Evaluation of a combined respiratory-gating system comprising the TrueBeam linear accelerator and a new real-time tumor-tracking radiotherapy system: a preliminary study

Abstract

A combined system comprising the TrueBeam linear accelerator and a new real-time, tumor-tracking radiotherapy system, SyncTraX, was installed in our institution. The goals of this study were to assess the capability of SyncTraX in measuring the position of a fiducial marker using color fluoroscopic images, and to evaluate the dosimetric and geometric accuracy of respiratory-gated radiotherapy using this combined system for the simple geometry. For the fundamental evaluation of respiratory-gated radiotherapy using SyncTraX, the following were performed:1) determination of dosimetric and positional characteristics of sinusoidal patterns using a motor-driven base for several gating windows; 2) measurement of time delay using an oscilloscope; 3) positional verification of sinusoidal patterns and the pattern in the case of a lung cancer patient; 4) measurement of the half-value layer (HVL in mm AL), effective kVp, and air kerma, using a solid-state detector for each fluoroscopic condition, to determine the patient dose. The dose profile in a moving phantom with gated radiotherapy having a gating window ≤ 4 mm was in good agreement with that under static conditions for each photon beam. The total time delay between TrueBeam and SyncTraX was < 227 ms for each photon beam. The mean of the positional tracking error was < 0.4 mm for sinusoidal patterns and for the pattern in the case of a lung cancer patient. The air-kerma rates from one fluoroscopy direction were 1.93 ± 0.01, 2.86 ± 0.01, 3.92 ± 0.04, 5.28 ± 0.03, and 6.60 ± 0.05 mGy/min for 70, 80, 90, 100, and 110 kV X-ray beams at 80 mA, respectively. The combined system comprising TrueBeam and SyncTraX could track the motion of the fiducial marker and control radiation delivery with reasonable accuracy; therefore, this system provides significant dosimetric improvement. However, patient exposure dose from fluoroscopy was not clinically negligible.

Figure 1

Photograph of combined system (a) comprising TrueBeam and SyncTraX at the Yamaguchi University hospital. SyncTraX consists of two X‐ray tubes and two color image intensifiers (I.I.s). Using the X‐ray tubes and color I.I.s along two directions (b), the position of a fiducial marker close to the tumor is automatically extracted using a pattern recognition technique to calculate the three‐dimensional (3D) coordinates for color fluoroscopic images.

Figure 2

Example of color fluoroscopic image from one X‐ray tube: (a) when the actual fiducial marker's position is not within several millimeters of the planned 3D marker's position, the treatment MV beam is turned off; (b) when the actual fiducial marker's position is within several millimeters of the planned 3D marker's position, the MV treatment beam is turned on and the delivery proceeds.

Figure 3

Photograph of the experimental setup (a) for determining dosimetric and positional characteristics of sinusoidal patterns using a motor‐driven base for several gating windows. Example of an oscilloscope image (b) for respiratory‐gated radiotherapy using SyncTraX. The yellow, green, and pink lines show the fluoroscopic signal from SyncTraX, the gate signal from SyncTraX, and the beam‐on signal from TrueBeam, respectively. Photograph of the experimental setup (c) for the positional verification of sinusoidal patterns and the pattern in the case of a lung cancer patient. Photograph of the experimental setup (d) for the measurement of air kerma, kVp, and half‐value layer (HVL in mm AL) using a solid‐state detector for various fluoroscopic conditions.

Figure 4

Dose profiles of a $50\times 50\hspace{0.17em}{\text{mm}}^2$ field under static condition, gating with several gating windows, and nongating states for a sinusoidal pattern: (a) 6 MV‐FF, (b) 6 MV‐FFF, (c) 10 MV‐FF, and (d) 10 MV‐FFF. The gating delivery had gating windows of 2, 4, 6, 8, and 10 mm. The label “non‐gating” indicates nongating delivery measured on a respiratory motion phantom, while “static” corresponds to a static phantom for the nongating beam. To compare the profiles, all dose profiles are shifted to align their radiation field edge (50% level) with the static profile.

Figure 5

Relationship between the gamma pass rate of ${\gamma }_{2\%/2\hspace{0.17em}\text{mm}}$ and gating windows for (a) 6 MV‐FF and 6 MV‐FFF photon beams and (b) 10 MV‐FF and 10 MV‐FFF photon beams. When the gating window was $\le 4\hspace{0.17em}\text{mm}$, the gamma pass rate of of ${\gamma }_{2\%/2\hspace{0.17em}\text{mm}}$ was $\ge 90\%$ for each photon beam. On the other hand, when the gating window was $>4\hspace{0.17em}\text{mm}$, the gamma pass rate of ${\gamma }_{2\%/2\hspace{0.17em}\text{mm}}$ was $\le 90\%$ for each photon beam.

Figure 6

Variations in the measured and tracked positions of the fiducial marker for sinusoidal patterns with (a) $\mathrm{A}=20\hspace{0.17em}\text{mm}$, $\mathrm{T}=2\hspace{0.17em}\mathrm{s}$, and (b) $\mathrm{A}=20\hspace{0.17em}\text{mm}$, $\mathrm{T}=4\hspace{0.17em}\mathrm{s}$; (c) the pattern in the case of a lung cancer patient. For the sinusoidal patterns, the mean ± SD values of absolute ${\mathrm{E}}_{\mathrm{p}}$ were $0.31\pm 0.20\hspace{0.17em}\text{mm}$ ($\mathrm{A}=20\hspace{0.17em}\text{mm}$, $\mathrm{T}=2\hspace{0.17em}\mathrm{s}$) and $0.13\pm 0.09\hspace{0.17em}\text{mm}$ ($\mathrm{A}=20\hspace{0.17em}\text{mm}$, $\mathrm{T}=4\hspace{0.17em}\mathrm{s}$), and that for pattern in the case of the lung cancer patient was $0.16\pm 0.10\hspace{0.17em}\text{mm}$. The blue line and red dashed line show the measured and tracked positions of the fiducial marker, respectively, while the green line shows the positional tracking error.

Figure 7

Relationship between the tube current of the X‐ray tube and air‐kerma rate along one fluoroscopy direction. The air‐kerma rates were $1.93\pm 0.01$, $2.86\pm 0.01$, $3.92\pm 0.04$, $5.28\pm 0.03$, and $6.60\pm 0.05\hspace{0.17em}\text{mGy}/\text{min}$ for the nominal 70, 80, 90, 100, and 110 kV X‐ray beams at 80 mA, respectively.