Targeting inaccuracy caused by mechanical distortion of the Leksell stereotactic frame during fixation

The Effectiveness and Toxicity of Frameless CyberKnife Based Radiosurgery for Parkinson's Disease-Phase II Study

Frame-based stereotactic radiosurgery (SRS) has an established role in the treatment of tremor in patients with Parkinson's disease (PD). The low numbers of studies of frameless approaches led to our prospective phase 2 open-label single-arm clinical trial (NCT02406105), which aimed to evaluate the safety and efficacy of CyberKnife frameless SRS. Twenty-three PD patients were irradiated on the area of the thalamic ventral nuclei complex with gradually increasing doses of 70 to 105 Gy delivered in a single fraction. After SRS, patients were monitored for tremor severity and the toxicity of the treatment. Both subjective improvement and dose-dependent efficacy were analysed using standard statistical tests. The median follow-up was 23 months, and one patient died after COVID-19 infection. Another two patients were lost from follow-up. Hyper-response resulting in vascular toxicity and neurologic complications was observed in two patients irradiated with doses of 95 and 100 Gy, respectively. A reduction in tremor severity was observed in fifteen patients, and six experienced stagnation. A constant response during the whole follow-up was observed in 67% patients. A longer median response time was achieved in patients irradiated with doses equal to or less than 85 Gy. Only two patients declared no improvement after SRS. The efficacy of frameless SRS is high and could improve tremor control in a majority of patients. The complication rate is low, especially when doses below 90 Gy are applied. Frameless SRS could be offered as an alternative for patients ineligible for deep brain stimulation; however, studies regarding optimal dose are required.

Stereotactic Positioning System: Towards a Mechanism Used in Thermal Ablation Therapy

In microwave thermal ablation, placing the antenna on a specific coordinate is one of the most critical steps. Several stereotactic systems can place an instrument on a specific point with great accuracy. However, these systems are developed for neurosurgery; moreover, a stereotactic system used in microwave thermal ablation must not disturb the electromagnetic (EM) pattern generated by the antenna. A stereotactic positioning system was designed, built, and tested. Different types of materials were proposed to manufacture the proposed stereotactic system to locate the microwave antennas. The stereotactic system can displace the microwave antenna around the Z-axis and Theta-axis. Displacements were generated by stepper motors and controlled by the user through a graphical interface. The system tests consist of programming displacements along the two axes in steps of 5 mm on the Z-axis and 5 degrees on the Theta-axis. Results showed that the system is capable of moving using cylindrical coordinates over a 70 mm displacement with an average error of ±0.85 mm for sensors on the Z-axis, while in the Theta-axis it reaches 180∘ displacement with an error of ±2.64∘. A stereotactic microwave antenna positioning system was developed and preliminarily tested. This first system can already be used to evaluate antenna performance either in phantoms or ex-vivo tissue. Moreover, this system can be extrapolated to different parts of the human body and be adapted to the required dimensions.

Discrepancies between frame- and CBCT-based stereotactic space definition on the Gamma Knife Icon

PURPOSE

To assess differences between frame-based and cone beam computed tomography (CBCT)-defined stereotactic space and to identify predictors of the observed findings.

METHODS AND MATERIALS

Differences between frame-based and CBCT-defined stereotactic space after image co-registration were reviewed for 529 patients. Treatment planning system reported the information about the shifts in X, Y, and Z coordinates of the center of the stereotactic space (i.e., coordinate X = 100 mm, Y = 100 mm, and Z = 100 mm) defined by the frame, and the maximum shot displacement (MSD) in mm. We collected the potential predictors of the differences. In total, 19 factors were investigated. We used multiple linear regression to evaluate associations with the increased differences.

RESULTS

Rotational and translational shifts greater than 1° and 1 mm, respectively, were observed in 2.6% of patients. At the same time, a decrease in tumor coverage of more than 5% was detected in 8.3% of cases. It was revealed that the higher fiducial errors (both mean and maximum), the greater weight of the patient, and the lower Karnofsky Performance Scale were predictors of increased rotational, translational shifts, and the MSD.

Accuracy of Robotic and Frame-Based Stereotactic Neurosurgery in a Phantom Model

Background

The development of robotic systems has provided an alternative to frame-based stereotactic procedures. The aim of this experimental phantom study was to compare the mechanical accuracy of the Robotic Surgery Assistant (ROSA) and the Leksell stereotactic frame by reducing clinical and procedural factors to a minimum.

