When measuring entrance surface air kerma Ke, the radiation incident on the patient or phantom
and the backscattered radiation are included in the measurement.
However, the effect of backscatter radiation is hardly noticeable with regard to normal use as the XR detector is shielded well enough for a wide variety of measurements, measurements for entrance dose require the application of backscatter correction to include the backscatter radiation. The entrance surface air kerma can be calculated as:
Ke = Ki·B,
where Ki is the incident air kerma, B is the backscatter factor depends on the field size, radiation
quality and backscatter material. Below is a table for backscatter factor B for water, ICRU tissue and PMMA for 21 diagnostic X ray beam qualities and for three field size at a focus to skin distance of 1000mm [1].
Reference:
[1] PETOUSSI -HENNS, N., ZANKL, M., DREXLER, G., PANZER, W.,
REGULLA, D., Calculation of backscatter factors for diagnostic radiology using
Monte Carlo methods, Phys. Med. Biol. 43 (1998) 2237–2250.