Delivery efficiency of an Elekta linac under gated operation

The accurate delivery of respiratory‐gated volumetric modulated arc therapy (VMAT) treatment plans presents a challenge since the gantry rotation and collimator leaves must be repeatedly stopped and set into motion during each breathing cycle. In this study, we present the commissioning process for an Anzai gating system (AZ‐733VI) on an Elekta Versa HD linear accelerator and make recommendations for successful clinical implementation. The commissioning tests include central axis dose consistency, profile consistency, gating beam‐on/off delay, and comparison of gated versus nongated gamma pass rates for patient‐specific quality assurance using four clinically commissioned photon energies: 6 MV, 6 FFF, 10 MV, and 10 FFF. The central axis dose constancy between gated and nongated deliveries was within 0.6% for all energies and the analysis of open field profiles for gated and nongated deliveries showed an agreement of 97.8% or greater when evaluated with a percent difference criteria of 1%. The measurement of the beam‐on/off delay was done by evaluating images of a moving ball‐bearing phantom triggered by the gating system and average beam‐on delays of 0.22–0.29 s were observed. No measurable beam‐off delay was present. Measurements of gated VMAT dose distributions resulted in decrements as high as 9% in the gamma passing rate as compared to nongated deliveries when evaluated against the planned dose distribution at 3%/3 mm. By decreasing the dose rate, which decreases the gantry speed during gated delivery, the gamma passing rates of gated and nongated treatments can be made equivalent. We present an empirically derived formula to limit the maximum dose rate during VMAT deliveries and show that by implementing a reduced dose rate, a gamma passing rate of greater than 95% (3%/3 mm) was obtained for all plan measurements.

Section: Discussionsupporting

confidence: 92%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Implementation of respiratory‐gated VMAT on a Versa HD linear accelerator

Snyder

Flynn

Hyer

2017

“…In gated beam delivery, the linear accelerator beam is typically triggered on and off at either full‐inspiration or end‐expiration. The user determines a gating window, and radiation is only delivered during a specified phase of the breathing cycle 10. One common approach to gated beam delivery is the use of a deep‐inspiration breath hold (DIBH) technique for left‐sided breast cancers with a goal of minimizing the dose to the heart and lung 9.…”

Section: Introductionmentioning

confidence: 99%

“…More recently, the gating characteristics (e.g., beam profile and beam delivery efficiency) have been evaluated for Elekta Precise and Synergy linacs 10, 19. In this study, we have focused on the beam startup characteristics for gated delivery of an Elekta linac and the overall accuracy of the delivery for a variety of gating scenarios.…”

Section: Introductionmentioning

confidence: 99%

Does gated beam delivery impact delivery accuracy on an Elekta linac?

Jermoumi¹,

Xie²,

Cao³

et al. 2017

In this study, we evaluated the performance of an Elekta linac in the delivery of gated radiotherapy. Delivery accuracy was examined with an emphasis on the impact of using short gating windows (low monitor unit beam‐on segments) or long beam hold times. The performance was assessed using a 20cm by 20cm open field with the radiation delivered using a range of beam‐on and beam‐off time periods. Gated delivery measurements were also performed for two SBRT plans delivered using volumetric modulated arc therapy (VMAT). Tests included both free‐breathing based gating (covering a variety of gating windows) and simulated breath‐hold based gating. An IBA MatriXX 2D ion chamber array was used for data collection, and the gating accuracy at low MU was evaluated using gamma passing rates. For the 20 cm by 20 cm open field, the measurements generally showed close agreement between the gated and non‐gated beam deliveries. Discrepancies, however, began to appear with a 5‐to‐1 ratio of the beam‐off to beam‐on times. The discrepancies observed for these tight gating windows can be attributed to the small number of monitor units delivered during each beam‐on segment. Dose distribution analysis from the delivery of the two SBRT plans showed gamma passing rates (± 1%, 2%/1 mm) in the range of 95% to 100% for gating windows of 25%, 38%, 50%, 63%, 75%, and 83%. Using a simulated sinusoidal breathing signal with a 4 second period, the gamma passing rate of free‐breathing gating and breath‐hold gating deliveries were measured in the range of 95.7% to 100%. In conclusion, the results demonstrate that Elekta linacs can accurately deliver respiratory gated treatments for both free‐breathing and breath‐hold patients. Some caution should be exercised with the use of very tight gating windows.

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

Edmunds

Gothard

Khabra

et al. 2018

Voluntary inspiration breath hold (VIBH) for left breast cancer patients has been shown to be a safe and effective method of reducing radiation dose to the heart. Currently, VIBH protocol compliance is monitored visually. In this work, we establish whether it is possible to gate the delivery of radiation from an Elekta linac using the Microsoft Kinect version 2 (Kinect v2) depth sensor to measure a patient breathing signal. This would allow contactless monitoring during VMAT treatment, as an alternative to equipment–assisted methods such as active breathing control (ABC). Breathing traces were acquired from six left breast radiotherapy patients during VIBH. We developed a gating interface to an Elekta linac, using the depth signal from a Kinect v2 to control radiation delivery to a programmable motion platform following patient breathing patterns. Radiation dose to a moving phantom with gating was verified using point dose measurements and a Delta4 verification phantom. 60 breathing traces were obtained with an acquisition success rate of 100%. Point dose measurements for gated deliveries to a moving phantom agreed to within 0.5% of ungated delivery to a static phantom using both a conventional and VMAT treatment plan. Dose measurements with the verification phantom showed that there was a median dose difference of better than 0.5% and a mean (3% 3 mm) gamma index of 92.6% for gated deliveries when using static phantom data as a reference. It is possible to use a Kinect v2 device to monitor voluntary breath hold protocol compliance in a cohort of left breast radiotherapy patients. Furthermore, it is possible to use the signal from a Kinect v2 to gate an Elekta linac to deliver radiation only during the peak inhale VIBH phase.