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Dose area product

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Dose area product

Dose area product (DAP) is a quantity used in assessing the radiation risk from diagnostic x-ray examinations and interventional procedures. It is defined as the absorbed dose multiplied by the area irradiated, expressed in gray square centimetres (Gy*cm²)[1] (sometimes mGy*cm² or cGy*cm²). Manufacturers of DAP meters usually calibrate them in terms of absorbed dose to air. DAP reflects not only the dose within the radiation field but also the area of tissue irradiated. Therefore, it may be a better indicator of the overall risk of inducing cancer than the dose within the field. It also has the advantages of being easily measured, with the permanent installation of a DAP meter on the x-ray set. Due to the divergence of a beam emitted from a ‘point source’, the area irradiated (A) increases with the square of distance from the source (A ∝ d²), while radiation intensity (I) decreases according to the inverse square of distance (I ∝ 1/d²). Consequently, the product of intensity and area, and therefore DAP, is independent of distance from the source.

How is DAP measured

An ionization chamber is placed beyond the X-ray collimators and must intercept the entire X-ray field for an accurate reading. Different X-ray technique parameters, such as kVp, mA, or time, or the area of the field can also be changed.
For example, a 5x5 cm X-ray field with an entrance dose of 1 mGy will yield a 25 mGycm^2 DAP value. When the field is increased to 10x10 cm with same entrance dose, then DAP increases to 100 mGycm^2, which is 4 time the previous value.[2]

Kerma Area Product

Kerma area product (KAP)[1] is a related quantity which for all practical radiation protection purposes is equal to dose area product. However, strictly speaking DAP = KAP x (1-g) where g is the fraction of energy of liberated charged particles that is lost in radiative processes in the material,[3] and the dose is expressed in absorbed dose to air. The value of g for diagnostic x-rays is only a fraction of a percent.

Adult Coronary angiography and PCI produces expose patient to an average DAP in the range of 20 to 106 Gy*cm² and 44 to 143 Gy*cm² respectively.[4]

References

  1. ^ a b Kim S, Toncheva G, Anderson-Evans C, Huh BK, Gray L, Yoshizumi T (June 2009). "Kerma area product method for effective dose estimation during lumbar epidural steroid injection procedures: phantom study". AJR Am J Roentgenol 192 (6): 1726–30.  
  2. ^ http://www.orau.org/ptp/PTP%20Library/library/CRCPD/10-01_QA_DAP.pdf
  3. ^ ICRU Report 50, Fundamental Quantities and Units for Ionizing Radiation, International Commission on Radiation Units and Measurements 1998 In Seltzer, Stephen M. Journal of the ICRU (Revised ed.) 11 (1). doi:10.1093/jicru/ndr012.
  4. ^ priory.com RADIATION EXPOSURE IN THE CATH LAB – SAFETY AND PRECAUTIONS Dr S M S Raza, MB BS, MD. First Published October 2006
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