Study of shielding requirements for Linacs with Flattening Filter-Free beams in IMRT techniques
DOI:
https://doi.org/10.32685/2590-7468/invapnuclear.5.2021.587Keywords:
flattening filter, TVL, workload, leakage radiation, use factor
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Abstract
In this work, the parameters used for shieding calculation for modern electron linear accelerators (linac) that can operate with or without a flattening filter in the beam (FF and FFF beams), in intensity modulated radiotherapy (IMRT) modality, were studied. In particular, the tenth-value layers (TVL), the workload (W) and the primary beam use factors (U) were analyzed, in order to determine the differences in both space requirements and costs for a bunker. The spectral properties of 6 MV FF and FFF beams, from published spectra, and their effect on parameters such as mean beam energy and TVLs were studied. It was found that the average energy for FF beams can be 1.15 higher than that of FFF beams, while the TVLs for FFF beams may be 16% lower than those for FF beams, results consistent with published data. A study of the workload and primary beam use factors was carried out for two last generation accelerators operated in a high-workload center in treatments with IMRT and volumetric modulated arc therapy (VMAT), with the objective of performing shielding calculations representative for a modern radiotherapy center. With an example of a bunker, it was found that the thickness of the primary barriers for a linac operated with FFF beams can be up to 19% less than that required for FF beams; also, the thickness of the necessary secondary barriers is reduced, which is partly explained by the decrease in the workload of leakage radiation, WL, in FFF beams of 54%, as a consequence of the decrease in the number of monitor units (MU) required for treatment with FFF beams compared to FF beams. When using combined beams, shielding benefits are reduced depending on the combination used; for example, it was established that by increasing the operating mode of a linac from 20% to 40 % with FFF beam, the advantage in shielding reduction can reach up to 9% for primary and secondary barriers. Using primary beam use factors, corresponding to a modern linac modes of operation, such as those studied, and in combination with single FFF beams, the thickness of the primary and secondary barriers can be reduced by up to 30%, thus optimizing costs and spaces, without sacrificing the protection of the public and workers. Finally, as practical consequences of the reduction of shielding thicknesses it is possible to reduce between 13% and 17% the amount of ordinary concrete for a bunker like the one in the analyzed example, and a specific savings in space between 6 m2 to 11.8 m2 can be achieved.
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