Quantification of end-to-end beam hold latency using Surface Guided Radiotherapy gating, and cross-vendor linac...

Measuring latency of radiation beam-hold is challenging and published guidance on thresholds/tolerances is inconsistent. Two independent Radiotherapy Centres in the South of England collaborated to determine an accurate and simple method applicable to linacs from any vendor.

There are an assortment of published methods for measuring SGRT latency, and it is not clear which is optimal (accuracy and efficiency). This project's aim was to develop a simple method that could accurately measure beam-hold latency, that could be shared with other centres for SGRT commissioning and QA without the need for specialist equipment.

A simple phantom was created using a cardboard bowl and a lead marker. This was placed on a moving platform that was programmed with a variety of constant velocities. A 10 mm vertical tolerance was set on the SGRT software (v6.3, AlignRT), which when exceeded would trigger a beam-hold. Images of the phantom were taken using the megavoltage panel for each velocity. The displacement of the lead marker from its initial reference position was plotted against the velocity. The gradient of the best-fit line was used to determine the latency.

The average latency measured on four Elekta Versa HD Linacs with Horizon cameras was 149 ms and that measured across four Varian TrueBeams with HD cameras was 131 ms.

The collaboration between the two centres yielded a simple and reproducible method for measuring latency using AlignRT for gating which is accessible to most centres using both Elekta and Varian linacs.