Spencer Attix review of how it differs

The Bragg–Gray cavity theory does not take into account the creation of
secondary (delta) electrons generated as a result of hard collisions in the
slowing down of the primary electrons in the sensitive volume of the dosimeter.

The Spencer–Attix cavity theory is a more general formulation that accounts
for the creation of these electrons that have sufficient energy to produce
further ionization on their own account. Some of these electrons released in the
gas cavity would have sufficient energy to escape from the cavity, carrying some
of their energy with them. This reduces the energy absorbed in the cavity and
requires modification of the stopping power of the gas. The Spencer–Attix
theory operates under the two Bragg–Gray conditions; however, these
conditions now even apply to the secondary particle fluence in addition to the
primary particle fluence.