Density compensated diodes for small field dosimetry: comprehensive testing and implications for design

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 k Q c l i n , Q m s r f c l i n , f m s r 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 k Q c l i n , Q m s r f c l i n , f m s r 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.

The PTW microSilicon diode: Performance in small 6 and 15 MV photon fields and utility of density compensation

“…For some detectors, there were notable differences between responses determined experimentally and computationally in 0.5 × 0.5 cm 2 fields, a finding attributed to detector-to-detector variations in the thickness of the dense epoxy resin housing of the sensitive volume. 10 This was supported by Monte Carlo calculations showing that the response of the 60017-type detector in a 6 MV 0.5 × 0.5 cm 2 field relative to that in a 4 × 4 cm 2 field would be 4% higher if the epoxy housing was 0.3 mm thicker, a change within manufacturing tolerance.It follows that detector response in small fields can only be reproducibly fine-tuned using airgaps engineered to a tenth of a millimeter if comparable tolerances are placed on the dimensions of dense detector components, or if these components are replaced with less dense materials.…”

“…21,24 Photon cross-section enhancement was used to accelerate calculations. 10 Doses absorbed by detector sensitive volumes located on-axis at 5 and 15 cm depths in water in 6 and 15 MV fields of size 0.5 × 0.5, 0.7 × 0.7, and 4 × 4 cm 2 were computed to a precision of ≤ ± 0.2% (2 standard deviations,s.d. ).To simulate field profiles measured at these depths, sensitive volume doses were calculated for detectors computationally shifted across the 0.5 × 0.5 cm 2 field in 0.25 mm steps, holding precision to ≤ ± 0.7% up to 1 mm beyond the field-edge.…”

“…For comparison, corresponding values previously com-puted for the 60017-type diode were 0.910 ± 0.005 and 0.971 ± 0.007. 10 Table 2 also lists Monte Carlo k…”

“…In comparison, the 60017-type detector over-responded by up to 9.7%. 10 For the 0.6 mm airgap detector, maximum…”

“…The DOSXYZnrc code (version: v20017) 25 was used to calculate doses absorbed from 6 and 15 MV fields by water voxels within a 50 × 50 × 50 cm 3 water phantom located at 100 cm source-to-surface distance (SSD). Radiation transport parameters were selected as described previously, with ECUT and PCUT set to 521 and 1 keV, 10 electrons and photons with lower energies typically travelling < 10 µm through water before stopping or interacting. 21,24 On-axis doses were calculated at 5 and 15 cm depths in water for the three fields.…”