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Determination of the beam quality index of high-energy photon beams under nonstandard reference conditions.

Palmans, H (2012) Determination of the beam quality index of high-energy photon beams under nonstandard reference conditions. Med. Phys., 39 (9). pp. 5513-5519.

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Purpose: At some modern radiotherapy machines it is not possible to achieve reference conditions for the measurement of beam quality indices used in dosimetry codes of practice such as IAEA TRS-398 and AAPM TG-51. This work aims at providing self-consistent and simpler expressions and more accurate fits for a limited range of beams of interest than have been proposed previously for deriving these beam quality indices from measurements.
Method: The starting point is a formula proposed by Sauer [Med. Phys. 36, 4168-4172 (2009)] for deriving the beam quality index used in IAEA TRS-398, TPR20,10, from a measurement of the tissue phantom ratio at depths of 20 cm and 10 cm in water for an s cm × s cm (equivalent) square field, TPR20,10(s). First, a self-consistent version of this formula is established followed by a simpler version by making a linear approximation. A similar approach is proposed to derive the beam quality index used in AAPM TG-51, %dd(10)X, from a measurement of PDD10(s), the percentage depth dose at 10 cm for a square field with size s. All models were fitted to subsets of relevant data from BJR supplement 25.
Results and Conclusions: The linear models for TPR20,10(s) and exponential models for PDD10(s) as a function of the equivalent) square field size (both with three parameters) can reproduce the beam quality within 0.3% and beam quality correction factors within 0.05% for square field sizes ranging from 4 cm to 12 cm and nominal photon energies from 4 MV to 12 MV. For higher energy beams the errors are only slightly worse but for %dd(10)X, an additional uncertainty component has to be considered for the electron contamination correction.
Conclusions: The models proposed here can be used in practical recommendations for the dosimetry of small and nonstandard fields.

Item Type: Article
Keywords: beam quality, photon beams, nonstandard reference conditions, analytical model
Subjects: Ionising Radiation
Ionising Radiation > Dosimetry
Identification number/DOI: 10.1118/1.4745565
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/5609

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