Commissioning of a motion system to investigate dosimetric consequences due to variability of respiratory waveforms

Abstract

A commercially available six-dimensional (6D) motion system was assessed for accuracy and clinical use in our department. Positional accuracy and respiratory waveform reproducibility were evaluated for the motion system. The system was then used to investigate the dosimetric consequences of respiratory waveform variation when an internal target volume (ITV) approach is used for motion management. The maximum deviations are 0.3 mm and 0.22° for translation and rotation accuracy, respectively, for the tested clinical ranges. The origin reproducibility is less than±0.1 mm. The average differences are less than 0.1 mm with a maximum standard deviation of 0.8 mm between waveforms of actual patients and replication of those waveforms by HexaMotion for three breath-hold and one free-breathing waveform. A modified gamma analysis shows greater than 98% agreement with a 0.5 mm and 100 ms threshold. The motion system was used to investigate respiratory waveform variation and showed that, as the amplitude of the treatment waveform increases above that of the simulation waveform, the periphery of the target volume receives less dose than expected. However, by using gating limits to terminate the beam outside of the simulation amplitude, the results are as expected dosimetrically. Specifically, the average dose difference in the periphery between treating with the simulation waveform and the larger amplitude waveform could be up to 12% less without gating limits, but only differed 2% or less with the gating limits in place. The general functionality of the system performs within the manufacturer's specifications and can accurately replicate patient specific waveforms. When an ITV approach is used for motion management, we found the use of gating limits that coincide with the amplitude of the patient waveform at simulation helpful to prevent the potential underdosing of the target due to changes in patient respiration.

Figure 1

HexaMotion system including the Delta⁴ phantom and the motion platform.

Figure 2

Example of waveforms for two gating limits: open limits to allow uninterrupted treatment (a) and closed limits to interrupt beam if treatment amplitude is greater than simulation amplitude (b). Horizontal lines represent the upper (blue) and lower (orange) limits. The black curve is an example of a respiratory waveform during treatment greater than the original, and the green curve is the waveform from CT simulation. The yellow shaded area indicates that the beam is on.

Figure 3

Example of translation difference for HexaMotion with movement in each of the directions (lateral, longitudinal, and vertical).

Figure 4

Measured difference in rotation for pitch and roll using two‐axis inclinometer.

Figure 5

Comparison of the original patient baseline waveform with the reproduced waveform from HexaMotion measured in the anterior–posterior, right–left, and superior–inferior directions for a breath‐hold waveform ((a)‐(c)) and free‐breathing waveform ((d)‐(f)). Please notice the different scales for the y‐axes to maximize the use of the plot area.

Figure 6

Dosimetric results of varying the amplitude from the original waveform without gating limits (a) and with gating limits (b).