End‐to‐end film‐based characterization of gated delivery latencies in a spot‐scanned synchrotron‐based proton therapy system
Z. S. Stottler, J. J. Kruse, A. J. DeisherAbstract
Background
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.
Purpose
To implement a novel strategy for precise end‐to‐end gating latency measurements of a spot‐scanned proton beam using Gafchromic film.
Methods
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.
Results
An average beam‐on latency of 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 . The average gate‐on and gate‐off latencies of the RPM system are calculated as and , respectively. The associated average contributions of the accelerator to the beam‐on latencies are measured as (first gate in new spill) and (subsequent gates in existing spill) and similarly for beam‐off latency.
Conclusion
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.