From phase‐based to displacement‐based gating: a software tool to facilitate respiration‐gated radiation treatment

Abstract

The Varian Real‐time Position Management (RPM) system allows respiratory gating based on either the phase or displacement (amplitude) of the breathing waveform. A problem in clinical application is that phase‐based gating, required for respiration‐correlated (4D‐CT) simulation, is not robust to irregular breathing patterns during treatment, and a widely used system version (1.6) does not provide an easy means to change from a phase‐based gate into an equivalent displacement‐based one. We report on the development and evaluation of a robust method to convert phase‐gate thresholds, set by the physician, into equivalent displacement‐gate thresholds to facilitate its clinical application to treatment. The software tool analyzes the respiration trace recorded during the 4D‐CT simulation, and determines a relationship between displacement and phase through a functional fit. The displacement gate thresholds are determined from an average of two values of this function, corresponding to the start and end thresholds of the original phase gate. The software tool was evaluated in two ways: first, whether in‐gate residual target motion and predicted treatment beam duty cycle are equivalent between displacement gating and phase gating during 4D‐CT simulation (using retrospective phase recalculation); second, whether residual motion is improved with displacement gating during treatment relative to phase gating (using real‐time phase calculation). Residual target motion was inferred from the respiration traces and quantified in terms of mean and standard deviation in‐gate displacement measured relative to the value at the start of the recorded trace. For retrospectively‐calculated breathing traces compared with real‐time calculated breathing traces, we evaluate the inaccuracies of real‐time phase calculation by measuring the phase gate position in each trace as well as the mean in‐gate displacement and standard deviation of the displacement. Retrospectively‐calculated data from ten patients were analyzed. The patient averaged in‐gate mean ± standard deviation displacement (representing residual motion) was reduced from 0.16±0.14cm for phase gating under simulation conditions to 0.12±0.08cm for displacement gating. Evaluation of respiration traces under treatment conditions (real‐time phase calculation) showed that the average displacement gate threshold results in a lower in‐gate mean and residual motion (variance) for all patients studied. The patient‐averaged in‐gate mean ± standard deviation displacement was reduced from 0.26±0.18cm for phase gating (under treatment conditions) to 0.15±0.09cm for displacement gating. Real‐time phase gating sometimes leads to gating on incorrect portions of the breathing cycle when the breathing trace is irregular. Displacement gating is less prone to such errors, as evidenced by the lower in‐gate residual motion in a large majority of cases.In terms of duty cycle and residual motion, displacement‐based gating is equivalent to phase‐based gating for retrospectively‐calculated phase informatio...

Quantifying the image quality and dose reduction of respiratory triggered 4D cone-beam computed tomography with patient-measured breathing

“…We have used the real-time phase signal from the RPM system because it is in current use for clinical gating systems. The real-time phase from the RPM system is known to contain significant errors (Santoro et al 2009); however, the work of Ruan et al (2009) on real-time phase calculation methods demonstrate improvements that would help address these issues. The 'ground truth' image for the XCAT simulations is taken to be the tumour in the middle of its trajectory between inhale and exhale and is taken from the middle of the range of the set of 50 XCAT phantoms.…”

“…The dots represent where projections are acquired for a selected mid-inhale phase. The spread of dots is reflective of the difficulties in calculating real-time phase which leads to intermittent assignment of the gating phase interval to an incorrect portion of the respiratory trace (Santoro et al 2009). The simulations incorporate these difficulties associated with real-time phase calculation and the accompanying inconsistencies of tumour motion by directly using the real-time RPM data.…”

Pancreatic cancer and SBRT: A new potential option?

“…The purpose of respiratory gating is to define treatment margins to account for motion, select appropriate gate intervals for gated treatment and limit treatment-related toxicities. 38 Alternatively, abdominal compression using a pneumatic abdominal compression belt developed in-house can be used to reduce respiratory motion of patients undergoing SBRT for LAPC. 39 Treatment is delivered using intensity-modulated radiation therapy (IMRT), which allows selective delivery of high doses of radiation to the region of interest with steep dose gradients at the transition to adjacent normal tissues (Fig.…”

Radiation Treatment for Gastric Cancer

“…More recently, there are novel motion-management techniques to account for respiratory motion, such as respiratory gating. Respiratory gating allows for reduction in the treatment margin and the consequent radiation exposure to normal tissues, thereby limiting treatment-related toxicities [26] by just turning the radiation beam on at a certain time during the breathing cycle when the tumor or postoperative bed is in a certain position. To facilitate confirmation that the radiation beam is turning on at the appropriate time during the respiratory cycle, radio-opaque markers or operative clips can be visualized on daily pretreatment x-rays to confirm that the position of the tumor or postoperative bed is in the treatment field.…”