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Dosimetric and Biologic Differences in Flattened and Flattening-Filter-Free Beam Treatment Plans

Purpose: To quantitatively compare the dosimetric and biologic differences in treatment plans from flattened and flattening-filter-free (FFF) beam for three anatomic cancer sites. Methods and Materials: Treatment plans with static intensity-modulated radiotherapy beams and volumetric modulated arc therapy beams were generated for 13 patients for both the flattened beam and the FFF beam of the TrueBeam system. Beam energies of 6 MV and 10 MV were chosen for planning. A total of 104 treatment plans were generated in 13 patients. In order to analyze the biological effectiveness of treatment plans, dose volume histograms (DVH) were utilized. Flattened and FFF beam plans are quantitatively compared. Results: In head and neck cases, for VMAT plans, dose reduction in the FFF beam plans compared to the flattened beam in left cochlea, right submandibular gland and right parotid gland reached up to 2.36 Gy, 1.21 Gy and 1.45 Gy, respectively. Similarly, for static IMRT plans, the dose reduction of the FFF beam plans compared to the flattened beam plans for the same organs reached up to 0.34 Gy, 1.36 Gy and 1.46 Gy, respectively. Overall, for head and neck, the FFF beam plans achieved mean dose reduction of up to 5%, 7% and 9%, respectively for above organs at risk. For lung and prostate cases, the FFF beams provided lower or comparable NTCP values to organ-at-risk (OAR) compared to the flattened beam for all plans. Conclusions: In general, we observed treatment plans utilizing FFF beams can improve dose sparing to OARs without compromising the target coverage. Significant dose sparing effect is obtained for head and neck cancer cases, especially for the cases with relatively large field sizes (about 16x20 cm^2). For lung and prostate cases, compared to the flattened beam, the FFF beam based treatment plans provide lower or comparable dose to most OARs.