|Year : 2019 | Volume
| Issue : 3 | Page : 119-125
The clinical feasibility and effect of online ExacTrac 6 degree-of-freedom system for head-and-neck cancer
Li-Rong Zhao, Jin-Dong Qian, Xiao-Juan Duan, Ding-Qiang Yang, Yi-Bing Zhou, Guang-Hui Li, Jian-Guo Sun
Department of Radiation Oncology, Cancer Institute of PLA, Xinqiao Hospital, Army Medical University, Chongqing, PR China
|Date of Web Publication||30-Dec-2019|
Department of Radiation Oncology, Cancer Institute of PLA, Xinqiao Hospital, Army Medical University, Chongqing 400037
Department of Radiation Oncology, Cancer Institute of PLA, Xinqiao Hospital, Army Medical University, Chongqing 400037
Source of Support: None, Conflict of Interest: None
Purpose: Online adaptive correction in image-guided intensity-modulated radiotherapy appears to be a promising approach for precision radiation treatment in head-and-neck tumors. This research is designed to evaluate the setup uncertainties in the left-right (L-R), superior-inferior (S-I), and anterior-posterior (A-P) directions and rotational variations: pitch, roll, and yaw for head-and-neck cancer (HNC) patients with the ExacTrac 6 degree-of-freedom (6D) image-guided radiotherapy (IGRT) system. Materials and Methods: The setup errors measured by ExacTrac 6D IGRT system at the treatment unit with respect to the planning computed tomography were recorded for 40 patients with head-and-neck tumors. The residual setup errors were computed and quantitatively analyzed. Results: The results indicated that the setup errors measured in the S-I direction were larger than the other two directions. For the three rotational angles, the results were very close. The verifications showed that after the first correction, the overall setup errors were generally <0.32 mm in the L-R, S-I, and A-P directions and <0.2° in the three rotational variations: pitch, roll, and yaw. According to the results of verifications, we know that ExacTrac 6D IGRT system was accurate and clinical feasibility. Conclusion: The results of our study have shown that daily image guidance with ExacTrac 6D image-guided system for HNC patients is effective. These data suggest it allows a high accurate of setup errors.
Keywords: ExacTrac 6 degree.of.freedom image.guided radiotherapy system, image.guided radiotherapy, intensity.modulated radiation therapy, setup errors
|How to cite this article:|
Zhao LR, Qian JD, Duan XJ, Yang DQ, Zhou YB, Li GH, Sun JG. The clinical feasibility and effect of online ExacTrac 6 degree-of-freedom system for head-and-neck cancer. Digit Med 2019;5:119-25
|How to cite this URL:|
Zhao LR, Qian JD, Duan XJ, Yang DQ, Zhou YB, Li GH, Sun JG. The clinical feasibility and effect of online ExacTrac 6 degree-of-freedom system for head-and-neck cancer. Digit Med [serial online] 2019 [cited 2020 Mar 28];5:119-25. Available from: http://www.digitmedicine.com/text.asp?2019/5/3/119/274381
| Introduction|| |
The implement of intensity-modulated radiation therapy (IMRT) in the setting of nasopharyngeal cancer has resulted in the ability to produce exquisitely shaped radiation doses that closely conform to the tumor dimensions while sparing sensitive normal structures.,, The development of IMRT places more stringent requirements on beam targeting accuracy. The demand of greater accuracy in IMRT has driven the development of more advanced verification systems in image-guided radiotherapy (IGRT)., Various approaches have been used to address the infiuence of the setup errors on the target dose coverage. One method is to incorporate the errors into margin design. However, larger margins may increase normal tissue toxicities, and the advantages of IMRT may be compromised. Therefore, improving patient immobilization device and setup accuracy by using IGRT techniques is essential for IMRT treatments, in which a tighter margin is employed.
In the most common setup and alignment procedure, the lasers are aligned to external marks on the face mask and followed by megavoltage (MV) portal imaging to assess the positioning accuracy. However, the major challenge of treatment verification with conventional portal imaging technology is the low subject contrast of poor subject image at MV energies. New generation, both two-dimensional (2D) kV radiographic imaging and three-dimensional (3D) kV cone-beam computed tomography (CBCT) imaging are now available in radiation therapy community. CBCT provides high-resolution 3D information of the patient in the treatment position and thus has great potential for accurate target localization and irradiation dose verification., In reality, however, its applications are limited by several practical concerns including relatively long image acquisition time,, relatively high imaging dose,, high nonmedical insure cost, and uncomfortable for patients due to long scanning time. Li et al. recently demonstrated that no statistically significant difference was shown in patient alignment between weekly 3D CBCT and kV 2D imaging.
