Monte Carlo modeling of diode detectors for small field MV photon dosimetry

Full text: Objective To model a comprehensive set of diode detectors for a recently proposed Monte-Carlo-based small field dosimetry formalism and to establish the level of model detail required to produce accurate small field output factors. The group's fully-validated BEAMnrc model of a Varian 6 MV linac was extended to jaw-collimated small fields down to 0.5 cm. The lBA stereotactic field diode (SFD), as well as PTW T60008, T60012, T60016 and T60017 diodes, were modelled using DOSRZnrc. Isocentric output factors (OFs) were simulated at depths of 1.5, 5.0 and 10.0 cm. All simulations were validated experimentally. Detector specific replacement correction factors were calculated following the small field dosimetry formalism proposed by Alfonso et al. (Med. Phys. 2008;35:5179-86). The detector models were then simplified to quantify the influence of the internal components as percent differences in OF compared to the complete model. The corrections required for 0.5 cm field size were 0.95 (±1.5%) for the unshielded diodes (SFD, T60012 and T60017) and 0.90 (±1.5%) for the shielded diodes (T60008 and T60016), i.e. measured OFs are 5% and 10% high compared to OFs in water. The SFD diode can be approximated as a chip in water, the unshielded diodes as a chip in water including the thin filter plate located upstream and the shielded diodes approximated as a chip in water plus the entire shielding geometry. Correction factors required for measured small field OFs are diode detector design dependent and need to be modelled as more than just a chip in water.