The recommended depth for photon beam calibration is 10 cm.
The calibration depth must be much beyond the depth of dose maximum, ie. the transient equilibrium region, where the electron contamination does not reach.
A depth of 10 cm will satisfy this criterion for the whole range of clinically used photon beam qualities. Earlier recommendations gave depth of 5 cm, 7 cm and 10 cm for different beam qualities, but recent protocols recommend a single depth of 10 cm for all beam qualities.
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