Photon beams usually have a metal filter on their path to make photon fluence uniform (or flat). With proliferation of accurate treatment planning algorithms and Intensity-Modulated Radiation Therapy (IMRT) the uniform fluence is not a concern. When the filter is removed from the photon beam, the intensity increases by a factor of 2.3 for 6 MV photons and by a factor of 4.0 for 10 MV photons.
The chart below illustrates this for 10 MV photon beam. The line labeled 10X (red) is the diagonal profile of the beam for the usual mode with a metal filter. The line labeled 10FFF (blue) is the new Flattening Filter Free (FFF) mode. The number of beams pulses per second remains the same (360 pulses/second at the maximum dose rate), however, the absence of attenuation due to the metal filter results in higher photon dose in the central portion of the beam.
In practical sense this means that the same dose could be delivered up to 4 times faster for this energy.
