To determine detector-specific output correction factors, , in 6 MV small photon beams for air and liquid ionization chambers, silicon diodes, and diamond detectors from two manufacturers.
Field output factors, defined according to the international formalism published byAlfonso et al. [Med. Phys.35, 5179–5186 (2008)], relate the dosimetry of small photon beams to that of the machine-specific reference field; they include a correction to measured ratios of detector readings, conventionally used as output factors in broad beams. Output correction factors were calculated with the PENELOPE Monte Carlo (MC) system with a statistical uncertainty (type-A) of 0.15% or lower. The geometries of the detectors were coded using blueprints provided by the manufacturers, and phase-space files for field sizes between 0.5 × 0.5 cm2 and 10 × 10 cm2 from a Varian Clinac iX 6 MV linac used as sources. The output correction factors were determined scoring the absorbed dose within a detector and to a small water volume in the absence of the detector, both at a depth of 10 cm, for each small field and for the reference beam of 10 × 10 cm2.
The Monte Carlo calculated output correction factors for the liquid ionization chamber and the diamond detector were within about ±1% of unity even for the smallest field sizes. Corrections were found to be significant for small air ionization chambers due to their cavity dimensions, as expected. The correction factors for silicon diodes varied with the detector type (shielded or unshielded), confirming the findings by other authors; different corrections for the detectors from the two manufacturers were obtained. The differences in the calculated factors for the various detectors were analyzed thoroughly and whenever possible the results were compared to published data, often calculated for different accelerators and using the EGSnrc MC system. The differences were used to estimate a type-B uncertainty for the correction factors. Together with the type-A uncertainty from the Monte Carlo calculations, an estimation of the combined standard uncertainty was made, assigned to the mean correction factors from various estimates.
The present work provides a consistent and specific set of data for the output correction factors of a broad set of detectors in a Varian Clinac iX 6 MV accelerator and contributes to improving the understanding of the physics of small photon beams. The correction factors cannot in general be neglected for any detector and, as expected, their magnitude increases with decreasing field size. Due to the reduced number of clinical accelerator types currently available, it is suggested that detector output correction factors be given specifically for linac models and field sizes, rather than for a beam quality specifier that necessarily varies with the accelerator type and field size due to the different electron spot dimensions and photon collimation systems used by each accelerator model.
This work was partly supported by the Swedish Radiation Safety Authority (SSM) through Project Nos. SSM2011-1105 and SSM2012-2331. The authors gratefully acknowledge PTW and IBA for providing detailed descriptions of the detectors used and for kindly answering questions on specific details for building the geometries accurately. We gratefully acknowledge partial financial support from the Spanish Ministerio de Economía y Competitividad (Grant No. FIS2012-38480). J.S. also acknowledges financial support from the Spanish Networking Research Center CIBER-BBN.
II. MATERIALS AND METHODS
II.A. The formalism for relative dosimetry of small fields
II.B. Monte Carlo simulations
II.C. Radiation sources and detectors
III. RESULTS AND DISCUSSION
III.A. Beam characteristics
III.B. Output correction factors
III.C. Ionization chambers
III.D. Solid-state detectors
III.D.1. Silicon diodes
III.D.2. Diamond detectors
III.E. Estimation of uncertainties of corrections factors
IV. SUMMARY AND CONCLUSIONS
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