End-to-end film-based characterization of gated delivery latencies in a spot-scanned synchrotron-based proton therapy...

Temporal mismatches between gating signals and beam activation-beam-on and beam-off latencies-in gated particle beam therapy can compromise dose integrity. Yet standardized measurement techniques are lacking, and system-specific characterization, particularly for synchrotron-based proton systems, remains limited.

To implement a novel strategy for precise end-to-end gating latency measurements of a spot-scanned proton beam using Gafchromic film.

Gating latencies have been measured for a Hitachi ProBeat accelerator equipped with a Varian RPM gating system. EBT4 films were positioned within a stack of acrylic blocks on a QUASAR respiratory motion platform operated with a 1.5 cm amplitude and 12 bpm sinusoidal pattern. A stainless-steel ball bearing (BB), positioned atop the film, facilitated localization. During irradiation, the BB produced a dose shadow on the film that indicated the phantom position. A proton plan consisting of a single 85.3 MeV spot was delivered to isocenter during a single gate-on/off interval initiated near the 25% phase, to maximize phantom velocity. Fifty-one measurements were made with a variety of gating intervals. Beam-on latency and delivery time produced a separation between the BB-shadow and the centroid of the Gaussian proton spot. Spot duration information was extracted from accelerator log files. BB-shadow/spot separation is modeled analytically based on phantom motion enabling the direct calculation of the total beam-on and beam-off latencies. Accelerator latency was measured directly with an oscilloscope, enabling isolation of the relative contributions of the RPM gating system.

An average beam-on latency of $142\pm 27$ ms is observed for the RPM+ProBeat system for the first gate of a delivery, which includes a known 69 ms beam extraction judgment time for the first proton spot in a spill from the Hitachi synchrotron. The corresponding average beam-off latency is $87\pm 30\text{ms}$ . The average gate-on and gate-off latencies of the RPM system are calculated as $72\pm 27\text{ms}$ and $81\pm 30\text{ms}$ , respectively. The associated average contributions of the accelerator to the beam-on latencies are measured as $69.3\pm 0.2\text{ms}$ (first gate in new spill) and $0.53\pm 0.07\text{ms}$ (subsequent gates in existing spill) and similarly $6\pm 4\text{ms}$ for beam-off latency.

A novel, high-precision method for gating-latency measurements has been implemented using film to characterize gating latencies in a spot-scanned particle beam. Commissioning and regular quality assurance methods to precisely characterize the temporal accuracy of a gating system ensure treatment accuracy and will become increasingly vital to consider for higher-frequency applications, such as cardiac gating or FLASH-capable systems.