Dosimetric Implications of Residual Tracking Errors During Robotic SBRT of Liver Metastases

doi:10.1016/j.ijrobp.2016.11.041

International Journal of Radiation OncologyBiologyPhysics

Volume 97, Issue 4, 15 March 2017, Pages 839-848

https://doi.org/10.1016/j.ijrobp.2016.11.041 Get rights and content

Purpose

Although the metric precision of robotic stereotactic body radiation therapy in the presence of breathing motion is widely known, we investigated the dosimetric implications of breathing phase–related residual tracking errors.

Methods and Materials

In 24 patients (28 liver metastases) treated with the CyberKnife, we recorded the residual correlation, prediction, and rotational tracking errors from 90 fractions and binned them into 10 breathing phases. The average breathing phase errors were used to shift and rotate the clinical tumor volume (CTV) and planning target volume (PTV) for each phase to calculate a pseudo 4-dimensional error dose distribution for comparison with the original planned dose distribution.

Results

The median systematic directional correlation, prediction, and absolute aggregate rotation errors were 0.3 mm (range, 0.1-1.3 mm), 0.01 mm (range, 0.00-0.05 mm), and 1.5° (range, 0.4°-2.7°), respectively. Dosimetrically, 44%, 81%, and 92% of all voxels differed by less than 1%, 3%, and 5% of the planned local dose, respectively. The median coverage reduction for the PTV was 1.1% (range in coverage difference, −7.8% to +0.8%), significantly depending on correlation (P=.026) and rotational (P=.005) error. With a 3-mm PTV margin, the median coverage change for the CTV was 0.0% (range, −1.0% to +5.4%), not significantly depending on any investigated parameter. In 42% of patients, the 3-mm margin did not fully compensate for the residual tracking errors, resulting in a CTV coverage reduction of 0.1% to 1.0%.

Conclusions

For liver tumors treated with robotic stereotactic body radiation therapy, a safety margin of 3 mm is not always sufficient to cover all residual tracking errors. Dosimetrically, this translates into only small CTV coverage reductions.

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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Cited by (26)

