Effects of collimator angle, couch angle, and starting phase on motion-tracking dynamic conformal arc therapy (4D DCAT)

Abstract

Purpose: The aim of this study was to find an optimized configuration of collimator angle, couch angle, and starting tracking phase to improve the delivery performance in terms of MLC position errors, maximal MLC leaf speed, and total beam-on time of DCAT plans with motion tracking (4D DCAT).

Method and materials: Nontracking conformal arc plans were first created based on a single phase (maximal exhalation phase) of a respiratory motion phantom with a spherical target. An ideal model was used to simulate the target motion in superior-inferior (SI), anterior-posterior (AP), and left-right (LR) dimensions. The motion was decomposed to the MLC leaf position coordinates for motion compensation and generating 4D DCAT plans. The plans were studied with collimator angle ranged from 0° to 90°; couch angle ranged from 350°(-10°) to 10°; and starting tracking phases at maximal inhalation (θ=π/2) and exhalation (θ=0) phases. Plan performance score (PPS) evaluates the plan complexity including the variability in MLC leaf positions, degree of irregularity in field shape and area. PPS ranges from 0 to 1, where low PPS indicates a plan with high complexity. The 4D DCAT plans with the maximal and the minimal PPS were selected and delivered on a Varian TrueBeam linear accelerator. Gafchromic-EBT3 dosimetry films were used to measure the dose delivered to the target in the phantom. Gamma analysis for film measurements with 90% passing rate threshold using 3%/3 mm criteria and trajectory log files were analyzed for plan delivery accuracy evaluation.

Results: The maximal PPS of all the plans was 0.554, achieved with collimator angle at 87°, couch angle at 350°, and starting phase at maximal inhalation (θ=π/2). The maximal MLC leaf speed, MLC leaf errors, total leaf travel distance, and beam-on time were 20 mm/s, 0.39 ± 0.16 mm, 1385 cm, and 157 s, respectively. The starting phase, whether at maximal inhalation or exhalation had a relatively small contribution to PPS (0.01 ± 0.05).

Conclusions: By selecting collimator angle, couch angle, and starting tracking phase, 4D DCAT plans with the maximal PPS demonstrated less MLC leaf position errors, lower maximal MLC leaf speed, and shorter beam-on time which improved the performance of 4D motion-tracking DCAT delivery.

Keywords: DCAT; MLC; motion tracking.

Figure 1

Respiratory motion phantom and setup. (a) Cedar insert with two off‐center hemispherical tumor phantoms. (b) The Cedar insert was hinged to the motor in the QUASAR phantom with a rotational stage to simulate 3D motion (photos were retrieved from http://modusqa.com/radiotherapy/phantoms/respiratory-motion, Modus Medical Device Inc., on October 22, 2016). (c) Two red markers were attached to the insert so that their positions could be tracked by two cameras.

Figure 2

Illustration of spherical target motion according to the 3D rigid motion model. Blue circles represent the positions of target mass center at different time points. If the target starts to move from position “a,” then if follows the sequence of “a‐b‐c‐d‐e‐f‐g‐h‐a.”

Figure 3

Flowchart of generating a deliverable 4D DCAT plan.

Figure 4

The effect of collimator and couch angle configuration on PPS of 4D DCAT plans. Each pixel value represents the PPS of a 4D DCAT plan with specific collimator and couch angles combination.

Figure 5

Passing rates of ArcCHECK measurements of each arc in 4D DCAT plans and nontracking DCAT plans.

Figure 6

RMSE of MLC leaves (1–120) for 4D DCAT plans. Blue: 4D DCAT with the minimal PPS (0.197). Orange: 4D DCAT with the maximal PPS (0.554). Gray: nontracking DCAT plans (PPS≈1).

Figure 7

An example of gamma analyses for film measurements of 4D DCAT plans. (a) Passing rates of 4D DCAT plan with the maximal PPS: 99.1%. (b) Passing rates of 4D DCAT plan with the minimal PPS: 97.1%. Failed points are those pixels in the figure with gamma index >1.

Figure 8

Gamma Passing rates of three repeated film measurements of 4D DCAT plans. Soft square: 4D with the maximal PPS; Solid square: 4D DCAT with the minimal PPS.

Figure 9

Evaluation of dosimetric effect caused by MLC errors during tracking. Circular: nontracking DCAT plan. Triangular: nontracking DCAT plan with MLC errors from 4D DCAT (PPS = 0.554) delivery; Square: nontracking DCAT plan with MLC errors from 4D DCAT (PPS = 0.197) delivery. Red lines: target. Blue lines: left lung. Green lines: spine. Purple lines: body. Orange lines: heart.

Figure Figure A1

Illustration of room coordinates in motion tracking.