  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.