To improve accuracy and efficiency of radiation therapy, commercial orthogonal X-ray imaging guidance system (ExacTrac, Brainlab) could be useful. The ExacTrac X-ray 6 degree-of-freedom (6D) is use to guide patient setup using only two oblique X-ray images. Due to a limited view of projections, the ExacTrac cannot provide as much information as CBCT. However, the ExacTrac could offer other clinical benefits including faster patient alignment using the 6D robic couch, faster image acquisition time, the ability to monitor patient motion during treatment, and a relatively low radiation to the patient. At present, the report of daily image guidance with ExacTrac X-ray 6D system for head-and-neck cancer (HNC) patients is in its infancy.
We herein investigated the setup errors measured with daily ExacTrac X-ray 6D system for retrospective HNC patients. The values of setup errors are quantitatively analyzed and presented in the current study.
| Materials and Methods|| |
A total of 1320 daily ExacTracs of 40 patients were analyzed. Of the 40 HNC patients, 9 (22.5%) had T4 disease and 7 (17.5%) had N3 disease. All patients were fixed in treatment position with MedTech head-and-neck frame and individualized thermoplastic facial masks. All underwent virtual CT simulation, using helical CT scans spanning from midbrain to sternoclavicular articulation, with 3.0 mm slices. Magnetic resonance imaging (MRI) scans were also obtained accordingly. The CT and MRI fusion images were transferred to Varian Eclipse (Release 10.0) work station to create IMRT plans.
Image-guided radiotherapy procedure
At our institution, all treatments of HNC patients are performed using the Novalis TX system  which is a combination of a modern linear accelerator from Varian (Varian Medical Systems, Inc., Palo Alto, USA) and the ExacTrac system from Brainlab (BrainLABAG, Feldkirchen, Germany). [Figure 1] is the Novalis TX image-guided system showing oblique configurations of the X-ray imaging devices. The X-ray component consists of two fioor-mounted kV X-ray tubes, projecting medial, anterior, and inferior obliquely into two corresponding fiat panel detectors mounted on the ceiling. ExacTrac X-ray 6D IGRT system  uses a combination of optical positioning and kV radiographic imaging to position patients and perform online positioning corrections. The system consists of two main subsystems: (1) an infrared (IR)-based optical positioning system (ExacTrac) for initial patient setup and precise control of couch movement, using a robotic couch, and (2) a radiographic kV X-ray imaging system (X-ray 6D) for position verification and readjustment based on the internal anatomy or implanted fiducial markers. In addition, the IR system can be used to monitor a patient's respiration and provide a signal to the linac for tracking and gating of the treatment beam. Therefore, the X-ray image can be performed in position readjustment between two fields if position drift is a concern.
|Figure 1: The prototype Novalis TX image-guided system which is a combination of a modern linear accelerator from Varian (Varian Medical Systems, Inc., Palo Alto, USA) and the ExacTrac system from Brainlab (BrainLABAG, Feldkirchen, Germany). The ExacTrac system consists of two oblique configurations of the X-ray imaging devices which can project medial, anterior, and inferior obliquely into 2 corresponding fiat panel detectors mounted on the ceiling|
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To verify the treatment position, two orthogonal images of the treatment region were taken and matched with the Digitally Reconstructed Radiographs (DRRs) generated from the planning CT data. We defined the anatomical borders as below:
- Superior border and posterior border are the occipital protuberant borders
- Inferior border is the lower border of the mandible
- Anterior border is the anterior border of the mandible.
In addition, typical clinical settings for cranial cases to acquire X-ray images at our institution are 100 kV, 100 mA, and 100 ms.
Atfirst, the patient was positioned on the 6D couch. Afterward, the patient was prepositioned with the aid of the IR guidance such that the patient was shifted to treatment position by moving the couch. The initial patient setup was then verified by using the X-ray component of ExacTrac: two kV-images were acquired (one image per X-ray tube of the ExacTrac). Then, these corresponding X-ray images were compared with DRRs. The best match was thus determined and the three translational and three rotational position variations used to generate the set of DRRs were the 6D offsets to fuse the images. Therefore, the 6D fusion method was actually a 2D (X-ray) to 3D (CT) image fusion algorithm. If the initial setup errors were outside of a specified tolerance (±0.5 mm ±0.5°), two kV-images with ExacTrac have been acquired again until the setup error was within the specified tolerance (±0.5 mm ±0.5°). For translational error >5 mm or rotational error >5°, the patient would be repositioned by technician's reentering the treatment room and adjusting the offset. 6D ExacTrac scan increased the patient's time on the table by no more than 1 min. It had been well tolerated by patients, with no complications in our series. Physicians review occurred in real time for thefirst 6D ExacTrac scan. On subsequent treatment days, the daily corrections were performed by the therapist according to the shift generated by physicians. However, the physicians were contacted for “large” shifts.
