FFF — Flattening-Filter-Free
Linac beam mode where the flattening filter is removed from the treatment head. Profile is forward-peaked instead of flat, but dose rate is several times higher than a flattened beam at equivalent energy.
Why it matters to buyers: FFF is the enabler for high-dose-per-fraction treatments — SRS single-fractions, SBRT lung / liver. A linac without FFF licensing doesn't deliver those programs at modern throughput.
Why it matters to engineers: Available on TrueBeam / TrueBeam STx / Edge and on Elekta Versa HD. Commissioning is a separate beam model from flattened; monitor-unit linearity, output-factor, and small-field dosimetry all require FFF-specific data collection.