Changes

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:

The unit for entrance surface air kerma is the Gy. Entrance surface air kerma Ke <mathm>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)