Methods

To precisely compare mechanical accuracy, a stereotactic system was chosen as reference for both methods. A thin layer CT scan with an acrylic phantom fixed to the frame and a localizer enabling the software to recognize the coordinate system was performed. For each of the five phantom targets, two different trajectories were planned, resulting in 10 trajectories. A series of five repetitions was performed, each time based on a new CT scan. Hence, 50 trajectories were analyzed for each method. X-rays of the final cannula position were fused with the planning data. The coordinates of the target point and the endpoint of the robot- or frame-guided probe were visually determined using the robotic software. The target point error (TPE) was calculated applying the Euclidian distance. The depth deviation along the trajectory and the lateral deviation were separately calculated.

Results

Robotics was significantly more accurate, with an arithmetic TPE mean of 0.53 mm (95% CI 0.41–0.55 mm) compared to 0.72 mm (95% CI 0.63–0.8 mm) in stereotaxy (p < 0.05). In robotics, the mean depth deviation along the trajectory was −0.22 mm (95% CI −0.25 to −0.14 mm). The mean lateral deviation was 0.43 mm (95% CI 0.32–0.49 mm). In frame-based stereotaxy, the mean depth deviation amounted to −0.20 mm (95% CI −0.26 to −0.14 mm), the mean lateral deviation to 0.65 mm (95% CI 0.55–0.74 mm).

Conclusion

Both the robotic and frame-based approach proved accurate. The robotic procedure showed significantly higher accuracy. For both methods, procedural factors occurring during surgery might have a more relevant impact on overall accuracy.

Noninvasive Thalamotomy for Refractory Tremor by Frameless Radiosurgery

OBJECTIVE

We sought to determine whether a more widely accessible, non-invasive frameless approach to radiosurgical thalamotomy would improve objective measures of refractory essential or parkinsonian tremor without added toxicity compared to reports of frame-based radiosurgery.

METHODS

We conducted a single-arm pilot observational prospective trial of adult patients with essential or parkinsonian tremor from 2013 to 2019 and report results at one-year follow-up. Patients were treated with frameless unilateral radiosurgical ablation of the thalamic ventral intermediate nucleus to a maximum dose of 160 Gy. Treatment response was measured by the Fahn-Tolosa-Marin (FTM) tremor rating scale and the Quality of Life in Essential Tremor or Parkinson's Disease Questionnaire obtained prior to treatment and at 3, 6, 9, and 12 months.

RESULTS

Thirty-three patients, including 23 with essential tremor and 10 with Parkinson's disease, were enrolled. Overall treatment response rate per FTM was 83% (n=15/18) at 6 months. There was a marked improvement in tremor, with average total FTM reduction of 21% at 3 months (from 46 to 30 points, p=0.003) and 41% at 6 months (from 46 to 24 points, p=0.001). At 6 months, functional decline had regressed by 54% (from 15 to 7 points, p=0.001). Quality of life improved by 57% (p=0.001) at 6 months in patients with essential tremor, and patients with Parkinson's Disease had unchanged quality of life. At one-year follow-up, grade 2 neurologic adverse events were observed in 6% (n=2/33) of patients without any grade ≥ 3 events.

CONCLUSION

Noninvasive, frameless radiosurgical thalamotomy may be a feasible treatment for patients with refractory tremor and demonstrates short-term safety at one-year follow-up. This pilot study provides promising preliminary descriptions of efficacy, and definitive estimates of long-term safety and benefit require further study with longer follow-up.

Utilization of CBCT to improve the delivery accuracy of Gamma Knife radiosurgery with G-frame

Purpose

To quantify the G‐frame based stereotactic coordinate definition accuracy of Leksell coordinate G‐frame‐based Gamma Knife radiosurgery (GKRS) by the on‐board cone‐beam CT (CBCT) and establish remedial action rules to minimize the delivery errors.

Methods

We analyzed the data of 108 patients (a total of 201 tumors) treated by GKRS with G‐frame for head fixation. After co‐registering the CBCT images and plan reference images, the Leksell GammaPlan (LGP) treatment planning system provided the amount of geometric translation and rotation required to minimize the position difference between the plan and treatment. The software also calculated maximum displacement, which characterizes the position shift more clearly. We studied how much these predicted dosimetric quantities changed if the treatment was delivered without correcting the patient's position.

Results

The maximum displacement of the patient position obtained from the co‐registration of CBCT and plan reference images was 0.81 ± 0.38 mm (0.24–2.03 mm). The target coverage decreased by 3.3 ± 7.0% on average (−48.5% to +35.7%). The decrease of the target coverage, however, became smaller as the target volume increased. In particular, if the volume was greater than 2 cm³, the %change in target coverage was always less than −5%.

