Commissioning of and preliminary experience with a new fully integrated computed tomography linac

Abstract Purpose A new medical linear accelerator (linac) platform integrated with helical computed tomography (CT), the uRT‐linac 506c, was introduced into clinical application in 2019 by United Imaging Healthcare (UIH) Co., Ltd. (Shanghai, China). It combines a Carm linac with a diagnostic‐quality 16‐slice CT imager, providing seamless workflow from simulation to treatment. The aim of this report is to assess the technical characteristics, commissioning results and preliminary experiences stemming from clinical usage. Methods The mechanical and imaging test procedures, commissioning data collection and TPS validation were summarized. CTIGRT accuracy was investigated with different loads and couch extensions. A series of end‐to‐end cases for different treatment sites and delivery techniques were tested preclinically to estimate the overall accuracy for the entire treatment scheme. The results of patient‐specific QA and machine stability during a one‐year operation are also reported. Results Gantry/couch/collimator isocentricity was measured as 0.63 mm in radius. The TPS models were in agreement with the beam commissioning data within a deviation of 2%. An overall submillimeter accuracy was demonstrated for the CT‐IGRT process under all conditions. The absolute point dose difference for all the preclinical end‐to‐end tests was within 3%, and the gamma passing rate of the 2D dose distribution measured by EBT3 film was better than 90% (3% DD, 3 mm DTA and 10% threshold). Pretreatment QA of clinical cases resulted with better than 3% point dose difference and more than 99% gamma passing rate (3% DD/2 mm DTA/10% threshold) tested with Delta4. The output of the linac was mostly within 1% of variation in a one‐year operation. Conclusion The commissioning results and clinical QA results show that the uRT‐linac 506c platform exhibits good and stable performance in mechanical and dosimetric accuracy. The integrated CT system provides an efficient workflow for image guidance with submillimeter localization precision, and will be a good starting point to proceed advanced adaptive radiotherapy.


