To quantitatively assess the plan quality of conventional fractionated volumetric modulated arc therapy (VMAT) plans for liver cancer treatments using flattening filter-free (FFF) and flattening filter (FF) modes based on plan quality metric (PQM).

A total of ten patients with liver tumor were selected. The patients were planned in FF and FFF mode of 6 MV X-ray respectively. The dose distribution of the planning target volume (PTV) and the organ at risk (OAR) were evaluated, and the monitor units (MUs) and the beam on time (BoT) were compared. According to the clinical requirements, a PQM with 16 submetrics was defined to evaluate the plan quality of the two modes.

The maximal dose of FFF plan was less than that of FF plan (t=3.828, P<0.05). The normalized volume of 5 Gy (V₅) and mean dose (D_mean) of the normal liver of FFF plan were lower than those of the FF plan (t=2.716, 3.007, P<0.05). The average MU of FFF plan [(574±130) MU] was higher than that of FF plan [(518±81) MU](t=-2.782, P<0.05), while the average BoT of FFF plan [(108±36)s] was significantly lower than that of FF plan [(160±29) s](t=6.767, P<0.05). The score of FFF plan was higher than that of FF plan (t=-2.746, P<0.05).

FFF mode can better protect the OAR at low dose levels. The MU of FFF mode plan were higher, but the beam on time was significantly lower than FF mode plan. With reference to the PQM evaluation criteria, the overall plan quality of the FFF mode was slightly higher.

Plan quality metric; Flattening filter-free; Liver cancer; Volumetric modulated arc therapy; Dosimetry