(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: To investigate the possibility of improving lung SBRT treatment plans by replacing conventional beams, having a flattening filter (FF) in the beamline, with flattening filter-free (FFF) beams, i.e. no flattening filter.
Methods: We selected ten patients treated with SBRT for early stage lung cancer. The treatments were with conventional 6 MV photon beams. We imported the treatment plans in Eclipse 13.6. The AAA dose algorithm was configured using representative beam data for TrueBeam accelerator operated in 6 MV FFF mode. After replacing conventional FF beams with FFF beams, we re-optimized all plans.
Results: For all plans, equivalent PTV coverage was achieved. For the ratio D95(FFF)/D95(FF), the median and interquartile range were 1.006 (1.003,1.014). Dose distributions were equally homogeneous. For the GTV ratio D98(FFF)/D98(FF), the statistics were better: 1.02 (1.00, 1.04). In most cases, doses to organs at risk were lowered. For the total lung, the statistics for the FFF/ FF ratio of mean doses were 0.98 (0.95, 1.00). V20 FFF/FF ratios improved for both the total and ipsilateral lung, the values were 0.94 (0.88, 0.98) and 0.95 (0.93, 0.98), respectively. The maximum doses for esophagus, cord and heart were in some cases reduced substantially. For the difference Dmax(FFF) - Dmax(FF) the statistics were -0.4 (-0.9, -0.2), -0.4 (-1.2, 0.1), -1.4 (-5.1, -0.3) Gy, respectively. The total number of monitor units for FFF plans was higher, whereas the beam-on time was lower. An increased number of monitor units is unlikely to result in a higher scattered dose, because FFF beams lack scatter from the flattening filter.
Conclusion: Using FFF beams has a potential for improving the therapeutic ratio for some patients. Choosing between the FF and FFF modalities requires comparison of two plans. The indications for considering FFF beams are a small tumor, thin chest wall, large dose per fraction.
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