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

• Published in: Physics in medicine & biology Vol. 63; no. 12; p. 125003
• Main Authors: Fenwick, John D, Georgiou, Georgios, Rowbottom, Carl G, Underwood, Tracy S A, Kumar, Sudhir, Nahum, Alan E
• Format: Journal Article
• Language: English
• Published: England IOP Publishing 08.06.2018
• Subjects: atomic number; density; detector response; Monte Carlo; small field
• ISSN: 0031-9155; 1361-6560; 1361-6560
• DOI: 10.1088/1361-6560/aac478

Differences in detector response between measured small fields, fclin, and wider reference fields, fmsr, can be overcome by using correction factors 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 correction factor from computed doses absorbed by water and by the detector sensitive volume in 0.5  ×  0.5 and 4  ×  4 cm2 fields. In addition to the 'real' detector, fully modelled according to the manufacturer's blue-prints, we calculated doses and 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−3, leaving mass stopping-powers and interaction coefficients at real levels. equalled 0.910  ±  0.005 (2 standard deviations) for the real detector, was insignificantly different at 0.912  ±  0.005 for the 'Z  →  H2O' variant, but equalled 1.012  ±  0.006 for the 'density  →  1' variant. For the 60017 diode in a 6 MV beam, then, 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 .