Constancy checks of well‐type ionization chambers with external‐beam radiation units

Abstract

Constancy checks of a well‐type ionization chamber should be performed regularly as part of a quality assurance regime. The goal of this work was to test the feasibility of using a linear accelerator and an orthovoltage unit to check the constancy of a well‐type chamber's response to an external radiation source. The reproducibility, linearity with dose, variation with dose‐rate, and variation between energy‐matched units of the well‐type chamber response when exposed to 6 MV beams was examined. The robustness to errors in establishing the measurement conditions, including setting the source‐to‐surface distance and gantry angle, rotation of the chamber around the central axis of the beam, and the effect of changing the length of the chamber cable exposed to the field, were tested. The reproducibility and linearity with dose of the chamber response, and robustness to errors in establishing the measurement conditions for 100 kVp and 250 kVp beams from an orthovoltage unit, were also examined. The combined uncertainty, including contributions from errors in establishing the reference conditions, for well‐type chamber measurements using a 6 MV beam from a linear accelerator is 1.0%. The combined uncertainties for measurements using 100 and 250 kVp beams were 1.8% and 1.5%, respectively. When focus‐source distance errors were reduced to ≤1mm, the combined uncertainties for the 100 and 250 kVp beams were 1.2% and 1.1%, respectively, when the dose to the chamber was confined to the linear region of the dose‐response curve. The response of a well‐type chamber should remain constant to within 1.2% when exposed to a constant dose from an external beam unit, if reference conditions can be reproducibly established. However, the uncertainty for establishing reference conditions for output measurements for an orthovoltage unit can be reduced, which would justify a reduction of the tolerance for constancy measurements.PACS numbers: 87.55.Qr, 87.56.Fc

GEC-ESTRO ACROP recommendations on calibration and traceability of HE HDR-PDR photon-emitting brachytherapy sources at the hospital level

“…(III) A phantom (solid, Krieger phantom or jig) where the source and Farmer chamber have fixed positions [28]. (IV) A linac beam irradiation of the WIC in a well-defined geometric setup [9,33]. However, the associated uncertainty can be too large.…”

Evaluation of Well-Type Chamber Calibration Factor for Measurement of Ir-192 Brachytherapy Source Over 10 Institutions

“…This radiation measuring instrument was calibrated in terms of air Kerma strength at the University of Wisconsin Accredited Dosimetry Calibration Laboratory traced to the National Institute of Standards and Technology (NIST) with a calibration factor N AKS = 4,690 × 10 5 Gy m 2 h −1 A −1 [24]. This well-type chamber was kept stable in the range 0.5% of deviation according to recommendations [21], [22].…”

GEC-ESTRO ACROP recommendations on calibration and traceability of LE-LDR photon-emitting brachytherapy sources at the hospital level

“…The constancy of electrometer, thermometer and pressure gauge must also be regularly checked. The constancy of a WIC can also be checked through use of a long-lived source (e.g., a 137 Cs or 90 Sr source) or through linac or kV-beam irradiation in a well-defined geometric setup [60,20], however the associated uncertainty can be an issue. The latter alternatives do not check the BT source insert which would hence have to be checked separately using, e.g., a ruler.…”