[en] A cavity theory is used to relate the dose deposited in the cavity sensitive volume of the detector) to that in the surrounding medium which may be of different atomic number or composition. Burlin proposed a general cavity theory to include all cavity sizes. The Burlin theory ignores all secondary-electron scattering effects which results in large discrepancies in dose to the cavity compared with the experimental results in high atomic number media. Kearsley proposed a new general cavity theory which includes secondary-electron scattering at the cavity boundary. The Kearsley theory showed excellent agreement with experimental results for ⁶⁰Co γ-rays ut poor correlation for 10 MV x-rays. The Kearsley theory has numerous parameters and the magnitude of the input parameters is arbitrary; therefore the dose to the cavity depends on the choice of parameters. We have developed new cavity theory which includes secondary-electron backscattering from the medium into the cavity. The strengths of this proposed theory are that it contains few parameters and a methodical way of determining the magnitude of the parameters experimentally. The proposed theory gives better agreement with experimental results in lithium fluoride thermoluminescence dosimeters or ⁶⁰Co γ-rays and 10 MV x-rays in aluminium, copper and lead than do the Burlin and Kearsley cavity theories. (author)