Characteristics of gated treatment using an optical surface imaging and gating system on an Elekta linac

Abstract

BackgroundKnowing the technical characteristics of gated radiotherapy equipment is crucial for ensuring precise and accurate treatment when using techniques such as Deep-Inspiration Breath-Hold and gating under free breathing. With one of the first installations of the novel surface imaging system Catalyst™ (C-RAD AB, Sweden) in connection with an Elekta Synergy linear accelerator (Elekta AB, Sweden) via the Elekta Response Interface, characteristics like dose delivery accuracy and time delay were investigated prior to clinical implementation of gated treatments in our institution.MethodsIn this study a moving phantom was used to simulate respiratory motion which was registered by the Catalyst™ system. The gating level was set manually. Within this gating window a trigger signal is automatically sent to the linac initiating treatment delivery. Dose measurements of gated linac treatment beams with different gating levels were recorded with a static 2D-Diode Array (MapCheck2, Sun Nuclear Co., USA) and compared to ungated reference measurements for different field sizes. In addition, the time delay of gated treatment beams was measured using radiographic film.ResultsThe difference in dose delivery between gated and ungated treatment decreases with the size of the chosen gating level. For clinically relevant gating levels of about 30%, the differences in dose delivery accuracy remain below 1%. In comparison with other system configurations in literature, the beam-on time delay shows a large deviation of 851 ms ± 100 ms.ConclusionsWhen performing gated treatment, especially for free-breathing gating, factors as time delay and dose delivery have to be evaluated regularly in terms of a quality assurance process. Once these parameters are known they can be accounted and compensated for, e.g. by adjusting the pre-selected gating level or the internal target volume margins and by using prediction algorithms for breathing curves. The usage of prediction algorithms becomes inevitable with the high beam-on time delay which is reported here.

A novel electron accelerator for MRI‐Linac radiotherapy

“…In the case of systems which do not use triode guns, rather long beam on/off latencies have been observed. 28 The current system would likely exhibit similar latencyhowever, it is worth noting that the same criticism can be made of currently proposed MRI-Linac systems, which appear to be using diode rather than triode guns at this stage. There have also been triode based RF guns proposed-such a structure may be able to improve the system latency to that achievable with DC triode guns.…”

Low‐cost Kinect Version 2 imaging system for breath hold monitoring and gating: Proof of concept study for breast cancer VMAT radiotherapy

“…Hardware latency causes a delay between a beam on signal and the commencement of radiation delivery, and vice versa for beam off. Previous studies have found beam‐on delays ranging between 220 ms18 and nearly 1 s 17. This raises questions about the dosimetric consequences of gating an Elekta linac during free breathing.…”

“…A separate set of experiments was carried out to investigate whether the depth signal from a Kinect v2 system could be used to gate an Elekta linac via the previously tested Elekta Response ™ gating interface 16, 17. See Fig.…”

Performance of a clinical gridded electron gun in magnetic fields: Implications for MRI‐linac therapy

“…On linear accelerators which use diode guns, beam gating must be carried out by radiofrequency (RF) pulse interruption methods, and this appears to result in quite long beam on/off latency. 9,10 Triode guns are also important for treatment machines which provide multiple treatment energies, and both photon and electron treatment modes. This is because both the target heating and the dose rate are correlated with the product of beam current and beam energy.…”