The determined correction was applied with the aid of the 6D couch including all six axes (in the left-right [L-R], superior-inferior [S-I], and anterior-posterior [A-P] direction, and rotational variations: Pitch, roll, and yaw). Pitch, roll, and yaw corresponded to the rotations around the X (i.e., L-R), Y (i.e., S-I), and Z (i.e., A-P) axes, respectively. Informed consent for the study was obtained from each patient before simulation.
In this study, all shifts indicated in the image-guided ExacTrac systems were considered as displacements between the planned treatment isocenter in the patient and the radiation isocenter of the linear accelerator. The setup errors of the 40 patients were analyzed using SPSS 19.0 (SPSS Inc., Chicago, IL, USA) and Origin 7.0 (OroginLab Corparation, Northampton, MA01060 USA) software. The mean and standard deviation (SD) of the residual setup errors in the L-R, S-I, and A-P directions, and rotational variations: pitch, roll, and yaw were calculated.
| Results|| |
A total of 40 HNC patients underwent ExacTrac imaging-guided position readjustment once for each fraction before treatments. A total of 1320 sessions of scans were analyzed for the present study. An example of a patient's residual setup errors image registrations was shown in [Figure 2], which showed a set of orthogonal X-ray images of a HNC in one of the fractional treatments acquired by the Novalis TX system and fused with DRRs. [Figure 2]a showed the head fusion image, while [Figure 2]b was the neck fusion image.
|Figure 2: Two verification kV X-ray images for evaluating the localization accuracy which is determined by taking two orthogonal images of the treatment region and matching with the DRRs generated from the planning computed tomography data. (a) The head fusion image (b) the neck fusion image|
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The mean and SD of patient setup errors measured from ExacTrac systems were also listed in [Table 1], both before and after ExacTrac corrections. The mean translational setup errors (±1 SD) were −1.52 ± 0.60 mm, 4.33 ± 1.40 mm, and −1.62 ± 1.17 mm for the L-R, S-I, and A-P, respectively. When considering the results according to the three axes, the setup errors were significantly larger in the S-I direction compared with the L-R and A-P directions (P < 0.05). The setup errors were found to be anisotropic in three translational directions. We observed the largest SD in S-I direction (SD = 1.40), while SD was the smallest in L-R direction (SD = 0.60).
|Table 1: The table summarizes the treatment position deviations measured both beforeand after corrections by ExacTrac 6 degree-of-freedom image-guided radiotherapy system in the translational left-right (X), superior-inferior (Y) and anterior-posterior (Z) directions, and rotational variations: Pitch, roll, and yaw|
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The mean correction setup errors (±1 SD) for the pitch, roll, and yaw rotational errors are 0.8° ± 0.7°, 1.0° ± 1.4°, and 0.9° ± 0.5°, respectively. We observed the largest SD in roll rotational error (SD = 1.4°), which was significantly larger than other two rotational directions (P < 0.05). For pitch and yaw rotational errors, a statistically significant difference was not reached (P > 0.05).
Finally, when comparing correction data (before ExacTrac corrections) and verification data (after ExacTrac corrections), it showed that submillimeter (<0.5 mm) as well as subdegree (<0.5°) could be achieved after ExacTrac corrections [Table 1]. The mean translational setup errors (±1 SD) were 0.32 ± 0.17 mm, −0.15 ± 0.32 mm, and −0.15 ± 0.18 mm for the L-R, S-I, and A-P, respectively. The mean correction setup errors (±1 SD) for the pitch, roll, and yaw were 0.0° ± 0.2°, −0.1° ±0.2°, and 0.0° ± 0.2°, respectively. Both mean and SD errors decreased significant after online adjustment in this study.
The frequency distribution of observed residual translational errors caused by L-R (X), S-I (Y), and A-P (Z) directions and residual rotational errors caused by pitch, roll, and yaw in degree was demonstrated in [Figure 3]. Obviously, the values of translational and rotational errors decreased with ExacTrac 6D IGRT system online correction. It was also shown in [Table 2]. In addition, the overlay scatter plot of the target translational displacement and rotational position in the sagittal, coronal, and transverse planes was demonstrated in [Figure 4]. Before correction, the target translational displacement was large in the transverse, coronal, and sagittal planes [Figure 4]a due to initial patient setup with skin markers. Most of the displacements of the three planes were congregated within the range of ±5 mm, with the largest displacement about 9 mm. The postcorrection cluster of displacements showed a much more congregated distribution with small displacements in all three planes [Figure 4]b. While for the rotational position, before correction, the rotational displacement was large in above three planes [Figure 4]c. Most of the values were congregated within the range of ±3°, with largest rotational degree was close to ±5°. After correction, the cluster of rotational displacements shows a much more congregated distribution with small displacements (about ±1.0°) in all three planes [Figure 4]d.