| INTRODUCTION
Since the widespread adoption of advanced radiotherapy (RT) technologies like intensity-modulated RT (IMRT) in the 1990s, 1 accurate and highly conformal dose delivery as well as efficient clinical workflow have become increasingly essential to medical linear accelerators (linacs). In this regard, the image-guidance technique is indispensable for managing the geometric variations in patient set-up and internal organ position. [2][3][4] Various image-guided RT (IGRT) systems have been employed on modern linacs, such as kilovolt (kV) and megavolt (MV) cone-beam computed tomography (CT) (CBCT), 5,6] fan-beam MVCT, 7 CT-on-rails, 8,9 and magnetic resonance imaging (MRI). 10 As image guidance has become a clinical routine and the need for adaptive RT (ART) has grown, better image quality with sufficient soft-tissue contrast is in greater demand than ever before. While MRI is the best soft-tissue imaging modality, MRI-guided RT systems may not be able to replace conventional X-ray imaging systems due to technical challenges 11,12 and high startup costs.
Recently, a newly designed CT-integrated linac named uRT-linac 506c was introduced into the market by United Imaging Healthcare (UIH) Co., Ltd. Different from the design of a sliding CT gantry and two rotation axes of couch top in CT-on-rails systems, 8 it has a diagnostic-quality helical CT system compactly fixed behind the gantry of a C-arm linac, and the patient is sent through the scanner by moving the couch longitudinally, as shown in Figure 1 Table 1. It has a 16-slice helical CT imager coaxially attached to the gantry of a C-arm linac, with a designated longitudinal distance of 2100 mm between the treatment isocenter and CT origin. The enclosed CT bore has a diameter of 70 cm. The gantry of the linac is capable of one and a half revolutions, namely, from −362°to 182°, which is achieved by using a long slack in the cables wrapping around the barrel of the gantry. Diagram of the beam line inside the treatment head is shown in Figure 2. The accelerating tube is mounted with its axis parallel to the central axis of the radiation beam such that electron beam is accelerated and strikes the target without bending. The linac is designed to generate and deliver photon beams of two energies, i.e., the 6-MV treatment beam and the 1.5-MV imaging beam, which are produced by electron beams with peak energy of 6-or 1.5-MeV F I G . 1. Schematic view of the uRT-linac 506c platform.
incident on a high-Z or low-Z target, respectively. Since there is no beam steering, focusing nor bending, it is assumed that the electron beams of two energies hit the same position of the targets, in other words, the imaging beam and treatment beam are generated equivalently by the same source. The 6-MV treatment beam is delivered in flattened (with flattening filter, FF) and unflattened (flattening-filterfree, FFF) modes with a maximum dose rate of 600 and 1400 MU/ min, respectively, while the 1.5-MV imaging beam is in FFF mode with a dose rate of 40 MU/min. Clinical electron beam modality is not available on this machine. The treatment head is equipped with dual-layered collimating jaws and 60 pairs of MLCs with a 0.5-cm width at the isocenter in the inner 20 cm and a 1.0-cm width in the outer 20 cm, projecting a maximum field size of 40 × 40 cm 2 . The available delivery techniques include three-dimensional conformal radiation therapy (3D-CRT), step-and-shoot IMRT (sIMRT), dynamic IMRT (dIMRT), and volumetric modulated arc therapy (VMAT, named uARC on this platform with "u" the initial of UIH). Virtual wedges of 10°/15°/20°/25°/30°/45°/60°are provided using dynamic jaws. The image acquisition system consists of MV portal and MV CBCT by employing the 1.5-MV imaging beam and an amorphous silicon electronic portal imaging device (aSi-EPID, 40 × 40 cm 2 active area) and kV FBCT offered by the integrated CT system. A treatment couch with movable base is utilized for patient transportation and IGRT automatic correction. It is a four-degrees-of-freedom (4DOF) couch allowing only translations (lateral, longitudinal, vertical) and yaw rotation. To mitigate risks of collision, there are triple protection modules in software and hardware: one is a trajectory monitoring module used to avoid collisions between mechanical components of the machine (patient is not taken into account in this module), another is an infrared laser proximity sensor mounted on the gantry to assure enough clearance for patients, and the third is a ring of springloaded mechanical switches fixed on the EPID holder that supports the panel to protect the detector from being touched by the casing.
An interlocking signal will be triggered immediately to stop motion in case that any of the modules predicts/detects a collision. The gantry isocenter clearance is 46.7 cm in radius at most and the patient clearance is limited to 43 cm by the proximity sensor.
The integrated CT scanner is available as a conventional simulator when required. In other cases, it is used for image guidance prior to treatment. Figure 3 illustrates the clinical IGRT workflow of the three imaging techniques. The amount of time taken for each step is indicated in brackets. MV portal/CBCT procedures are similar to those on other linacs. In regard to the FBCT acquisition, one needs to press a button of "Go to CT position" to send the patient into the

| 211
For the running of uRT-linac 506c, existing commercial treatment planning systems (TPSs) or oncology information systems (OISs) are currently not supported for use. The linac has its own control system, consisting of an integrated TPS + OIS platform named uRT-TPOIS and a treatment delivery system (TDS). The former manages the clinical workflow including patient registration, contouring, plan creation and evaluation, patient QA, plan scheduling, and image review, and the latter is used for interactions with the linac and the CT scanner, as well as for radiation delivery.
3 | ME TH ODS AND METERIALS

3.A | Isocenter verification
A conventional spoke-shot test was performed to investigate the coincidence between the mechanical isocenter and the treatment isocenter during gantry and collimator rotations. The mechanical isocenter was indicated using the front pointer and marked by a pinhole on the film, while the intersection of the central axes of the beams (or the point with the shortest distance to all beam axes) defined the treatment isocenter. As required in TG-142, 20 deviation between the two isocenters should be less than 1 mm. For couch rotation, a radiopaque ball bearing (BB) was placed on the couch near the isocenter, and portal images of a 10 × 10-cm 2 field were acquired by EPID during couch rotation. The distance between the center of the BB trajectory and the center of the radiation field was required to be no more than 0.25 mm, which is in compliance with the manufacturer's specifications.
We further verified the isocentric coincidence by an independent EPID-based Winston-Lutz (WL) test at various gantry and couch angles. The WL phantom (BB of 5 mm in diameter) was set to the mechanical isocenter by aligning to the crosshair at gantry 0°and 90°, and then moved to the calculated isocenter location by using images at four cardinal gantry angles with opposing collimator angles. 28,30 After aligning the BB location to the radiation isocenter precisely, we sampled six oblique gantry angles (30°, 60°, 130°, 230°, 300°, and 330°) and three couch angles (0°, 90°, and 270°).
On each EPID image, the crossline and inline deviations (ΔU and ΔV) between the center of the jaw-defined 2 × 2-cm 2 aperture and the center of the BB were determined. The confinement radius of central beam axis variation during gantry and couch rotations was defined as the maximum 2D centroid distance , rescaled to the isocenter plane).

