International Journal of Radiation Oncology*Biology*Physics
Physics ContributionVerification of Accuracy of CyberKnife Tumor-tracking Radiation Therapy Using Patient-specific Lung Phantoms
Introduction
Stereotactic ablative radiation therapy (SABR) has been used for the past few years to treat medically inoperable non-small cell lung cancer, and it has shown an encouraging result of a survival rate of 70% and a local control rate of 91% at 2 years in patients with localized stage I non-small cell lung cancer (1). Because of its promising rates of local control and acceptable toxicity, the use of SABR is on the increase. The CyberKnife robotic radiosurgery system (Accuray Inc, Sunnyvale, CA) is a frameless SABR equipment system. The CyberKnife has 2 real-time Synchrony respiratory tracking systems: a fiducial-based target tracking system (FTTS) and the Xsight Lung Tracking System (XLTS), which is a fiducial-free tracking system. When the real-time FTTS is used, there are several major and minor adverse events related to fiducial insertion, such as pneumothorax, pulmonary hemorrhage, and systemic toxicity of local anesthetic drugs. By contrast, the XLTS is a real-time direct respiratory motion tracking method that eliminates invasive fiducial implantation procedures, thereby eliminating the risk of fiducial insertion-related adverse events.
However, sufficient tumor contrast against surrounding tissues in x-ray images is essential for direct soft-tissue tracking; thus, tumors over 15 mm in diameter located in the peripheral or apex lung regions and away from the spine are recommended for XLTS. A recent study also showed that tumor size with its density is a key factor for XLTS, but the tracking error will be dependent on soft-tissue contrast on x-ray images, which is expected to be greater than the error of FTTS. Several studies have investigated the tracking accuracy of either XLTS or FTTS based on clinical data or phantom study 2, 3, 4, 5, 6. The tracking error of XLTS or FTTS has multiple sources; correlation error between internal tumor locations versus external respiratory surrogate positions, prediction error for system delay compensation, and tumor localization error. Most studies were to evaluate overall tracking accuracy based on log file analysis or dosimetric phantom analysis 2, 4, 5, 6. The key difference between XLTS and FTTS is the localization (or segmentation) performance of soft tissue versus radiopaque fiducial marker.
Although some investigators have used simple anthropomorphic phantoms containing a spherical solid target surrounding radiographically equivalent lung, spine, and ribs surrounded by soft tissue, the lung was composed of homogeneous material, which could not present the complex structure of lung parenchyma, vessels, and air (2). This is the main reason for the lack of studies concerning the accuracy of the XLTS. Recently, 3-dimensional (3D) printing technologies have opened up the possibility of customization of a wide variety of applications in the medical field. We realized that 3D printing has the capability to produce individualized lung-mimicking phantoms and is therefore potentially useful for investigating the accuracy of XLTS. By overlaying patient information acquired during computed tomographic (CT) simulation with the phantom, we reproduced real treatment situations. We were able to generate 3D lung phantoms containing a tumor visible by orthogonal x-ray images that moved with the respiration motion of the patient.
In this study, based on the lung phantoms we created by precision 3D printing technology, we verify the accuracy of the CyberKnife XLTS compared with the FTTS.
Section snippets
Methods and Materials
The correlation model describes the mathematical relationship between the position of external breathing optical signals and the location of the internal fiducial marker. Segmentation error refers to error in the ability of the system to define the tumor. Prediction error is the discrepancy between the real position and the predicted position caused by a time delay of 115 ms. Error resulting from deformation of the body and organs relative to the center of the implanted fiducial markers is also
Results
The discrepancies between the measured and modeled target positions, regarded as the total tracking errors, for all 6 experiments are summarized in Table 2. The overall mean values and standard deviations for the FTTS were 0.36 ± 0.39 mm, 0.15 ± 0.64 mm, and 0.15 ± 0.62 mm for the craniocaudal (CC), left–right (LR), and anteroposterior (AP) components, respectively. For the XLTS, the mean values and standard deviations were 0.38 ± 0.54 mm, 0.13 ± 0.18 mm, and 0.14 ± 0.37 mm for the CC, LR, and
Discussion
Several studies have demonstrated the clinical efficacy of the CyberKnife using the FTTS 7, 8, 9, 10. However, several major and minor adverse events are related to fiducial insertion. Trumm et al (11) evaluated the technical outcomes and safety of CT fluoroscopy-guided percutaneous fiducial maker placement before CyberKnife stereotactic radiosurgery. They reported that major pneumothorax requiring chest tube or pigtail drainage placement occurred in 13.3% of patients, minor pneumothorax
References (16)
- et al.
Stereotactic radiotherapy (SABR) for the treatment of primary non-small cell lung cancer: Systematic review and comparison with a surgical cohort
Radiother Oncol
(2013) - et al.
Clinical accuracy of the respiratory tumor tracking system of the CyberKnife: Assessment by analysis of log files
Int J Radiat Oncol Biol Phys
(2009) - et al.
Stereotactic radiotherapy with real-time tumor tracking for non-small cell lung cancer: Clinical outcome
Radiother Oncol
(2009) - et al.
CyberKnife radiosurgery for stage I lung cancer: Results at 36 months
Clin Lung Cancer
(2007) - et al.
CT fluoroscopy-guided percutaneous fiducial marker placement for CyberKnife stereotactic radiosurgery: Technical results and complications in 222 consecutive procedures
J Vasc Interv Radiol
(2014) - et al.
Insertion and fixation of fiducial markers for setup and tracking of lung tumors in radiotherapy
Int J Radiat Oncol Biol Phys
(2005) - et al.
Xsight lung tracking system: A fiducial-less method for respiratory motion tracking
- et al.
Accuracy of tumor motion compensation algorithm from a robotic respiratory tracking system: A simulation study
Med Phys
(2007)
Cited by (0)
Supported by a grant (2013-472) from the Asan Institute for Life Sciences, Seoul, Korea and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013R1A1A2011346).
Conflict of interest: none.