Session Item

To investigate (i) the dosimetric leaf gap (DLG) and the effect of the ‘trailing distance' between leaves from different multi-leaf collimator (MLC) layers in Halcyon™ systems and (ii) the ability of the currently available treatment planning systems (TPS) to replicate this effect.

DICOM plans with transmission and sweeping gap beams were created in Python for measuring the DLG for each MLC layer independently and for both layers combined. To characterize the effect of the leaf trailing patterns seen in clinical Halcyon plans, new tests called ‘trailing sweeping gaps’ were designed and created where the leaves from one layer follow the leaves from the other layer at a fixed ‘trailing distance' between the tips (see Fig 1a). Measurements were carried out on five Halcyons SX2 from different institutions and calculations from both the Eclipse and RayStation TPSs were compared with measurements.

The dose accumulated during a sweeping gap delivery progressively increased as the trailing distance t increased (see representative case in Fig 1b).

We call this ‘the trailing effect’. It was most pronounced for t between 0 and 5 mm, although some changes were obtained up to 20 mm. The dose difference was independent of the gap size. The measured DLG values also increased with t with the steepest variation for t between 0 and 5 mm and finally reaching a plateau after t=20 mm (see Fig 2). Measured DLG values were negative at t=0 (the leaves from both layers at the same position) but changed sign for t ≥1 mm, in line with the positive DLG sign usually observed with single-layer rounded-end MLCs.

The Eclipse TPS does not explicitly model the leaf tip and, as a consequence, could not predict the dose reduction due to the trailing effect. This resulted in dose discrepancies from -8% to +10% for the 5 mm sweeping gap and from -5% to +5% for the 10 mm one depending on the distance t. RayStation implements a simple model of the leaf tip that was able to approximate the trailing effect and improved the agreement with measured doses. In particular, with a prototype version of RayStation (11P) that assigned a higher transmission at the leaf tip the agreement with measured doses was within ±3% even for the 5 mm gap. The five Halcyon systems behaved very similarly but differences in the DLG around 0.2 mm were found across different treatment units and between MLC layers from the same system. The DLG for the proximal layer was consistently higher than for the distal layer, with differences ranging between 0.10 mm and 0.24 mm.

The trailing distance between the leaves from different layers substantially affected the doses delivered by sweeping gaps and the measured DLG values. Stacked MLCs introduce a new level of complexity in TPSs, which ideally need to implement an explicit model of the leaf tip in order to reproduce the trailing effect. Dynamic tests called ‘trailing sweeping gaps' were designed that are useful for characterizing and commissioning dual-layer MLC systems.