Preoperative single fraction breast radiotherapy: Intra-fraction geometric uncertainties and dosimetric implications
2023, Physica Medica
A preoperative breast robotic radiosurgery trial was concluded in our centre. Purposes of the present study were to evaluate retrospectively over the enrolled patients: i) respiratory patterns ii) tracking uncertainties iii) necessity of respiratory compensation iv) tracking errors dosimetric effects.
22 patients were treated in 21 Gy single fraction using CyberKnife (CK) respiratory modelling and tracking (SynchronyResp) and data extracted from log-files. Respiratory motion and baseline drifts (BD) were analyzed. SynchronyResp uncertainties were computed and compared with errors simulated for CK fiducial tracking without respiratory compensation. Plans were perturbed by tracking errors and perturbed doses calculated on the planning CT scan in order to simulate the dosimetric consequences of intra-fraction errors.
After BD correction, respiratory amplitudes were below 5.5 mm except one value of 8 mm. 50% of patients showed BD above 3 mm. Standard deviations of SynchronyResp errors remained within 2.1 mm. Standard deviations of tracking errors without respiratory compensation were comparable and below 2.5 mm. Using a 3 mm PTV margin, perturbed CTV coverage was below 95% (93.7%) just for one patient. The latter case presented a large CTV-Skin interface. Perturbed OAR doses were always judged clinically acceptable.
Intra-fraction geometric uncertainties and their effects were quantified for breast neoadjuvant CK treatments. Data indicated that in the majority of cases respiratory compensation may be disabled without increasing uncertainties and reducing treatment time, provided that fiducial intra-fraction tracking is performed to account for BD. Dosimetric effects are mostly not clinically relevant.
Liver SBRT with active motion-compensation results in excellent local control for liver oligometastases: An outcome analysis of a pooled multi-platform patient cohort
2021, Radiotherapy and Oncology
Citation Excerpt :
The patient alignment was performed on liver contour and surrogate markers, if present (surgical clips). Patients with robotic-based SBRT (RS) were treated with CyberKnife (Accuray Inc., Sunnyvale, CA, USA) with real-time motion compensation (Synchrony, Accuray Inc.) [20,37,39]. Briefly, after fiducial implantation in the anatomic proximity of the target lesions, treatment planning was performed utilizing Sequential Multi-Objective Optimization (MultiPlan Vers.
Local treatment of metastases in combination with systemic therapy can prolong survival of oligo-metastasized patients. To fully exploit this potential, safe and effective treatments are needed to ensure long-term metastases control. Stereotactic body radiotherapy (SBRT) is one means, however, for moving liver tumors correct delivery of high doses is challenging. After validating equal in-vivo treatment accuracy, we analyzed a pooled multi-platform liver-SBRT-database for clinical outcome.
Local control (LC), progression-free interval (PFI), overall survival (OS), predictive factors and toxicity was evaluated in 135 patients with 227 metastases treated by gantry-based SBRT (deep-inspiratory breath-hold-gating; n = 71) and robotic-based SBRT (fiducial-tracking, n = 156) with mean gross tumor volume biological effective dose (GTV-BED_α/β=10Gy) of 146.6 Gy₁₀.
One-, and five-year LC was 90% and 68.7%, respectively. On multivariate analysis, LC was significantly predicted by colorectal histology (p = 0.006). Median OS was 20 months with one- and two-year OS of 67% and 37%. On multivariate analysis, ECOG-status (p = 0.003), simultaneous chemotherapy (p = 0.003), time from metastasis detection to SBRT-treatment (≥2months; p = 0.021) and LC of the treated metastases (≥12 months, p < 0.009) were significant predictors for OS. One- and two-year PFI were 30.5% and 14%. Acute toxicity was mild and rare (14.4% grade I, 2.3% grade II, 0.6% grade III). Chronic °III/IV toxicities occurred in 1.1%.
Patient selection, time to treatment and sufficient doses are essential to achieve optimal outcome for SBRT with active motion compensation. Local control appears favorable compared to historical control. Long-term LC of the treated lesions was associated with longer overall survival.
In-vivo treatment accuracy analysis of active motion-compensated liver SBRT through registration of plan dose to post-therapeutic MRI-morphologic alterations
2019, Radiotherapy and Oncology
Citation Excerpt :
For all RS patients, treatment planning was performed on end-expiration breath-hold CT (1.5 mm slice-thickness) after marker/fiducial implantation in the anatomic proximity of the treated lesions and locoregional matching with the planning MRI. GTV was expanded of 5 mm in the liver and additionally 3 mm isotropically to generate the PTV [1,25]. Median prescription dose was 40 Gy (29–45 Gy) in 3–5 fractions utilizing Sequential Multi-Objective Optimization, with the main objective of reaching GTV mean biological equivalent doses (BED with α/β = 10 Gy) of ≥151.2 Gy10 [1,23].
In-vivo-accuracy analysis (IVA) of dose-delivery with active motion-management (gating/tracking) was performed based on registration of post-radiotherapeutic MRI-morphologic-alterations (MMA) to the corresponding dose-distributions of gantry-based/robotic SBRT-plans.
Forty targets in two patient cohorts were evaluated: (1) gantry-based SBRT (deep-inspiratory breath-hold-gating; GS) and (2) robotic SBRT (online fiducial-tracking; RS). The planning-CT was deformably registered to the first post-treatment contrast-enhanced T1-weighted MRI. An isodose-structure cropped to the liver (ISL) and corresponding to the contoured MMA was created. Structure and statistical analysis regarding volumes, surface-distance, conformity metrics and center-of-mass-differences (CoMD) was performed.
Liver volume-reduction was −43.1 ± 148.2 cc post-RS and −55.8 ± 174.3 cc post-GS. The mean surface-distance between MMA and ISL was 2.3 ± 0.8 mm (RS) and 2.8 ± 1.1 mm (GS). ISL and MMA volumes diverged by 5.1 ± 23.3 cc (RS) and 16.5 ± 34.1 cc (GS); the median conformity index of both structures was 0.83 (RS) and 0.80 (GS). The average relative directional errors were ≤0.7 mm (RS) and ≤0.3 mm (GS); the median absolute 3D-CoMD was 3.8 mm (RS) and 4.2 mm (GS) without statistically significant differences between the two techniques. Factors influencing the IVA included GTV and PTV (p = 0.041 and p = 0.020). Four local relapses occurred without correlation to IVA.
For the first time a method for IVA was presented, which can serve as a benchmarking-tool for other treatment techniques. Both techniques have shown median deviations <5 mm of planned dose and MMA. However, IVA also revealed treatments with errors ≥5 mm, suggesting a necessity for patient-specific safety-margins. Nevertheless, the treatment accuracy of well-performed active motion-compensated liver SBRT seems not to be a driving factor for local treatment failure.
Intra-breath-hold residual motion of image-guided DIBH liver-SBRT: An estimation by ultrasound-based monitoring correlated with diaphragm position in CBCT
2018, Radiotherapy and Oncology
Craniocaudal motion during image-guided abdominal SBRT can be reduced by computer-controlled deep-inspiratory-breath-hold (DIBH). However, a residual motion can occur in the DIBH-phases which can only be detected with intrafractional real-time-monitoring. We assessed the intra-breath-hold residual motion of DIBH and compared residual motion of target structures during DIBH detected by ultrasound (US). US data were compared with residual motion of the diaphragm-dome (DD) detected in the DIBH-CBCT-projections.
US-based monitoring was performed with an experimental US-system simultaneously to DIBH-CBCT acquisition. A total of 706 DIBHs during SBRT-treatments of metastatic lesions (liver, spleen, adrenal) of various primaries were registered in 13 patients. Residual motion of the target structure was documented with US during each DIBH. Motion of the DD was determined by comparison to a reference phantom-scan taking the individual geometrical setting at a given projection angle into account. Residual motion data detected by US were correlated to those of the DD (DIBH-CBCT-projection).
US-based monitoring could be performed in all cases and was well tolerated by all patients. Additional time for daily US-based setup required 8 ± 4 min. 385 DIBHs of 706 could be analyzed. In 59% of all DIBHs, residual motion was below 2 mm. In 36%, residual motion of 2–5 mm and in 4% of 5–8 mm was observed. Only 1% of all DIBHs and 0.16% of all readings revealed a residual motion of >8 mm during DIBH. For DIBHs with a residual motion over 2 mm, 137 of 156 CBCT-to-US curves had a parallel residual motion and showed a statistical correlation.
Soft-tissue monitoring with ultrasound is a fast real-time method without additional radiation exposure. Computer-controlled DIBH has a residual motion of <5 mm in >95% which is in line with the published intra-breath-hold-precision. Larger intrafractional deviations can be avoided if the beam is stopped at an US-defined threshold.
Retrospective assessment of a single fiducial marker tracking regimen with robotic stereotactic body radiation therapy for liver tumours
2019, Reports of Practical Oncology and Radiotherapy
Citation Excerpt :
It should be noted that spine realignments are necessary for single marker tracking to confirm rotational errors when patient movement is detected visually during beam delivery. The impact of rotation by breath motion on overall intra-fractional rotational errors during liver treatment seems to be more significant than that of spine rotation.20,26 Xu et al. evaluated tumour motion in the case of multiple fiducial markers and demonstrated that the mean intra-fractional rotation of the roll, pitch, and yaw angles was 1.2 ± 1.8°, 1.8 ± 2.4°, and 1.7 ± 2.1°, respectively.27
To investigate tumour motion tracking uncertainties in the CyberKnife Synchrony system with single fiducial marker in liver tumours.
In the fiducial-based CyberKnife real-time tumour motion tracking system, multiple fiducial markers are generally used to enable translation and rotation corrections during tracking. However, sometimes a single fiducial marker is employed when rotation corrections are not estimated during treatment.
Data were analysed for 32 patients with liver tumours where one fiducial marker was implanted. Four-dimensional computed tomography (CT) scans were performed to determine the internal target volume (ITV). Before the first treatment fraction, the CT scans were repeated and the marker migration was determined. Log files generated by the Synchrony system were obtained after each treatment and the correlation model errors were calculated. Intra-fractional spine rotations were examined on the spine alignment images before and after each treatment.
The mean (standard deviation) ITV margin was 4.1 (2.3) mm, which correlated weakly with the distance between the fiducial marker and the tumour. The mean migration distance of the marker was 1.5 (0.7) mm. The overall mean correlation model error was 1.03 (0.37) mm in the radial direction. The overall mean spine rotations were 0.27° (0.31), 0.25° (0.22), and 0.23° (0.26) for roll, pitch, and yaw, respectively. The treatment time was moderately associated with the correlation model errors and weakly related to spine rotation in the roll and yaw planes.
More caution and an additional safety margins are required when tracking a single fiducial marker.
Dosimetric impact of rotational errors in trigeminal neuralgia radiosurgery using CyberKnife
2024, Journal of Applied Clinical Medical Physics

