Dosimetric impact of flattening filter and flattening filter-free beams on IMRT planning of NSCLC

Abstract

Purpose or Objective: This retrospective study aimed to compare and determine the potential dosimetric benefits of intensity-modulated radiotherapy (IMRT) treatment plans with (FF) and without flattening filter (FFF) as well as to explore the dosimetric differences in 6MV FFF and 10MV FFF plans for non-small-cell lung carcinoma (NSCLC). Material and Methods: Ten cases of CT data were selected from NSCLC patients. 4 sets of 5-field-IMRT plans were computed with FFF beams (X6FFF, X10FFF) and flattened beams (X6FF, X10FF) with the prescription of total 60Gy in 30 fractions. Planning constraints were based on the Radiation Therapy Oncology Group (RTOG) protocol 1306. Determination of isocentre, beam arrangement and dose constraints were kept constant in each case. All plans were computed using Varian Eclipse version 11.0 treatment planning system. The plans were then evaluated based on the target coverage, homogeneity, conformity, number of monitor units (MU) to be delivered and dose-volume constraints for various organs at risk (OARs). Results: All plans exhibited comparable PTV homogeneity (HI≤ 7.5) and conformity (CI > 96%) with a steep dose fall-off outside the PTVs but at the expense of increased MUs by 39.4% (p=0.007) and 44.7% (p=0.005) for FFF beams at 6 MV and 10 MV respectively. FFF beams offered better dose sparing of OARs than flattened beams. Spinal cord+5mm and volume of 'whole lung (WL) – Gross tumour volume (GTV)' (WL-GTV) that received 20Gy (V20) were reduced by 2% (p=0.017) and 2.8% (p=0.016) respectively in X10FFF plans when compared with X10FF plans. There was also a 16.4 % dose reduction to brachial plexus in X10FFF plans than X6FFF plans. Conclusion: The application of FFF IMRT for NSCLC yielded quantitatively comparable dosimetric distribution with better sparing of the OARs including ‘spinal cord+5mm’, V20 of ‘WLGTV’ and brachial plexus than using FF beams.