Physics Contribution
Verification of Accuracy of CyberKnife Tumor-tracking Radiation Therapy Using Patient-specific Lung Phantoms

https://doi.org/10.1016/j.ijrobp.2015.02.055Get rights and content

Purpose

To investigate the accuracy of the CyberKnife Xsight Lung Tracking System (XLTS) compared with that of a fiducial-based target tracking system (FTTS) using patient-specific lung phantoms.

Methods and Materials

Three-dimensional printing technology was used to make individualized lung phantoms that closely mimicked the lung anatomy of actual patients. Based on planning computed tomographic data from 6 lung cancer patients who underwent stereotactic ablative radiation therapy using the CyberKnife, the volume above a certain Hounsfield unit (HU) was assigned as the structure to be filled uniformly with polylactic acid material by a 3-dimensional printer (3D Edison, Lokit, Korea). We evaluated the discrepancies between the measured and modeled target positions, representing the total tracking error, using 3 log files that were generated during each treatment for both the FTTS and the XLTS. We also analyzed the γ index between the film dose measured under the FTTS and XLTS.

Results

The overall mean values and standard deviations of total tracking errors 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. Those for the XLTS were 0.38 ± 0.54 mm, 0.13 ± 0.18 mm, and 0.14 ± 0.37 mm for the CC, LR, and AP components, respectively. The average of γ passing rates was 100% for the criteria of 3%, 3 mm; 99.6% for the criteria of 2%, 2 mm; and 86.8% for the criteria of 1%, 1 mm.

Conclusions

The XLTS has segmentation accuracy comparable with that of the FTTS and small total tracking errors.

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)

There are more references available in the full text version of this article.

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.

View full text