Abstract
Dosimetry protocols for external beam radiotherapy currently in use, such as the IAEA TRS-398 and AAPM TG-51, were written for conventional linear accelerators. In these accelerators, a flattening filter is used to produce a beam which is uniform at water depths where the ionization chamber is used to measure the absorbed dose. Recently, clinical linacs have been implemented without the flattening filter, and published theoretical analysis suggested that with these beams a dosimetric error of order 0.6% could be expected for IAEA TRS-398, because the TPR20,10 beam quality index does not accurately predict the stopping power ratio (water to air) for the softer flattening-filter-free (FFF) beam spectra. We measured doses on eleven FFF linacs at 6 MV and 10 MV using both dosimetry protocols and found average differences of 0.2% or less. The expected shift due to stopping powers was not observed. We present Monte Carlo kQ calculations which show a much smaller difference between FFF and flattened beams than originally predicted. These results are explained by the inclusion of the added backscatter plates and build-up filters used in modern clinical FFF linacs, compared to a Monte Carlo model of an FFF linac in which the flattening filter is removed and no additional build-up or backscatter plate is added.
Highlights
In recent years linear accelerators with flattening filter free capability have become common in radiotherapy clinics
The Australian Clinical Dosimetry Service (ACDS) uses beam quality data provided by the radiotherapy facility
In this work we investigated the differences between dosimetry for FFF and with flattening filter (WFF) beams by comparing TG-51 with TRS-398 for a series of clinical beams
Summary
In recent years linear accelerators with flattening filter free capability have become common in radiotherapy clinics. In particular the unflattened beams have a larger spectral spread with a greater low-energy component They are non-uniform over the length of a typical ionisation chamber used in reference dosimetry, and can be significantly higher in dose rate. Xiong and Rogers (2008) used Monte Carlo models of accelerators without flattening filters to determine the spectrum-averaged (Spencer-Attix restricted) stopping power ratios as a function of TPR20,10 and %dd(10)x. They found that the stopping powers for FFF follow a different curve when plotted against TPR20,10 compared to conventional with flattening filter (WFF) beams. Other work by Dalary et al (2014) saw a smaller difference in the stopping power ratios of FFF and flattened beams of 0.3% with similar TPR20,10
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