Small field dosimetry on a ring-shape Linac machine with a dual-layer MLC

The Varian Halcyon linac, equipped with a dual-layer MLC and a single 6 MV FFF beam, is increasingly used for treatments involving small target volumes. The accuracy of dose calculation for small static fields on this platform remains a matter of concern.

This study investigates the dosimetric accuracy of small static fields generated by the Halcyon dual-layer MLC and to evaluate their modeling in Eclipse through Acuros XB versions 16.1, 18.0, and 18.1.

On-axis field out put factors (FOFs) were measured for square and rectangular fields (0.5-4 cm) according to TRS-483, using the equivalent square field formalism by Fogliata. Measurements with microDiamond and microSilicon detectors were compared with Acuros XB calculations in Eclipse versions 16.1, 18.0, and 18.1. Profiles and penumbrae were analyzed, and a clinical single-lesion brain VMAT plan was evaluated.

The two detectors agreed within ± 0.5%. Differences from vendor Representative Beam Data were 1.1%-1.8% for 1 cm fields. Acuros XB v.16.1 underestimated FOFs by up to -27% for 0.5 × 0.5 cm², while v.18.x agreed within ∼2%. Y-direction penumbrae were larger for proximal bank or groove edges, not fully modeled in TPS. In the clinical case investigated, 50% of control points had fields < 1 cm, with central underdosage in v.16.1.

Acuros XB ≥ 18.x improves small-field accuracy for Halcyon, although Y-penumbra modeling remains incomplete. Constraints on minimum field sizes during treatment optimization are important to ensure high accuracy in treatment delivery.