|Figure 3: Histograms showing the frequency distribution of (a-c) the residual translational errors caused by left-right (X), superior-inferior (Y), and anterior-posterior (Z) directions and (d-f) the residual rotational errors caused by pitch, roll, and yaw. Both before (red) and after (black) corrections with ExacTrac 6 degree-of-freedom image-guided radiotherapy system are shown|
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|Table 2: Frequency of residual setup error for thresholds ±1 mm and ±4 mm translations and 0.5° and 1.0° rotations for before and after corrections with ExacTrac 6 degree-of-freedom image-guided radiotherapy system|
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|Figure 4: Scatter plot of target displacements of translational and rotational on transverse, coronal and sagittal planes are evaluated from 1320 ExacTrac sessions. The target displacements are plotted against two-dimensions (the ordinate and abscissa represent superior-inferior and anterior-posterior orientations on the sagittal plane, superior-inferior and left-right orientations on the coronal plane and anterior-posterior and left-right orientations on the transverse plane). The cluster (a) represents the precorrection target translational position which had larger error; the cluster (b) represents the postcorrection target translational position, indicating reduced error with online correction; the cluster (c) represents the precorrection target rotational position which had larger error; the cluster (d) represents the postcorrection target rotational position, indicating reduced rotational error with online correction|
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| Discussion|| |
In our previous work, a home-made system, the XGS-10 system (Weidu Medical Systems, Inc., Chongqing, China), has been developed for noninvasive image-guided radiation treatments. The system also simultaneously acquires two orthogonal oblique view X-ray images for each patient after initial setup. A drawback of XGS-10 is the lack of rotational error correction. Hence in this study, the clinical range in which a 6D couch was used to correct patient setup errors in six dimensions was investigated. For this purpose, patient setup errors in six dimensions determined with the aid of ExacTrac in clinical routine were analyzed. The results of the present work have demonstrated the feasibility and utility of ExacTrac imaging data for clinical use. This is an important clinical use, as HNC patient IMRT can allow simultaneous sparing of organs at risk and dose escalation to gross and microscopic disease, but only if it is delivered accurately.
The ExacTrac X-ray IGRT system uses multiple and integrated imaging modalities to localize the patient. Our study provides one of the largest data sets (>1000 ExacTrac scans) on this topic and is one of the few prospective trials of the use of ExacTrac system. Positioning based on optical guidance has been shown to have submillimeter accuracy, which is also in line with the study by Bova et al. Logically based on these results, ExacTrac would be the choice of verification method and recommended for HNC patients.
The Novalis TX, introduced recently by Varian (Varian Medical Systems, Palo Alto, CA) couples the ExacTrac IGRT system with the Varian on-board imager, thereby facilitating volumetric imaging of soft tissues before treatment. The ExacTrac system is relatively new, and its pros and cons relative to other forms of IGRT have not been fully assessed. Chang et al. investigated setup discrepancies measured with ExacTrac X-ray 6D and CBCT for patients under treatments of stereotactic body radiation therapy (SBRT). They demonstrated in both phantom and patient studies that ExacTrac X-ray 6D represents a potential alternative to CBCT; however, precaution should be taken when only ExacTrac X-ray 6D is used to guide SBRT with small setup margins.
Studies by Schmidhalter et al. have analyzed the patient setup error after rotating the couch. In a perfect world, no additional setup error would be expected when rotate the couch. Nevertheless, different errors (instability of the couch rotation center or patient movement) lead to the fact that such additional setup errors occur in clinical routine.
The study has several limitations. The setup errors after rotating the couch were not taken into account in this system. Future study with 6D including setup errors after rotating the couch might be helpful, especially for noncoplanar irradiation. In addition, only head-and-neck patients were included in this study. It would be interesting to extend this study to thorax and abdomen patients where respiration is the principal cause for intrafraction internal anatomy motion. There could be a potential benefit of ExacTrac X-ray 6D system in these conditions.
| Conclusion|| |
The use of daily image guidance with ExacTrac 6D image-guided system is an effective modality to improve the accuracy of head-and-neck patients. This could allow for dose escalation to tumor and decreased radiation exposure to the normal tissues.
The study was supported in part by The National Key Research and Development Project (grant no. 2016YFC0106400), the Foundation of National Natural Science Foundation of China (No. 81272910).
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]