Assessment of 6MV flattening filter free (FFF) beam commissioning parameters for Versa HD and TrueBeam: A comparison...

Flattening filter-free (FFF) beams are increasingly deployed for stereotactic body radiotherapy (SBRT) and stereotactic radiosurgery (SRS). The Elekta Versa HD (VHD) and Varian TrueBeam (TB) are the two most widely used FFF-capable platforms; however, no prior study has compared their 6 MV FFF commissioning parameters under fully equipment-equivalent conditions.

To quantify the dosimetric differences between the Elekta VHD and Varian TB 6 MV FFF platforms under equipment-equivalent conditions and to assess their translation into patient treatment plan parameters.

Percentage depth doses (PDDs), lateral beam profiles, and output factors (OFs) were acquired using an identical-model IBA SMARTSCAN three-dimensional water tank, a unified detector set (IBA RAZOR for small fields; CC13 for standard fields), and a fixed source-to-surface distance of 90 cm for both platforms, in conformance with AAPM TG-45, AAPM TG-106, and IAEA TRS-483. A treatment planning translation study was performed in the Monaco TPS using the IBA IMRT phantom, with 3D-CRT plans generated for four spherical targets (1, 3, 6, and 12 cm diameter) on both beam models.

VHD demonstrated consistently deeper beam penetration, with R50 exceeding TB by a mean of 1.753 ± 0.048 cm across all field sizes (t(9) = 36.34, p < 0.001, Cohen's d = 11.49, 95% CI [1.643, 1.862] cm). TB delivered significantly higher surface doses than VHD (mean Ds: 63.64 ± 5.77% vs. 48.78 ± 7.72%; p = 0.002, |r| = 1.000). At 10 cm depth, VHD showed a higher percentage depth dose (mean difference = 5.005 ± 0.352%; t(9) = 14.22, p < 0.001, d = 4.498). The Unflatness conversion index (UCI) was strongly correlated with field size (r = 0.916, p = 0.001) and statistically independent of measurement depth (p = 0.292). At the plan level, TB required 5.9-17.6% more monitor units (MU) than VHD across all target sizes, with the differential diminishing as target diameter increased. Surface dose differences were preserved across all four targets (TB - VHD: 15.9-31.1%). Both the homogeneity index and gradient index reversed direction between the 1 cm target (VHD superior) and targets of 3 cm and larger (TB superior), consistent with the field-size dependence of UCI.

A harmonized, equipment-equivalent measurement approach demonstrated systematic differences between VHD and TB FFF beams in penetration, surface dose, and field-size-dependent unflatness. Treatment planning study across four target sizes (1-12 cm) showed these differences propagate to plan-level parameters in a target-size-dependent manner, with VHD advantageous for small targets and TB for larger ones. The UCI, depth-independent and strongly correlated with field size (r = 0.916), provides a simple cross-platform unflatness comparison tool supporting machine-specific planning strategies in multi-platform clinical settings.