Methodological study of spot-isocenter performance verification for pencil-beam scanning proton therapy systems

In this work, we developed a spot-isocenter quality assurance method for pencil-beam scanning (PBS) proton therapy systems using a custom-designed isocenter spot-analysis phantom. Since the phantom cannot be readily combined with existing 2D array detectors at non-horizontal/non-vertical gantry angles, EBT4 film has been introduced to measure the deviation between the spot isocenter and the room-laser isocenter, used a UV–visible spectrophotometer to determine the optimal color channel, and established a standardized image-analysis workflow and parameter-selection protocol. Experimental results at different gantry angles demonstrate that the combined use of the phantom and EBT4 film can accurately resolve deviations between the spot isocenter and the room-laser isocenter, with a maximum observed deviation of 0.88 mm, which is below the 1 mm tolerance limit. This method meets the “radiation-beam isocenter deviation” quality assurance criterion specified in WS 816—2023 and is particularly well suited for multi-angle isocenter verification in PBS systems, providing methodological guidance for proton therapy centers seeking to establish standardized, reproducible QA workflows.