Hugo Palmans1. 1. Acoustics and Ionising Radiation Division, National Physical Laboratory, Teddington , UK. hugo.palmans@npl.co.uk
Abstract
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. METHODS: 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, TPR(20,10), from a measurement of the tissue phantom ratio at depths of 20 cm and 10 cm in water for an s × s cm(2) (equivalent) square field, TPR(20,10)(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 PDD(10)(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: The linear models for TPR(20,10)(s) and exponential models for PDD(10)(s) as a function of the (equivalent) square field size 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.
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. METHODS: 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, TPR(20,10), from a measurement of the tissue phantom ratio at depths of 20 cm and 10 cm in water for an s × s cm(2) (equivalent) square field, TPR(20,10)(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 PDD(10)(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: The linear models for TPR(20,10)(s) and exponential models for PDD(10)(s) as a function of the (equivalent) square field size 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.