# Air-kerma, entrance surface

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The air kerma on the central x-ray beam axis at the point where the x-ray beam enters the patient. The contribution of backscatter radiation is included through backscatter factor ([math]B[/math]), thus:

[math]K_e= B⋅K_i[/math]

The unit for entrance surface air kerma is the Gy. Entrance surface air kerma [math]K_e[/math] can be calculated from incident air kerma [math]K_i[/math] using suitable backscatter factor [math]B[/math], or determined directly using small dosimeters (thermoluminescent or semiconductor) positioned at the representative point on the skin of the patients. (Adapted from ICRP Publication 117, 2010)

*ICRP Glossary entry - May 2019*

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