or national level. Being a reasonable indication for average sized patients, they provide guidance on what is achievable with current good practice. Quantities used tor the DRLs should be easily measurable. Report 74 of the International Commission of Radiation Unites and Measurements recommends, among other measurable quantities, incident art kerma (Ka,i) and the kerma area product (Pka or DAP) for the establishment of diagnostic reference levels.
The air kerma is the sum of the kinetic energies of the primary charged particles produced by the radiation per unit mass of air. The unit is joule per kilogram (J kg-1) or Gray (Gy). Incident (backscattering not taken into account) air kerma (K ai) is easily measured with an ionization chamber. It depends on the distance to focus and therefore the reference point as position of the measured is defined.
Kerma Area Product (Pka or DAP)
The product of the area of the useful beam and the air kerma over the cross section of the useful beam, both quantities being measured at the same distance from the focal spot. The unit is Gy m2 (µGy m2 = cGy cm2). The symbol recommended by ICRU Report 74 for the air kerma area product is Pka. The historical designation of this quantity “dose area product” (DAP), is still widely common. DAP is conveniently measured using a transmission ionization chamber. The independence on the distance to focus facilitates the comparison of measured DAP values in dose survey studies.
Effective Dose is a measure of the probability of stochastic effects (mainly cancer induction) due to radiation exposure. It can be calculated from the measured value of DAP using conversion coefficients that are specific for the applied procedure, the age and size of the patient. The unit is J kg-1 or Sv (Sievert).
Peak Skin Dose (PSD)
Peak Skin Dose is the maximum dose received by a portion of the exposed skin. This quantity is especially important in prolonged or higher dose-rate radiological examinations, such as fluoroscopy or internationally guided procedures, as it is related to skin injury. The PSD can be calculated from the incident air kerma at the reference point.
Several definitions of the reference point exist, all designated to represent the skin surface of the patient. One should be aware, that in some cases of real geometry, it is possible that the reference point is located at a different position related to the patient’s skin.