A Varian TrueBeam linear accelerator has been in full clinical operation with 6-MV and 10-MV FFF and flattened x-ray photons. Entrance dose at the surface was measured using a parallel plate ionization chamber in a solid water phantom with buildup depth = 0鈭?5 mm for 6X and 0鈭?5 mm for 10X. Different field size (FS) patterns were created in the Eclipse Treatment Planning System by multileaf collimator (MLC) rather than jaws (FS = 2 脳 2, 3 脳 3, 4 脳 4, 6 脳 6, and 10 脳 10 cm2 by MLC and jaw size = 2.2 脳 2.2, 3.2 脳 3.2, 4.2 脳 4.2, 6 脳 6, and 10 脳 10 cm2). The smallest FS was about four times larger than the ion chamber dimension. All buildup dose measurements were normalized to FS = 10 脳 10 cm2 at the depth of dose maximum (dmax).
Good repeatability was demonstrated and surface dose increased linearly with FS for both flattened and FFF photons. The entrance dose of the FFF photons was modestly larger than that of the corresponding flattened photons for both 6X and 10X for different FS ranging from 2 脳 2 cm2 to 10 脳 10 cm2.
The FFF photons have a higher entrance dose than that of the corresponding flattened photons for FS smaller than 10 脳 10 cm2. However, the difference is not substantial and may be clinically insignificant.