Technical Note: Preferred dosimeter size and associated correction factors in commissioning high dose per pulse, flattening filter free x-ray beams

• Published in: Medical physics (Lancaster) 2016-03, Vol.43 (3), p.1507-1513
• Main Authors: Sudhyadhom, A., Kirby, N., Faddegon, B., Chuang, C. F.
• Format: Article
• Language: English
• Subjects: absolute dosimetry; Addendum; Calibrating of instruments or apparatus; calibration; Devices sensitive to very short wavelength, e.g. x‐rays, gamma‐rays or corpuscular radiation; dosimetry; Dosimetry/exposure assessment; Error analysis; Ion beams; ion recombination; ionisation chambers; Ionization chambers; ionization chambers and flattening filter free; Particle beam detectors; Physicists; Polynomials; Radiation Dosage; Radiometry - instrumentation; Radiometry - methods; relative dosimetry; Scintigraphy; Standards and calibration; Testing or calibrating of apparatus or arrangements provided for in groups G01D1/00 to G01D15/00; Transforming x‐rays; Uncertainty; X-Rays; X‐ and γ‐ray instruments; X‐ray apparatus; X‐ray effects; X‐ray technique
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.4941691

Purpose: High dose rate flattening filter free (FFF) beams pose new challenges and considerations for accurate reference and relative dosimetry. The authors report errors associated with commonly used ion chambers and introduce simple methods to mitigate them. Methods: Dosimetric errors due to (1) ion recombination effects of high dose per pulse (DPP) FFF beams and (2) volume-averaging effects of the radial profile were examined on a TrueBeam STx. Four commonly used cylindrical ion chambers spanning a range of lengths (0.29–2.3 cm) and volumes (0.016–0.6 cm3) were used to determine the magnitude of these effects for 6 and 10 MV unflattened x-ray beams (6XFFF and 10XFFF, respectively). Two methods were used to determine the magnitude of ion collection efficiency: (1) direct measurement of the percent depth dose (PDD) for the clinical, high DPP beam in comparison to that obtained after reducing the DPP and (2) measurement of P ion as a function of depth. Two methods were used to quantify the magnitude of volume-averaging: (1) direct measurement of volume-averaging via cross-calibration and (2) calculation of volume-averaging from radial profiles of the beam. Finally, a simple analytical expression for the radial profile volume-averaging correction factor, P rp = [OAR(0.29L)]−1, or the inverse of the off-axis ratio of dose at 0.29L, where L is the length of the chamber’s sensitive volume, is introduced to mitigate the volume-averaging effect in Farmer-type chambers. Results: Errors in measured PDD for the clinical beams were 1.3% ± 0.07% and 1.6% ± 0.07% at 35 cm depth for the 6XFFF and 10XFFF beam, respectively, using an IBA CC13 ion chamber, due to charge recombination with a high DPP. Volume-averaging effects were 0.4% and 0.7% for the 6XFFF and 10XFFF beam, respectively, when measured with a Farmer-type chamber. For the application of TG-51, these errors combine when using a CC13 to measure the PDD and a Farmer for absolute output dosimetry for a total error of up to 2% at d max for the 10XFFF beam. Conclusions: Relative and absolute dosimetry in high DPP, unflattened x-ray beams of 10 MV or higher requires corrections for charge recombination and/or volume-averaging when dosimeters with certain geometries are used. Chambers used for PDD measurement are available that do not require a correction for charge recombination. A simple analytical expression of the correction factor P rp was introduced in this work to account for volume-averaging effects in Farm