Line scanning represents a faster and potentially more flexible form of pencil beam scanning than conventional step-and-shoot irradiations. It seeks to minimize dead times in beam delivery whilst preserving the possibility of modulating the dose at any point in the target volume. Our second generation proton gantry features irradiations in line scanning mode, but it still lacks a dedicated monitoring and validation system that guarantees patient safety throughout the irradiation. We report on its design and implementation in this paper. In line scanning, we steer the proton beam continuously along straight lines while adapting the speed and/or current frequently to modulate the delivered dose. We intend to prevent delivery errors that could be clinically relevant through a two-stage system: safety level 1 monitors the beam current and position every 10 μs. We demonstrate that direct readings from ionization chambers in the gantry nozzle and Hall probes in the scanner magnets provide required information on current and position, respectively. Interlocks will be raised when measured signals exceed their predefined tolerance bands. Even in case of an erroneous delivery, safety level 1 restricts hot and cold spots of the physically delivered fraction dose to  ±[Formula: see text] (±[Formula: see text] of [Formula: see text] biologically). In safety level 2-an additional, partly redundant validation step-we compare the integral line profile measured with a strip monitor in the nozzle to a forward-calculated prediction. The comparison is performed between two line applications to detect amplifying inaccuracies in speed and current modulation. This level can be regarded as an online quality assurance of the machine. Both safety levels use devices and functionalities already installed along the beamline. Hence, the presented monitoring and validation system preserves full compatibility of discrete and continuous delivery mode on a single gantry, with the possibility of switching between modes during the application of a single field.

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The step-and-shoot method of pencil beam scanning delivers the dose on a threedimensional grid in the target volume, with one dimension defined by the proton energy. While the dose per pencil beam may vary substantially within an iso-energy layer, the beam current typically remains constant. In this static operation mode, the inherent latency of the beam switchoff mechanism results in a lower limit for the deliverable spot dose, which may prevent the application of some of the low-weighted spots prescribed by the treatment planning system. To overcome this limitation, we introduced dynamic beam current control at the PSI Gantry 2, an innovative new approach successfully commissioned and in clinical operation since fall 2017. The control system was enhanced with a direct link to the vertical deflector located at the centre of the cyclotron. This connection allows much faster beam current changes (~ 0.1 ms) and hence opens up the possibility of dynamically reducing the current for individual low-dose spots. We demonstrate that with this new dynamic operation mode, all spots are delivered as planned without compromising treatment time. We show by two independent and complementary methods that the delivered dose distribution is improved.

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