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

Abstract

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.

Keywords: VMAT; beam delivery accuracy; linac gating performance; respiratory gating.

Figure 1

User interface of the Response kit gating software (Elekta AB, Stockholm, Sweden) used to simulate a variety of scenarios for beam‐on/off to perform automatic gating beam delivery using a square wave. The figure shows the signal pattern for a beam‐on of 1 second and beam‐off of 5 seconds.

Figure 2

An example of time‐resolved symmetry for gated beam delivery of 20 MU. (a) using beam‐on/off time of (1 s:1 s), (1 s:3 s), and (1 s:5 s) and (b) static beam delivery of 20 MU using segment of 2MU × 10, 4MU × 5, and 10MU × 2.