A novel and independent method for time-resolved gantry angle quality assurance for VMAT

Abstract

Volumetric-modulated arc therapy (VMAT) treatment delivery requires three key dynamic components; gantry rotation, dose rate modulation, and multi-leaf collimator motion, which are all simultaneously varied during the delivery. Misalignment of the gantry angle can potentially affect clinical outcome due to the steep dose gradients and complex MLC shapes involved. It is essential to develop independent gantry angle quality assurance (QA) appropriate to VMAT that can be performed simultaneously with other key VMAT QA testing. In this work, a simple and inexpensive fully independent gantry angle measurement methodology was developed that allows quantitation of the gantry angle accuracy as a function of time. This method is based on the analysis of video footage of a "Double dot" pattern attached to the front cover of the linear accelerator that consists of red and green circles printed on A4 paper sheet. A standard mobile phone is placed on the couch to record the video footage during gantry rotation. The video file is subsequently analyzed and used to determine the gantry angle from each video frame using the relative position of the two dots. There were two types of validation tests performed including the static mode with manual gantry angle rotation and dynamic mode with three complex test plans. The accuracy was 0.26° ± 0.04° and 0.46° ± 0.31° (mean ± 1 SD) for the static and dynamic modes, respectively. This method is user friendly, cost effective, easy to setup, has high temporal resolution, and can be combined with existing time-resolved method for QA of MLC and dose rate to form a comprehensive set of procedures for time-resolved QA of VMAT delivery system.

Keywords: VMAT; gantry angle; quality assurance.

Figure 1

Experimental setup for dynamic gantry angle QA (left) and printed Double Dot pattern (right).

Figure 2

Overview of methodology for gantry angle determination from each image frame of the Double Dot pattern.

Figure 3

An example of the output of the gantry angle QA system for a single frame of the video footage.

Figure 4

Design of gantry angle QA test plans: constant gantry speed (left), gantry speed transitions (middle), and maximum gantry speed inertia (right).

Figure 5

Comparison between Double Dot, encoder and inclinometer gantry angle for static gantry measurements.

Figure 6

Measured gantry angle results in test plan 1: Constant gantry speed.

Figure 7

Measured gantry angle results in test plan 2: Gantry speed transitions.

Figure 8

Measured gantry angle results in test plan 3: Maximum gantry speed inertia.