3.B | Commissioning data collection
The required data for beam commissioning included percent-depth dose (PDD), lateral profiles, output factors, MLC transmission factor, and leaf-tip offset for 6XFF and 6XFFF modes. An IBA Blue Phantom 2 water tank controlled by OmniPro-Accept 8 software (IBA Dosimetry GmbH, Germany) was used for beam scanning and data collection, following recommendations from TG-106. 18 The PDD, crossline/inline profiles and output factors were measured at a source-to-surface distance (SSD) of 100 cm for field sizes ranging In addition, a 0.6-cc Farmer chamber (PTW 30013) was used for absolute dose calibration at d max .

3.C | Beam modeling and IMRT commissioning
The measured beam data were imported into the uRT-TPOIS for beam modeling. The uRT-TPOIS provided dose calculations based on collapse cone convolution (CC) and MC methods and was commissioned following the recommendations of TG-53, 16 TG-157, 21 and MPPG 5.a. 23 Here, we focus on validation of the beam models, without going into too much detail about nondosimetric commissioning.
In basic beam modeling, the TPS model parameters were iteratively adjusted to optimally agree with the measured data in the high-dose region, penumbra, and low-dose tail regions, within specific tolerance values and evaluation criteria. 21,23 Note that the statistical uncertainty in the MC calculation was user-defined and it was set to 1% throughout this article unless otherwise specified.
Special considerations have been given for modeling the characteristics of curved-end MLC, as it is known that small changes in leaf-tip position may lead to large dose deviations in IMRT plans. 31,32 As proposed by Vial et al., 33  and geometric leaf position C (defined by the light field), as illustrated in Figure 5. In the MLC control system of the uRT-linac 506c, the leaf position was calibrated so that the physical leaf position corresponded to the nominal leaf position, namely, the radiation field size was consistent with the digital field setting, by using a vendorsupplied calibration table. The light field was adjusted to coincide with the radiation field by shifting the light source. In order to take into account the limitations of the standard calibration procedure, the MLC leaf-tip offset was determined by the radiation field measurements mentioned above, and the leaf position used for planning was shifted by the value of the offset so that the calculated treatment field in uRT-TPOIS matched the actual radiation field. Furthermore, the tongue-and-groove effect and the additional transmission through the rounded leaf edge were accounted for by another two parameters called tongue-and-groove width and leaf-tip width, respectively, which were predefined by the vendor in the MLC model.

3.D | Imaging tests
Technical specifications of the imaging devices on the uRT-linac 506c unit are described in Table 2

3.D.1 | Image quality
The image quality of the IGRT systems was assessed by evaluating the spatial resolution and soft-tissue contrast. in the X, Y, and Z directions, respectively. The test procedures were as follows.
1. Prior to IGRT use, the CT image of the MIMI phantom was acquired as a reference, and a 3DCRT plan was generated based on the contouring of an arbitrary target volume.   3. At each couch position, the MV portal image, MV CBCT, and kV FBCT were acquired in turn and registered to the reference CT, respectively. The results of couch corrections for the three types of images were recorded accordingly.

4.
With loads of 100 kg and full couch extension, the phantom was displaced by the predefined distances with respect to the isocenter (by aligning the decentered marker to the laser), and step (3) was repeated.

3.E | End-to-end verification
We

4.A | Isocentricity
The results of the spoke-shot tests indicate that in all gantry and collimator settings, the distance between the mechanical isocenter and the treatment isocenter was well within 1 mm, approximately 0.6 mm during gantry rotation and 0.2 mm during collimator rotation. For couch rotation, the distance was measured as 0.14 mm by the BB test.
The EPID-based WL data were analyzed by an in-house MATLAB tool, as shown in Table 3. The radius of isocentricity was 0.63 mm as determined by the maximum 2D distance at gantry/collimator/couch angles of G330°/C0°/T270°.

