SU-E-T-61: Capabilities and Considerations for the Clinical Use of Flattening Filter Free (FFF) Beams for Volumetric Modulated Arc Therapy (VMAT)




Machines with FFF beams are becoming more prevalent. As technology evolves, guidelines for its use need to be developed. FFF beams have the potential for much greater dose rates and lower out-of-field dose. The use of VMAT with FFF adds additional complexity. This work attempts to define the capabilities and limitations of FFF when applied to VMAT fields and characterize the situations which this technology is best suited.


The total monitor units of a VMAT delivery is modified to determine the minimum deliverable dose rate for FFF and non-FFF beams on an Elekta VersaHD linear accelerator. The minimum dose rate limits the minimum dose per segments, thus reducing the available modulation. Time is assessed by recording the beam-on time for FFF and non-FFF beams. For 10 plans with standard fraction sizes (180–212 cGy/Fx) maximum dose rate per control point and number of control points exceeding 400 MU/min are recorded.


VMAT minimum dose rate is related to the pulse rate. The minimum deliverable dose rate is ∼36 MU/min (0.12 MU/deg) for 6x-FFF (max 1400 MU/min) and ∼75 MU/min (0.27 MU/deg) for 10x-FFF (max 2400 MU/min) beams. Delivery time is independent of dose rate for dose rates below 1500 MU/min as it is limited by gantry speed. The maximum dose rate across in all plans according to Pinnacle is 1170 MU/min for FFF plans and 800 MU/min for non-FFF plans. The percentage of control points exceeding 400 MU/min is 13.3% for FFF and 9% for non-FFF beams.


FFF does not improve delivery time of VMAT fields for conventional fraction sizes, since the dominant factor is gantry speed instead of dose rate. However, there may be potential for speed improvement for very large fraction sizes. Due to pulse rate dependency, FFF beams require a higher minimum dose rate.