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: Conflict of interest: none.

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Physics ContributionDosimetric Implications of Residual Tracking Errors During Robotic SBRT of Liver Metastases

Purpose

Methods and Materials

Results

Conclusions

Introduction

Section snippets

CyberKnife treatment

Maximum fiducial motion

Residual tracking errors

Discussion

Conclusions

Acknowledgments

Radiother Oncol

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Radiother Oncol

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Semin Radiat Oncol

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Multi-institutional phase I/II trial of stereotactic body radiation therapy for liver metastases

J Clin Oncol

Final results of a phase II trial for stereotactic body radiation therapy for patients with inoperable liver metastases from colorectal cancer

J Cancer Res Clin Oncol

Phase II study on stereotactic body radiotherapy of colorectal metastases

Acta Oncol

Stereotactic body radiotherapy for liver tumors: Principles and practical guidelines of the DEGRO Working Group on Stereotactic Radiotherapy

Strahlenther Onkol

Stereotactic body radiotherapy for colorectal liver metastases: A pooled analysis

Cancer

The management of respiratory motion in radiation oncology report of AAPM Task Group 76

Med Phys

The CyberKnife robotic radiosurgery system in 2010

Technol Cancer Res Treat

Physics Contribution
Dosimetric Implications of Residual Tracking Errors During Robotic SBRT of Liver Metastases