4.B.2 | MLC transmission factor and leaf-tip offset
The average MLC transmission factor was measured as approximately 0.9% for the 6XFF and 6XFFF modes, which indicated less than 2% tolerance as required in TG-50. 14 The measured MLC leaftip offsets of the left and right banks at different leaf positions are shown in Figure 9. The offset for position between the measured points was determined by linear interpolation in beam modeling. Figure 10 shows the measured output factors of the 6XFF and 6XFFF beams. As mentioned earlier, the 5 × 5-cm 2 field was measured using both an ion chamber and a diamond detector and served as an intermediate transition for the output factor derivation of smaller field sizes using a "daisy chain" strategy. 34

4.B.4 | Basic beam validation and IMRT commissioning
The basic beam validation involved photon tests for square, rectangular, asymmetrical, and irregularly shaped fields as well as for SSD dependence, virtual wedge and oblique incidence. 25 In each test, a selection of central-axis and off-axis points were measured at various depths. Good agreements were observed between the measurements and corresponding calculated values within the recommended tolerances for different regions. 23,25 The TG-119 tests used for IMRT commissioning included multitarget, prostate, H&N, and C-shaped modalities. 19 The commissioning evaluations were performed not only for sIMRT and dIMRT deliveries but also for uARC delivery in both 6XFF and 6XFFF modes. The TG-119 defined DD was expressed as a ratio of  prescription dose. For the CC method, the measured point DD ranged from −1.83% to 2.51% with an average of 0.59% for the highdose region and from −1.59% to 3.35% with an average of 0.59% for the low-dose region. The MC method ranged from −1.15% to 3.14% with an average of 0.40% for the high-dose region and from −1.74% to 2.84% with an average of 0.46% for the low-dose region.
For all the test cases, gamma passing rates of the two methods were better than 98% with criteria of 3% DD, 3-mm DTA and 10% threshold, and around 95% with criteria of 2%/2 mm/10%.
For low-dose protocols of FBCT, resolutions of 11 lp/cm and 8 mm (0.5% @ 3.5 mGy) were achieved with a reduction of dose by 90%.
All the testing items were within the manufacturer's specifications.

4.C.2 | Geometric accuracy
The results of the WL test using the imaging beam were almost same with those with the treatment beam shown in Table 3

4.D | End-to-end verification
The end-to-end tests were performed for breast (

5.B | uRT-linac 506c vs. CT-on-rails
The integration of a diagnostic-quality CT with a linac is not an original design. The CT-on-rails system has set a precedent for in-room CT imaging; however, it is not commercially available any more, mostly because of its doubtful accuracy and complex workflow for IGRT. Extra uncertainties may be introduced during couch rotation and CT gantry movement. Although the overall mechanical precision is predicted to be within 1 mm by evaluating all the sources of potential uncertainties, 9 as far as we know, vibration or miscalibration of the CT gantry moving on rails could lead to either poor image quality or spatially displaced objects, and the accuracy of patient alignment may be subject to variations of mechanical flex, couch sag, and positioning accuracy during couch rotation. That is why a fiducial marker method is recommended in the alignment workflow in order to transfer the isocenter information from the linac side to the CT images. By contrast, the uRT-linac 506c system exhibits new features in geometry and clinical workflow. It suggests a unique configuration in which the CT scanner is fixed behind the linac gantry. The long couch traveling distance to CT position has little impact on the geometric accuracy between the linac and CT coordinate coincidence, as we have validated earlier, which is attributed to the real-time correction of CT coordinates. It is of great importance to assure the reproducibility of geometric accuracy. And the CT-IGRT procedure on the uRT-linac 506c seems more efficient, as described in Figures 3 and 4. Generally, it takes approximately 1 min for the whole process, which is comparable to the standard kV-CBCT procedure.

| CONCLUSION
This study summarized the commissioning process of a new fully integrated CT-linac, the uRT-linac 506c, and preliminary experiences in clinical operation. The commissioning and QA results indicate that this treatment platform exhibits good performance in dosimetric and mechanical accuracies. As the first clinical model type, its long-term reproducibility and stability are still under inspection. The integrated CT system, as a highlight, allows a diagnostic-quality visualization of internal patient anatomical structures for accurate image guidance with a concise workflow, and paves the way towards online ART.

ACKNOWLEDG MENT
This study was supported by research funding from the Funding of Shanghai Committee of Science and Technology (19DZ1930902).

CONFLI CT OF INTEREST
The authors have no conflict of interest to disclose.

D A T A A V A I L A B I L I T Y S T A T E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.