Experimental validation of the dual parameter beam quality specifier for reference dosimetry in flattening-filter-free (FFF) photon beams

Removal of the flattening filter alters the energy spectrum of the photon beam such that current beam quality specifiers may not correctly account for this change when predicting the Spencer-Attix restricted water-to-air mass collision stopping-power ratio, ((L) over bar/rho)(air)(water). Johnsson et al (2000 Phys. Med. Biol. 45 2733-45) proposed a beam quality specifier, known as the dual parameter beam quality specifier, which was calculated via Monte Carlo (MC) simulations using transmission data of primary kerma through two differing thicknesses of water material. Ceberg et al (2010 Med. Phys. 37 1164-8) extended this MC study to include relevant flattening filter free (FFF) beam data. Experimental investigations of this dual parameter beam quality specifier have not previously been published, therefore the purpose of this work was to validate that the dual parameter beam quality specifier could be measured experimentally for clinical beams (both with a flattening filter (WFF) and without (FFF)). Transmission measurements of primary kerma were performed by employing the setup outlined in Johnsson et al (1999 Phys. Med. Biol. 44 2445-50). Varying absorber thicknesses, in 5 cm increments from 5 to 40 cm, were placed at isocentre with the chamber positioned at an extended source to chamber distance of 300 cm. Experimental setup for TPR20,10 and %dd(10)(x) followed the methodology outlined in IAEA TRS398 (2004) and TG-51 (1999) with AAPM Addendum to TG-51 (2014) respectively. The maximum difference of ((L) over bar/rho)(air)(water) determined using the different beam quality specifiers was found to be 0.35%. Analysis of the absorber thickness combination found that small thicknesses (< 10 cm) for the first absorber and absorbers similar in thickness (< 10 cm) should be avoided. Stopping-power ratios of the beams investigated were determined using three different beam quality specifiers. The results demonstrated successful experimental determination of the dual parameter beam quality specifier, indicating its potential as an alternate beam quality specifier for FFF beams.