Abstract：
PURPOSE:To quantify detector perturbation effects in megavoltage small photon fields and support the theoretical explanation on the nature of quality correction factors in these conditions. METHODS:In this second paper, a modern approach to radiation dosimetry is defined for any detector and applied to small photon fields. Fano's theorem is adapted in the form of a cavity theory and applied in the context of nonstandard beams to express four main effects in the form of perturbation factors. The pencil-beam decomposition method is detailed and adapted to the calculation of perturbation factors and quality correction factors. The approach defines a perturbation function which, for a given field size or beam modulation, entirely determines these dosimetric factors. Monte Carlo calculations are performed in different cavity sizes for different detection materials, electron densities, and extracameral components. RESULTS:Perturbation effects are detailed with calculated perturbation functions, showing the relative magnitude of the effects as well as the geometrical extent to which collimating or modulating the beam impacts the dosimetric factors. The existence of a perturbation zone around the detector cavity is demonstrated and the approach is discussed and linked to previous approaches in the literature to determine critical field sizes. CONCLUSIONS:Monte Carlo simulations are valuable to describe pencil beam perturbation effects and detail the nature of dosimetric factors in megavoltage small photon fields. In practice, it is shown that dosimetric factors could be avoided if the field size remains larger than the detector perturbation zone. However, given a detector and beam quality, a full account for the detector geometry is necessary to determine critical field sizes.
journal_name
Med Physjournal_title
Medical physicsauthors
Bouchard H,Kamio Y,Palmans H,Seuntjens J,Duane Sdoi
10.1118/1.4930798subject
Has Abstractpub_date
2015-10-01 00:00:00pages
6048-61issue
10eissn
0094-2405issn
2473-4209journal_volume
42pub_type
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