Journal of applied clinical medical physics
End-to-end film-based characterization of gated delivery latencies in a spot-scanned synchrotron-based proton therapy system
By Stottler ZS, Kruse JJ and Deisher AJ.
Abstract
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.