Origins of the changing detector response in small megavoltage photon radiation fields



Fenwick, John D, Georgiou, Georgios, Rowbottom, Carl G ORCID: 0000-0002-5716-6840, Underwood, Tracy SA, Kumar, Sudhir and Nahum, Alan E
(2018) Origins of the changing detector response in small megavoltage photon radiation fields. PHYSICS IN MEDICINE AND BIOLOGY, 63 (12). 125003-.

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Abstract

Differences in detector response between measured small fields, f <sub>clin,</sub> and wider reference fields, f <sub>msr</sub> , can be overcome by using correction factors [Formula: see text] or by designing detectors with field-size invariant responses. The changing response in small fields is caused by perturbations of the electron fluence within the detector sensitive volume. For solid-state detectors, it has recently been suggested that these perturbations might be caused by the non-water-equivalent effective atomic numbers Z of detector materials, rather than by their non-water-like densities. Using the EGSnrc Monte Carlo code we have analyzed the response of a PTW 60017 diode detector in a 6 MV beam, calculating the [Formula: see text] correction factor from computed doses absorbed by water and by the detector sensitive volume in 0.5  ×  0.5 and 4  ×  4 cm<sup>2</sup> fields. In addition to the 'real' detector, fully modelled according to the manufacturer's blue-prints, we calculated doses and [Formula: see text] factors for a 'Z  →  water' detector variant in which mass stopping-powers and microscopic interaction coefficients were set to those of water while preserving real material densities, and for a 'density  →  1' variant in which densities were set to 1 g cm<sup>-3</sup>, leaving mass stopping-powers and interaction coefficients at real levels. [Formula: see text] equalled 0.910  ±  0.005 (2 standard deviations) for the real detector, was insignificantly different at 0.912  ±  0.005 for the 'Z  →  H<sub>2</sub>O' variant, but equalled 1.012  ±  0.006 for the 'density  →  1' variant. For the 60017 diode in a 6 MV beam, then, [Formula: see text] was determined primarily by the detector's density rather than its atomic composition. Further calculations showed this remained the case in a 15 MV beam. Interestingly, the sensitive volume electron fluence was perturbed more by detector atomic composition than by density; however, the density-dependent perturbation varied with field-size, whereas the Z-dependent perturbation was relatively constant, little affecting [Formula: see text].

Item Type: Article
Uncontrolled Keywords: small field, detector response, density, atomic number, Monte Carlo
Depositing User: Symplectic Admin
Date Deposited: 21 May 2018 06:51
Last Modified: 01 Feb 2024 20:59
DOI: 10.1088/1361-6560/aac478
Related URLs:
URI: https://livrepository.liverpool.ac.uk/id/eprint/3021505