SU‐E‐T‐552: Monte Carlo Calculation of Correction Factors for a Free‐Air Ionization Chamber in Support of a National Air‐Kerma Standard for Electronic Brachytherapy

Abstract

Purpose:To use Monte Carlo radiation transport methods to calculate correction factors for a free‐air ionization chamber in support of a national air‐kerma standard for low‐energy, miniature x‐ray sources used for electronic brachytherapy (eBx).Methods:The NIST is establishing a calibration service for well‐type ionization chambers used to characterize the strength of eBx sources prior to clinical use. The calibration approach involves establishing the well‐chamber's response to an eBx source whose air‐kerma rate at a 50 cm distance is determined through a primary measurement performed using the Lamperti free‐air ionization chamber. However, the free‐air chamber measurements of charge or current can only be related to the reference air‐kerma standard after applying several corrections, some of which are best determined via Monte Carlo simulation. To this end, a detailed geometric model of the Lamperti chamber was developed in the EGSnrc code based on the engineering drawings of the instrument. The egs_fac user code in EGSnrc was then used to calculate energy‐dependent correction factors which account for missing or undesired ionization arising from effects such as: (1) attenuation and scatter of the x‐rays in air; (2) primary electrons escaping the charge collection region; (3) lack of charged particle equilibrium; (4) atomic fluorescence and bremsstrahlung radiation.Results:Energy‐dependent correction factors were calculated assuming a monoenergetic point source with the photon energy ranging from 2 keV to 60 keV in 2 keV increments. Sufficient photon histories were simulated so that the Monte Carlo statistical uncertainty of the correction factors was less than 0.01%. The correction factors for a specific eBx source will be determined by integrating these tabulated results over its measured x‐ray spectrum.Conclusion:The correction factors calculated in this work are important for establishing a national standard for eBx which will help ensure that dose is accurately and consistently delivered to patients.