Anthropomorphic phantom

A PseudoPatient® Prime head phantom (RTsafe, Athens, Greece) was used (Figs. 1d and 1e), and a 3D-printed anatomical replica was created using the CT image of a human head. ^{Reference Han, Diagaradjane and Luo25} Nine metallic fiducials (5 mm diameter) were placed on plastic spheric holders (10 mm, 15 mm and 25 mm diameter to differentiate them on the CT images), as shown in Figs. 1a and 1b, and randomly distributed with clinical relevant positions in the phantom (they corresponded with sizes and distances to isocentre reported on average from our institution ^{Reference Rojas-López, Díaz Moreno and Venencia4} ), avoiding overlaps. To ensure that the fiducials do not move during and after the tests, the phantom was completely and carefully filled with silicone. CT images of the immobilised phantom were acquired using a SOMATOM goUp unit (Siemens Healthinners AG, Erlangen, Germany) with the Brainlab stereotactic mask (Figs. 1c and 1f), following the intracranial SRS institutional protocol with slice thickness of 0·6 mm and artefact correction for dental prosthesis.

Figure 1. Anthropomorphic phantom for off-axis WL test. (a) Plastic holders and fiducials. (b) Phantom interior and metallic fiducials and their plastic spherical holders. (c) Frontal view of the phantom with thermoplastic mask. (d) Lateral view. (e) Frontal view. (f) Axial CT image of the phantom.

Assessment of the linac isocentre

Conventional WL test allows us to determine the match between the mechanical and radiation isocentres directly by a radiopaque marker being irradiated. Usually, the irradiated object is a ball-bearing (BB) phantom with a millimetric radius. ^{Reference Yaqoub26} The BB is aligned to the isocentre using the treatment room positioning lasers. The BB is imaged by a circular or square collimated field. The projection of the BB should be in the field in the ideal situation. The good results should show a shift ≤ 0·5 mm. ^{Reference Shepard27}

Running the WL test is a time-consuming and tedious process. It could have human errors, such as accidental movement of the couch between beams and selection of inconsistent imaging templates. The automation of the WL test process and analysis shortens the execution test time. The Developer Mode module allows users to access this functionality and operate it through XML scripts. ^{Reference Yaqoub26,Reference Alarcón and Venencia28} The Developer Mode permits access to the full set of capabilities that have been built into the TrueBeam control system, not available in the clinical modes. ^{Reference Yaqoub26} It is driven by XML beams loaded from local storage or network on the TrueBeam control console workstation computer. XML beams are essentially text scripts in XML format where a rich instruction set allows Developer Mode users to construct, deliver and imaging non-standard beams. ²⁹ Therefore, an XML script was used in the development environment of Microsoft Visual Studio Community 2017. ^{Reference Alarcón and Venencia28} The XML contains automation sequences with specific control points, beam configurations and instantaneous image acquisition for each beam. The set of gantry, couch and collimator angles used is chosen based on RIT (Radiological Imaging Technology) software (RadImage, Colorado, USA) requirements and was automatically set up without any need to get into the treatment room between fields.

The EPID must be calibrated by the IsoCal calibration inasmuch as all images are acquired by it. It is an initial adjustment, and it is carried out with some frequency. IsoCal is a licence purchased from Varian. It determines the correct location and alignment between the treatment isocentre and rotation centre of the kV/MV imaging system. ¹² Furthermore, the correction for CBCT images is through the IsoCal calibration. ^{Reference Huang, Zhao and Chetty30,31}

In our case, we used a WL pointer alignment phantom (Brainlab AG, Munich, Germany). The pointer was imaged by a square collimated field using a set of 16 EPID images with different combinations ^{Reference Clements, Beres and Shastri32} of gantry (0º, 90º, 180º and 270º), collimator (0º, 45º, 135º, 225º, 90º and 270º) and couch (0º, 45º, 90º, 270º and 315º). The test was performed on Developer Mode by automatic execution, and the images were processed on RIT v6.7 to calculate the deviation between radiation and mechanical isocentre using an algorithm ^{Reference Low, Li and Drzymala33} and the projections. Following the WL test, the 3D displacements reported by RIT were used to perform the movements that must be carried out to place the pointer in the isocentre.

Once the optimal position of the pointer is obtained, then we calibrated the X-ray imaging system by the X-ray isocentre adjustment of ExacTrac^TM. This calibration depends on the initial adjustment of the pointer in the optimum isocentre position. It was performed following Brainlab’s procedures and by the use of the ET isocentre and ET XR calibration phantoms. The calibration of this system is described in Supporting Information.

