International Journal of Radiation Oncology*Biology*Physics
Physics ContributionDosimetric Implications of Residual Tracking Errors During Robotic SBRT of Liver Metastases
Introduction
The treatment of liver metastases with stereotactic body radiation therapy (SBRT) has been established through clinical trials 1, 2, 3, guidelines (4), and retrospective pooled analyses (5). SBRT in the presence of strong tumor motion, which can be prominent in some parts of the lung and liver, should be performed under image guidance, ideally with active motion compensation systems 6, 7.
Real-time tumor tracking can be performed with the robotic CyberKnife (Accuray, Sunnyvale, CA) 8, 9, which is one system capable of active motion compensation. The CyberKnife uses registration of stereoscopic x-ray images to the planning computed tomography (CT) scan to detect the position of the patient on the treatment couch (8). During the step-and-shoot irradiation of generally 100 to 200 small cylindrical beams, differences in patient position in reference to the calibrated imaging center are corrected by the robot. With repeated imaging, this method has been shown to ensure high-precision beam delivery in phantoms (10) and throughout the treatment 11, 12. Furthermore, the CyberKnife can compensate for respiration-induced motion by correlation and prediction modeling of the internal target motion detected on the x-ray images and external light-emitting diode (LED) signals on the patient's chest (9). This method also has been described as highly accurate in phantoms 7, 13, 14, in simulations 15, 16, and in patients 17, 18, 19, 20, 21, allowing the application of high radiation doses inside the gross tumor volume (GTV) and clinical tumor volume (CTV) while applying minimal safety margins to maximize sparing of the surrounding normal tissue. The resulting high local tumor control rates for liver lesions have been published repeatedly 22, 23, 24.
In the past, there has been great debate on the appropriate dimensions of the necessary planning target volume (PTV) safety margins, which should include systematic and random tracking errors alike. In previous studies, safety margins for correlation, prediction, and delivery errors between 3 and 5 mm were proposed for robotic lung and liver SBRT on the basis of statistical analysis of log files 17, 18, 19, 20, 21. However, these studies did not analyze the dosimetric impact of the estimated errors. Indeed, only a few dosimetric error studies for the CyberKnife are available, for example, for spinal (12) and prostate treatments (25). Our aim was to analyze the dosimetric effects caused by residual errors from inaccurate correlation and prediction modeling and from untracked tumor rotation during robotic liver SBRT.
Section snippets
CyberKnife treatment
Between March 2011 and March 2014, 46 patients (age range, 38-85 years) were treated with SBRT for metastatic liver lesions with the CyberKnife. Prior to treatment, either 1 to 5 Gold Anchor fiducial markers (Naslund Medical, Huddinge, Sweden) or 1 to 5 solid gold fiducial markers (IZI Medical Products, Owings Mills, MD) were implanted as close to the lesions as possible under CT guidance. Treatment planning was performed as described previously with a focus on optimizing the GTV mean dose (24)
Maximum fiducial motion
The maximum fiducial motion was estimated by use of the Synchrony models generated during each of the 90 analyzed fractions. The overall maximum patient fiducial motion was 17.4 mm (median over all fractions; range, 8.2-38.2 mm). The maximum fiducial motion in a single direction in any fraction was 40.5 mm (inferior-superior), 17.2 mm (left-right), and 21.4 mm (anterior-posterior).
Residual tracking errors
To estimate individual systematic correlation, prediction, and rotation errors, we used the average over the 10
Discussion
For the first time, dosimetric fluctuations due to breathing phase–dependent residual tracking errors of real-time motion-compensated robotic SBRT of liver metastases were investigated. Although in our patient analysis, the captured motion and systematic correlation and prediction errors were on the order of those published in the literature 17, 18, 19, 20, 26, 28, 30, we found larger uncorrected systematic rotation errors (>2°) in 23% of our patients. Breathing-dependent fiducial rotations in
Conclusions
The CyberKnife system precisely compensates for respiratory-induced translational tumor motion. For liver tumors, a safety margin of 3 mm may not always be sufficient to cover all residual tracking errors, mostly because of uncompensated tumor rotations. Dosimetrically, however, this translates into only small CTV coverage reductions.
Acknowledgments
The authors kindly thank Warren Kilby and Mikail Gezginci (Accuray) for their helpful remarks.
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In-vivo treatment accuracy analysis of active motion-compensated liver SBRT through registration of plan dose to post-therapeutic MRI-morphologic alterations
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Conflict of interest: none.