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Title: Dosimetric impact of flattening filter and flattening filter-free beams on Intensity Modulated Radiation Therapy Planning of Non-small Cell Lung Carcinomas
Authors: Lee, SWY 
Or, K
Kwong, J
Choy, S
Lee, YSW
Wu, VWC 
Issue Date: 2016
Source: 35th ESTRO Annual Meeting, Turin, Italy, 29 Apr-3 May 2016 (Poster) How to cite?
Abstract: Introduction: Flattening filter free (FFF) technique utilizes the benefit of faster beam-on time [1] to treat tumours that are susceptible to respiratory motion. Lung cancer was chosen for this study due to its high prevalence and the variation in tissue heterogeneity in the thoracic region that might interact with FFF beams differently. Locally advanced non-small cell lung carcinomas (NSCLCs) were selected in this study [2]. Our objectives are to compare the dosimetric outcomes of FFF and FF IMRT plans and to investigate the effect of 6MV and 10MV beams on these FFF plans.
Methods: Five-field IMRT planning was conducted retrospectively in 10 patients using flattened (X6FF, X10FF) and FFF (X6FFF, X10FFF) beams with maximum dose rate of 400 and 800 MU/min respectively. RTOG 0937 protocol was followed. Plan evaluation was based on dosimetric parameters such as planning target volume (PTV), monitor units (MU) and organs at risk (OARs). Ratios of dosimetric parameters using X6FFF/X6FF and X10FFF/X10FF were used to study the effect of FFF. The effect of beam energy on FFF plans was also studied using the same methodology.
Results and Discussion: All plans exhibited comparable homogeneity and conformity with a steep dose fall-off beyond PTVs. This can be explained by less off-axis dose contributed by characteristic conical dose profile of the FFF beam. More MUs were delivered in FFF beams with its conical dose profile, reduction in head leakage and scatter, causing a reduction in out-of-field dose leading to more contribution of primary beam. FFF beams also offered better sparing of OARs than flattened beams. Without beam hardening effect in FFF beams, the lower effective beam energy results in more patient scatter, which may increase the out-offield dose. However, reduced head scatter and leakage in FFF resulted in lower dose for ‘V20 of WL-GTV’ and ‘cord+5mm’ [3]. Reduction in brachial plexus dose was found in X10FFF plans over X6FFF plans due to higher energy photons with less patient scatter.
Conclusion: The majority of dosimetric parameters were comparable between FF and FFF IMRT plans. However, FFF IMRT plans demonstrated better sparing of OARs including ‘cord+5mm’, ‘V20 of WL-GTV’ and brachial plexus. With the limitations of significant variations in tumour characteristics, further prospective studies with clinical outcome assessment are recommended in order to bring the benefits of FFF IMRT for lung cancer patients to clinical settings.
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