For many years cavity ionization chambers have been the preferred detectors for measuring absorbed dose from ionizing radiation. Cavity theory formalisms relate the detector signal to a dose in the surrounding medium. Common cavity theories are straightforward to calculate but make limiting assumptions about charged particle fluences in the chamber and the medium. The choices that make them accurate are conceptually ambiguous and unrelated to physics fifirst principles. This prohibits their general application across a wide range of physical conditions. A novel general cavity theory is introduced which addresses some of the previous limiting assumptions of existing formalisms by explicitly determining the perturbation of charged particles in the medium due to the chamber cavity. This cavity theory removes the ambiguity and derives from fifirst principles at the expense of increased complexity. The formalism converges to the Spencer-Attix cavity theory in the limit of Bragg-Gray conditions and to the ratio of mass-energy absorption coeffcients in the large cavity limit. The EGSnrc Monte Carlo simulation software is used to determine the expected dose ratios from full chamber dose calculations and to generate the input quantities to the novel formalism with regard to the air kerma formalism under Fano conditions. For 1.25 MeV incident photons the formalism is within 0.1% of full chamber calculated dose ratios for materials with atomic numbers 4 < Z < 29, between 0.2-0.7% at 300 keV, and between 0.8-5.3% at 50 keV. Formalism dose ratios calculated from cobalt-60, and 120 kVp x-ray, spectra showed similar agreement with full chamber calculated dose ratios as the mono-energetic cases. This new cavity theory is shown to be fifive times more accurate, on average, than Spencer-Attix for cavity heights of 1.39 mm, and 2.3 times more accurate, on average, for cavity heights of 0.15 mm in 24 comparisons. Over all materials and incident energies investigated the new cavity theory tracks the trend of dose ratios as cavity size changes from 0.1 mm to 10 mm in height. Restrictions imposed by previous cavity theories are removed by this novel formalism derivation and it shows promise as a confirmation of modern Monte Carlo dose calculation methods.