Room: Track 3
Purpose: To evaluate the accuracy of a novel general cavity theory that is formulated to explicitly account for the effects of the cavity on the charged particle fluence in the surrounding medium. Of particular interest is the accuracy over the range of incident beam energies and detector configurations where traditional cavity theory formalisms are known to be unreliable.
Methods: The EGSnrc Monte Carlo (MC) code was used to calculate of the dose to the cavity of an idealized plane-parallel ion chamber (1-cm radius) free-in-air exposed to incident monoenergetic photon beams. Calculations were performed for various combinations of incident photon beam energies, chamber wall materials (4 = Z = 29), cavity sizes (0.1 – 10 mm cavity heights), and threshold energies for creating and tracking charged particles (1 – 10 keV) which, together, form a set of benchmark calculations that serve as the basis for comparison. Using a consistent set of interaction cross-sections, stopping powers, and transport parameters the EGSnrc code was also used to calculate charged particle spectra in homogeneous media and chamber-dependent charged particle energy deposition functions as input into the general cavity theory formalism. The integrals associated with this formalism were computed using a 256-point Gauss-Legendre quadrature.
Results: Cavity doses predicted by the general cavity theory formalism agree well with full MC simulations. For a 1.25 MeV beam incident on a chamber with copper walls, the largest difference from full MC simulations was 0.36% for a cavity height of 0.1 mm. All other general cavity theory predictions were within 0.2% or less of full MC dose calculations. Formalism calculations required a fraction of the computing time of full cavity dose calculations.
Conclusion: A general cavity theory was formulated to accurately predict ion chamber cavity doses over a broad range of physical conditions where traditional formalisms are considered inapplicable.