Abstract
Purpose. In small megavoltage photon fields, the accuracies of an unmodified PTW 60017-type diode dosimeter and six diodes modified by adding airgaps of thickness 0.6–1.6 mm and diameter 3.6 mm have been comprehensively characterized experimentally and computationally. The optimally thick airgap for density compensation was determined, and detectors were micro-CT imaged to investigate differences between experimentally measured radiation responses and those predicted computationally. Methods. Detectors were tested on- and off-axis, at 5 and 15 cm depths in 6 and 15 MV fields ≥ 0.5 × 0.5 cm2. Computational studies were carried out using the EGSnrc/BEAMnrc Monte Carlo radiation transport code. Experimentally, radiation was delivered using a Varian TrueBeam linac and doses absorbed by water were measured using Gafchromic EBT3 film and ionization chambers, and compared with diode readings. Detector response was characterized via the formalism, choosing a 4 × 4 cm2 reference field. Results. For the unmodified 60017 diode, the maximum error in small field doses obtained from diode readings uncorrected by factors was determined as 11.9% computationally at +0.25 mm off-axis and 5 cm depth in a 15 MV 0.5 × 0.5 cm2 field, and 11.7% experimentally at −0.30 mm off-axis and 5 cm depth in the same field. A detector modified to include a 1.6 mm thick airgap performed best, with maximum computationally and experimentally determined errors of 2.2% and 4.1%. The 1.6 mm airgap deepened the modified dosimeter’s effective point of measurement by 0.5 mm. For some detectors significant differences existed between responses in small fields determined computationally and experimentally, micro-CT imaging indicating that these differences were due to within-tolerance variations in the thickness of an epoxy resin layer. Conclusions. The dosimetric performance of a 60017 diode detector was comprehensively improved throughout 6 and 15 MV small photon fields via density compensation. For this approach to work well with good detector-to-detector reproducibility, tolerances on dense component dimensions should be reduced to limit associated variations of response in small fields, or these components should be modified to have more water-like densities.
Highlights
Radiation doses absorbed by water from small non-equilibrium megavoltage photon fields should ideally be measured using detectors with effective atomic numbers and densities sufficiently close to water to limit spectral and lateral electronic disequilibrium effects, and sensitive volumes small enough to minimize volume averaging (IAEA 2017)
Several studies have demonstrated that errors in small field dose measurements made directly from diode detector readings, without applying field-size-specific corrections, can be reduced by adding air cavities to the diodes (Charles et al 2013, 2014, Underwood et al 2015b)
The detectors were tested experimentally in a 6 MV photon beam, comparing diode readings with doses measured using Gafchromic EBT3 film (Ashland Inc, Covington, Kentucky), and were further characterized computationally via Monte Carlo radiation transport calculations. Data from both approaches showed that the maximum error in uncorrected dose measurements made on-axis at 5 cm depth in fields ≥0.5 × 0.5 cm2 was 8% for an unmodified 60017 detector calibrated in a 10 × 10 cm2 field, but fell to
Summary
Radiation doses absorbed by water from small non-equilibrium megavoltage photon fields should ideally be measured using detectors with effective atomic numbers and densities sufficiently close to water to limit spectral and lateral electronic disequilibrium effects, and sensitive volumes small enough to minimize volume averaging (IAEA 2017). The detectors were tested experimentally in a 6 MV photon beam, comparing diode readings with doses measured using Gafchromic EBT3 film (Ashland Inc, Covington, Kentucky), and were further characterized computationally via Monte Carlo radiation transport calculations. Data from both approaches showed that the maximum error in uncorrected dose measurements made on-axis at 5 cm depth in fields ≥0.5 × 0.5 cm was 8% for an unmodified 60017 detector calibrated in a 10 × 10 cm field, but fell to
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