To show the mechanical error components (gantry, collimator, couch and MLC) of the linac, we performed the gantry/collimator/couch angle indicator test for 0º, 90º, 180º and 270º using a digital spirit level held against the flat reference collimator/couch surface, the star shot test using radiochromic film and the static Picket-Fence tests for 0º, 90º, 180º and 270º using EPID. The images were processed on RIT v6.7.

ExacTrac™

The ExacTrac^TM X-ray room-mounted imaging system consists of an infrared camera-based tracking system, two kV X-ray tubes recessed into the room floor and two ceiling-mounted amorphous silicon flat panel detectors (512 × 512 pixels). ExacTrac^TM is a linac-independent system. Therefore, it is essential to calibrate it for the linac isocentre.

ETv6 consists of infrared positioning and set-up system, a video monitoring system, ETv6 control software, a six-dimensional treatment bed, a pair of kV-level X-ray tubes buried underground and two corresponding amorphous silicon flat detectors with a 204 × 204-mm² radiation-sensitive area. ^{Reference Brainlab34,Reference Hua, Xu and Zhang35} The geometrical radiation field size at the isocentre is 129 × 129 mm². Its use could be referring to a patient’s anatomy using bone references or fiducials.

Recently, the ETD system, version 1.0 (Brainlab AG, Munich, Germany) was installed in our institution. It is a combined SGRT and IGRT system used for patient positioning, monitoring and tumour targeting. The system can provide intrafractional positioning information of the bony anatomy via oblique stereoscopic X-ray imaging of the patient in parallel to real-time 3D surface imaging, including thermal information, for continuous motion detection during treatment delivery. ^{Reference Shepard27,36} In contrast to other systems, only one optical camera is used by structured light scanning, but thermal information creates an additional dimension, which is assumed to improve tracking accuracy. ^{Reference Da Silva Mendes, Reiner and Huang37}

Off-Axis WL test

The CT images were imported on Eclipse v15.6 (Varian Medical Systems, Palo Alto, CA, USA). The targets (fiducial + plastic holder) were carefully contoured keeping their spherical shape. Each target was designed an off-axis WL test with eight different gantry/collimator/couch combinations, as shown in Table 1. The 2·6 × 2·6 cm² square field was defined by the MLC, and primary jaws were fixed to define a 3 × 3 cm² square field as shown in Figs. 2a, 2b. The centre of the square field is the fiducial. The isocentre was set in the mass centre of the nine targets.

Table 1. Gantry, couch and collimator angle combinations at which images of the targets were obtained

Figure 2. Off-axis WL test. (a) Beam eye view (BEV) for the combination collimator 0º, gantry 0º and couch 0º with an MLC square field of 2·6 × 2·6 cm² and jaw field of 3 × 3 cm² and (b) BEV for the combination collimator 90º, gantry 90º and couch 0º. (c) Offline review of the image acquired and the DRR. The isocentre is defined in the intersection point of the yellow orthogonal axis. (d) Horizontal profile extracted from (c).

Before performing the tests, phantom pre-positioning was done using the ExacTrac™ systems at the couch 0º position. To position with the optical surface camera, it was mandatory to select an area to track with temperature for thermal 3D information registration by the Perspective-n-Point algorithm. ³⁸ Subsequently, it is necessary to correct the position with X-rays at each couch angle. The phantom used the frameless SRS positioning array with infrared spheres and the surface for ETv6 and ETD, respectively. Then, the final position was taken by stereoscopic X-ray images with tolerances of 0·5º/0·5 mm. These tolerances are defined at isocentre, and they are considered as clinical consensus ^{Reference Meeks, Mercado and Popple39,Reference Calmels, Blak Nyrup Biancardo and Sibolt40} for intrafraction uncertainties during single-fraction MBM SRS treatments. Moreover, these are the movements that ExacTrac^TM and the robotic couch can correct during the treatment. CBCTs were acquired at couch 0º position, as a second check for pre-positioning and at the final of the test, and registered to the planning CT, confirming that the displacements were in tolerance.

Portal images were acquired for all gantry and couch angles as described in Table 1. In the cases where the couch was moved, the couch’s position was verified through stereoscopic X-rays. If the imaged position is out of tolerance in any gantry–couch combination, 6D shifts were applied and the phantom was reimaged until the residual shifts are within tolerance.

