Two conditions for Bragg Gray

The Bragg–Gray cavity theory was the first cavity theory developed to
provide a relation between the absorbed dose in a dosimeter and the absorbed
dose in the medium containing the dosimeter.
The conditions for application of the Bragg–Gray cavity theory are:
(a) The cavity must be small when compared with the range of charged
particles incident on it, so that its presence does not perturb the fluence
charged particles in the medium;
(b) The absorbed dose in the cavity is deposited solely by charged particles
crossing it (i.e. photon interactions in the cavity are assumed negligible
and thus ignored).
The result of condition (a) is that the electron fluences in Eq. (2.22)
the same and equal to the equilibrium fluence established in the surrounding
medium. This condition can only be valid in regions of CPE or TCPE.
addition, the presence of a cavity always causes some degree of fluence perturbation
that requires the introduction of a fluence perturbation correction
factor.
Condition (b) implies that all electrons depositing the dose inside
cavity are produced outside the cavity and completely cross the cavity.
secondary electrons are therefore produced inside the cavity and no electrons
stop within the cavity.

Courtesy Pgorsak