Air-kerma, entrance surface

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 ($B$), thus:

$K_e= B⋅K_i$

The unit for entrance surface air kerma is the Gy. Entrance surface air kerma <mathm>K_e</math> can be calculated from incident air kerma $K_i$ using suitable backscatter factor $B$, 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