The 2D deviation between the fiducial centre and the radiation field centre, for each projection image, was found by off-line review, and comparison with the original digital reconstructed radiography (DRR) by the horizontal and vertical profiles (Figs. 2c and 2d) was done. The deviations were compared with the values reported by the toolkit QALMA. ^{Reference Rahman, Lei and Kalantzis41}

Using a formal reported method, ^{Reference Low, Li and Drzymala33} the optimal fiducial displacement from the current location in three dimensions (generally labelled dX, dY and dZ) was found that would minimise the vertical and horizontal deviations (x and y) at the projected locations. If we consider the deviations (x and y) in the gantry coordinate system where the gantry has been rotated to an angle of θ and φ defines the couch rotation angle, then it is possible to relate the deviations with the vertical (V), lateral (L) and anterior-posterior (AP) shifts as:

(1) $$\matrix{ {\left( {\matrix{ x \hfill \cr y \hfill \cr } } \right) = \;\left( {\matrix{ {vertical} \cr {horizontal} \cr } } \right)} \hfill \cr {\quad \quad \;\;\; = \left( {\matrix{ {\cos \varphi } & { - \sin \varphi } & 0 \cr {\cos \theta \sin \varphi } & {\cos \theta \cos \varphi } & {\sin \theta } \cr } } \right)\left( {\matrix{ V \cr L \cr {AP} \cr } } \right)} \hfill \cr {\quad \quad \;\;\; = A\left( {\theta ,\varphi } \right) \times \Delta .} \hfill \cr } $$

where A(θ, φ) is the 2 × 3 matrix above and Δ is the shift vector (dX, dY and dZ). For n combinations of gantry/collimator/couch, if $\eta = \left( {\matrix{ {A\left( {{\theta _1},\;{\varphi _1}} \right)} \cr \vdots \cr {A\left( {{\theta _n},\;{\varphi _n}} \right)} \cr } } \right)\;$ and $\xi = {\left( {{x_1},\;{y_1},\; \cdots ,{x_n},\;{y_n}} \right)^T},\;$ there is a unique solution that minimises Δ:

(2) $$\Delta = \;\left( {{{\left( {{\eta ^T}\eta } \right)}^{ - 1}}{\eta ^T}} \right)\xi .$$

The maximum 3D displacement was obtained following an isocentre optimisation. ^{Reference Clements, Beres and Shastri32,Reference Low, Li and Drzymala33} The details are described in RIT documentation.

1. (a) To convert 2D (x,y) into 3D (x’, y’, z’) deviations for gantry/collimator/couch combinations by polar transformation (non-rotated coordinate system).

2. (b) The gantry rotation isocentre point, P, is chosen as the midpoint of the extremes in each projection axis. This location was chosen because it minimises the maximum deviation at any point in space:

   (3) $$P = \;\left( {\matrix{ {{P_x}} \cr {{P_y}} \cr {{P_z}} \cr } } \right) = \left( {\matrix{ {{\raise0.7ex\hbox{${\left( {\max x' + \min x'} \right)}$} \!\mathord{\left/ {\vphantom {{\left( {\max x' + \min x'} \right)} 2}}\right.}\!\lower0.7ex\hbox{$2$}}} \cr {{\raise0.7ex\hbox{${\left( {\max y' + \min y'} \right)}$} \!\mathord{\left/ {\vphantom {{\left( {\max y' + \min y'} \right)} 2}}\right.}\!\lower0.7ex\hbox{$2$}}} \cr {{\raise0.7ex\hbox{${\left( {\max z' + \min z'} \right)}$} \!\mathord{\left/ {\vphantom {{\left( {\max z' + \min z'} \right)} 2}}\right.}\!\lower0.7ex\hbox{$2$}}} \cr } } \right)\;$$

3. (c) To take the differences between the gantry rotation isocentre point and each deviation.

4. (d) The effects of the gantry, collimator, and couch rotation have been minimised, and calculations of the deviations between beam centre and fiducial for all possible rotations can be determined and reported. The maximum deviation given all possible combinations can then be reported following (c).

The CBCT images were co-registered with the planning CT. The deviations with the vertical, lateral and AP shifts were measured, and the 3D displacement was calculated as the Euclidean distance.

The 3D displacements calculated between the two versions of ExacTrac^TM were compared by the use of Mann–Whitney–Wilcoxon test in Python v.3.11 with significance level of α  = 0·05.

Finally, rotational errors of 0·5º were induced in the three directions (roll, pitch or yaw) and the off-axis WL tests were taken and analysed.