In recent years, the number of high-precision radiotherapy procedures such as stereotactic radiosurgery (SRS) has been increasing due to the use of high-precision linear accelerators. In radiotherapy for multiple brain tumors, the risk of tumor dose reduction due to poor alignment has been reported [1-3]. Task Group 24,22,American Association of Physicists in Medicine (AAPM TG24,22) [4], the causes of poor beam alignment are reported as , displacement of the X-ray focus, asymmetry of the collimator, angle of incidence of accelerated electrons on the X-ray target surface, and displacement of the rotation axis of the gantry and collimator [5].

These test methods include the Winston-Lutz (WL) test [6-8], which measures the center of gravity, or radiation center, of MV X-rays when mechanically rotating components such as gantries and collimators are in operation. However, even if the mechanical rotation of the component is proper, any displacement in the focus of the MV X-ray may be measured as a displacement in the axis of rotation [9]. The relationship between alignment and components is similar for gantry-mounted image-guided radiation therapy (IGRT) devices [10-12]. The purpose of this study is to verify the feasibility of a new isocenter measurement method using the QCT test.

The device has the function to perform the light/radiation field coincidence test, and the isocenter test at the same time. In addition, the Winston-Lutz test can be performed [13].

Design of QCT

The QCT has a hollowed-out interior and walls of a cube. It is a square with a side length of 20 cm, and the wall consists of 5 cubic faces of 5.0 mm-thick meta-acrylic resin sheets (Figure 1). A 1.0 mm wide slit is carved crosswise in the center of each wall. A brass Ball bearing (BB) with a diameter of 2.0 mmφ is placed in the center of the QCT, and its position can be fine-adjusted using a micrometer. The QCT pedestal is equipped with an angle adjustment screw to correct the tilt of itself. Figure 2 shows QCT images taken with 6 MV X-rays and 80 kV X-rays. If there is a displacement of the isocenter, the center of the BB does not overlap with the cross slits on the beam injection wall and the injection wall of the QCT in the captured image.

Figure 1: Appearance details of the developed device of QCT phantom (a) QCT phantom has composed a cubic with five plates on which each plate with thickness 5.0 mm has cross slit of width 1.0 mm and geometric figure (b) Micrometer with 3 axes (c) A brass 2.0 mmφ to set up to isocenter with triaxial micrometer (d) Wall mounted horizontal Crossed laser (e) Adjusted the leveling of three adjustment devices at the bottom.

(a)  MVX-ray image at 270 ° (EPID)   (b) kVX-ray image at 270° (OBI)

Figure 2: QCT phantom images are kV and MVX-ray images (a) Linacgraphy at gantry angle 270° in proper alignment (b) kVX-ray image at source angle 270° in improper alignment. The shoulder of the Figure (a) (b) shows an enlarged view of the dashed line.

Theory

The displacement of the focal spot position in the X-ray target plane is obtained by correcting the magnification factor and calculating the gradient due to the slit and BB of the QCT projected on the image, the source to imager distance (SID) at the time of imaging10). Figure 3 shows a diagram of an accelerated electron beam striking an arbitrary point r, which is the focal spot point. When the magnification factor of QCT is E= (d₁+d₂)/d₁, the displacement of the focal spot point position Δr is obtained from the distance r' from the X-ray beam center axis (Figure 3).

Cross slit imaging through QCT phantom from focal spot

Figure 3: Diagram of a linac head (schematic and not to scale) and illustration of radiation focal spot position determination using the EPID. Vertical black line represents the vertical leaser axis as a matter of convenience. Dot line represents actual focal spot position and the geometric relationship of the EPID and QCT phantom at isocenter. Δr: Focal spot position on target plane Δx: Displacement distance for Cross-plane on target plane Δy: Displacement distance for In-plane on target plane d1: Distance from the X-ray target (focal spot) to the isocenter d2: Distance from the isocenter to the EPID θ: Angle from vertical laser to x-ray beam center axis.

The displacement Δr of the focal spot of the X-ray target plane is calculated from the components in the ???? (Cross plane) and ???? (In-plane) directions.

The relation between the displacement of the focal spot r and the displacement angle ???? is as follows, from the perpendicular line on the isocenter and the distance ????₁ to the X- ray focal plane.

QCT Setup Testing

The QCT was placed on the treatment couch using the three-screw adjuster placed on the bottom of the QCT to keep it horizontal. A digital level of Smart Tool (MD-Building products USA, accuracy: 1/10°) was used to check the level. The placement of the QCT was fine-tuned so that the cross slits on the beam injection and injection walls pass through the crosses of the wall and ceiling-mounted laser (TAKEX ALPC series, Japan). Figure 4 is the flow of QCT from setup on the treatment couch to Graticule measurement.

(a) Laser light is transmitted across the cross section of the QCT device and set up on the treatment couch.

(b) Measure the distance from Graticule to BB on the monitor image.

Figure 4: Flow of QCT device from setup on the treatment couch to Graticule measurement.

Couch Shifting Test Simulating Focal Shift Using QCT

Since the focus is usually at the center of the Jaw collimator, it is difficult to determine the displacement of the focus only. In order to simulate the displacement of the focal spot position, the couch with the QCT was moved for image collection. The distance of the couch was moved at 0.1 mm intervals and images were collected.

QCT Setup Accuracy

As for the setup accuracy, Figure 5 show the measured distances for the QCT test. The mean and standard error of the setup with respect to the laser beam was 0.17 ± 0.16 mm for 0-90°. The images in the monitor in Figure 6 are obtained by shifting the treatment couch by ±0.5 mm to the left and right, respectively, from the position where the crosshairs of the graticule and the center of the QCT are aligned.

Figure 5: Monitor image of Graticule and QCT phantom (a) BB center moved 0.5 mm to the right (b) Graticule crosshairs coincide with BB center of QCT phantom (c) BB center moved 0.5 mm to the left.

Figure 6: Setup accuracy in OBI images of kV X-rays.

Couch Shifting Test Simulating Focal Shift Using QCT

Figure 7 shows that the correlation between the distance the treatment couch was moved and the distance of the graticule on the monitor image was very high at 0.999.

Figure 7: Correlative relationship between couch and Graticule when the displacement of the QCT upper and lower slits is simulated of linacgraphy of 6 kV X-ray.

Verification of Focus Shift by Accelerator

Assuming that the distance of the accelerator couch coordinate shift is the displacement distance of the focal spot, it was coordinates measured from the isocenter were within 0.57 mm (Figure 8) (Table).

Figure 8: DICOM images of gantry angle for 6 MV X-ray.

Table 1: Focal spot position of 6 MV X-ray (Figures 3, 8).

In this study, we developed a QCT device that can measure the alignment of a medical linear accelerator and estimate the displacement of the focal spot of the accelerator. AAPM TG-142 [14] and guidelines [15-17] recommend mechanical/dosimetric isocenters for SRS within ± 1.0 mm from baseline for intensity-modulated radiation therapy and ± 2.0 mm for conventional irradiation methods. There are a wide variety of factors that contribute to the variation of accelerator baselines [6,7,18-21]. The measurement of isocenters is also subject to errors due to changes in mechanical/radiological properties of the accelerator, changes in the graticule due to calibration, and changes in the laser beam over time.

QCT for accelerator alignment has made it possible to measure both the isocenter and the displacement of the X-ray focus position from the baseline.

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