Response corrections for solid-state detectors in megavoltage photon dosimetry

Abstract

Solid-state detectors offer high sensitivity, stability and resolution and are frequently the dosimeter of choice for on-line dosimetry and small field therapies such as stereotactic radiosurgery. The departure from tissue equivalence of many solid-state devices, including diodes and MOSFETs, has to be carefully considered at lower energies and for Compton scattered radiation where the strongly Z-dependent photoelectric effect is significant. A modification of Burlin cavity theory is proposed that treats primary and scatter photon spectra separately and this has been applied to determine the correction factors for diode detector measurements of 6 and 15 MV linear accelerator beams. Uncorrected, an unshielded diode overestimates the dose at depth by as much as 15% for the 6 MV beam. The model predicts the effect to within 1% for both energies offering a basis for the correction of diodes for use in routine dosimetry.

The Radiological Physics Center's standard dataset for small field size output factors

“…The challenges of penumbra size versus detector size 6 , 7 and the impact of changes in the energy spectrum on detector response 6 , 7 , 8 all complicate the measurement process. Consequently, there have been multiple incidents recently of incorrect small field size output factors being measured resulting in the mistreatment of patients (9) …”

Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators

“…As a consequence, photons originating in the flattening filter itself are not likely to be the cause of changes in the detector response in small fields; however, this does not preclude changes arising from the absorption of low‐energy components of the beam by the presence of the flattening filter. Detectors with mass‐energy absorption coefficients different to that of water are known to exhibit photon energy dependence in their response (32) that can lead to an overresponse in large fields as a result of the presence of low‐energy photons 33 , 34 . In particular, unshielded silicon diodes are known to overrespond in large fields due to the mass density of silicon being different from water (32) .…”

“…Detectors with mass‐energy absorption coefficients different to that of water are known to exhibit photon energy dependence in their response (32) that can lead to an overresponse in large fields as a result of the presence of low‐energy photons 33 , 34 . In particular, unshielded silicon diodes are known to overrespond in large fields due to the mass density of silicon being different from water (32) . The presence of a steel central electrode in the cc01 chamber is also known to lead to an overresponse in large fields 35 , 36 .…”

The clinical impact of detector choice for beam scanning

“…Also, an inherent problem of using diodes is the overresponse to low‐energy scattered radiation, resulting from the relatively high photoelectric cross section of silicon $false(\mathrm{Z}=14false)$ 6 , 21 . This overresponse can be as high as 10% for large fields (22) . The overresponse is reduced by the use of metal shielding (such as in the case for the PTW 60008), which presents perturbation in measurement in the buildup region and small field sizes.…”