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Description
Objective
Upright linacs are expected to reach multileaf collimator (MLC) speeds of up to 10 cm/s. This study investigates how higher MLC speeds affect plan quality in upright versus supine plans.
Method
Volumetric modulated arc therapy plans for three lung and three prostate cancer patients were generated in RayStation v2025. Each patient had plans at MLC speeds of 2.5, 5, and 10 cm/s, using both standard sequencing (ST) and sliding-window sequencing (SW), giving six plans per patient. Within each cancer site, anatomy, prescription, beam geometry, optimisation objectives, and planning parameters were held constant to isolate the effect of the MLC-speed constraint. The primary metric was the RayStation Objective value (OV), where lower values indicate better fulfilment of the optimisation objectives. Prostate and lung results were analysed separately, comparing within each case and between configurations.
Results
Increasing MLC speed from 2.5 to 10 cm/s generally decreased OV for both sites and sequencing approaches, though the trend varied across individual cases. Prostate plans showed OV decreases of 29.4% (ST) and 13.4% (SW); lung plans showed 15.7% and 28.7%, respectively. For all lung plans, SW gave a considerable OV decrease from 2.5 to 5 cm/s but smaller gains from 5 to 10 cm/s, while ST showed no benefit in 5.6% of the lung cases. This is likely because SW OVs had a lower baseline than ST OVs. For prostate, SW showed no benefit in 11.1% of the cases, while ST showed benefit in all prostate cases.
Outlook & Discussion
These early findings suggest that more complex plans may benefit more from increased MLC speeds and SW, warranting further investigation. Higher MLC speeds lowered mean OV across prostate and lung plans, but the magnitude of improvement varied by treatment site and sequencing configuration. The inconsistent response in some cases suggests that increasing permitted MLC speed does not necessarily yield proportional optimisation benefits.
Further analysis will examine delivery parameters (maximum dose rate, maximum gantry/chair rotation time), the selected fractionation, individual objective values, target coverage and OAR doses to understand the mechanisms behind the observed changes.More plans will be created, and the study will be extended to additional cases and upright CT datasets when available to assess the relevance of these findings to future upright photon therapy.