Conclusions

The position differences reported by the registration module of LGP were within the accuracy limit of image registration for most clinical cases, but the errors could be larger in some cases. Therefore, we propose the following decision process. We do not advise position adjustment for G‐frame based GKRS if the maximum displacement is less than 1 mm. When this limit is exceeded, however, another criterion should be applied to the decision making by considering the tumor size (or the treatment volume) together with the acceptable change of the tumor coverage.

Mask-based immobilization in Gamma Knife stereotactic radiosurgery

The Gamma Knife Icon (Elekta AB, Stockholm) is a cobalt-based stereotactic radiosurgery (SRS) unit to support the use of a thermoplastic mask in lieu of a rigid frame, using an onboard cone-beam CT (CBCT) and an intrafraction motion management system (IFMM). We retrospectively reviewed 124 patients treated with Gamma Knife SRS from January 2018 to December 2019 at our institution using a mask-based immobilization system. Patient and treatment characteristics were collected and summarized as well as interfraction shifts and treatment-related outcomes. This dataset includes 124 patients with an associated 358 intracranial tumors. Twenty-four patients presented with primary brain tumors, which included 14 meningiomas and 10 other histologies, with 100 patients having brain metastases. Sixty tumors were post-operative, while 298 were intact. The median dose for primary tumors was 25 Gy in 5 fractions. Median doses to metastases were 20 Gy in 1 fraction, 27 Gy in 3 fractions, and 25 Gy in 5 fractions. Median interfraction CBCT shifts were submillimeter. Median patient follow-up was 6.28 months. 91% of patients with metastases maintained local control. Our early clinical experience has demonstrated limited toxicity profiles and high patient tolerance, which suggests that mask-based Gamma Knife SRS provides a safe alternative option for frameless SRS. Patients with large target volumes where fractionation is preferred or with small target volumes in non-eloquent areas can be considered for this approach. Response rates are encouraging, and continued follow-up is necessary to investigate long-term control and survival.

Assessment of unintended shifts during frame-based stereotactic radiosurgery using cone beam computed tomography image guidance

PURPOSE

To investigate the frequency, magnitude and possible causes of frame-shifts that may occur between treatment planning and treatment delivery when performing Gamma Knife radiosurgery with rigid frame-based immobilization.

METHODS

Differences between computed tomography (CT) framed fiducial stereotactic coordinate reference and cone beam computed tomography stereotactic coordinates after image registration were recorded for 49 frame-based GK radiosurgery cases performed using the Gamma Knife Icon. Parameters recorded include rotational shifts, translational shifts, and the GK-computed Maximum Shot Displacement (MSD) between the two stereotactic coordinate spaces. Other patient-specific parameters were collected and linear regression analysis was performed to evaluate predictors of increased displacement.

RESULTS

The median values of rotational shifts were: pitch 0.14°, yaw 0.17°, and roll 0.13°. The median absolute values of translational shifts were: left-right 0.39 mm, anteroposterior 0.14 mm, and superior-inferior 0. 22 mm. The median value of MSD was 0.71 mm. Twelve cases (24.5%) had a MSD of greater than 1.0 mm. Male gender was associated with increased MSD (p = 0.013) and translational shifts (root-mean-squared value, p = 0.017). Cases with large differences between right and left sided pin lengths were also associated with increased MSD (p = 0.011).

CONCLUSIONS

The use of CBCT image guidance in frame-based GK radiosurgery allows unintended frame shifts to be identified and corrected. A significant fraction (24.5%) of patients had large enough shifts to result in a MSD of greater than 1.0 mm. Male gender and eccentrically placed frames were associated with increased MSD, and particular care should be taken in these cases.

Gamma Knife® icon CBCT offers improved localization workflow for frame-based treatment

Object

The purpose of this study was to compare two methods of stereotactic localization in Gamma Knife treatment planning: cone beam computed tomography (CBCT) or fiducial. While the fiducial method is the traditional method of localization, CBCT is now available for use with the Gamma Knife Icon. This study seeks to determine whether a difference exists between the two methods and then whether one is better than the other regarding accuracy and workflow optimization.

Methods

Cone beam computed tomography was used to define stereotactic space around the Elekta Film Pinprick phantom and then treated with film in place. The same phantom was offset known amounts from center and then imaged with CBCT and registered with the reference CBCT image to determine if measured offsets matched those known. Ten frameless and 10 frame‐based magnetic resonance imaging (MRI) to CBCT patient fusions were retrospectively evaluated using the TG‐132 TRE method. The stereotactic coordinates defined by CBCT and traditional fiducials were compared on the Elekta 8 cm Ball phantom, an anthropomorphic phantom, and actual patient data. Offsets were introduced to the anthropomorphic phantom in the stereotactic frame and CBCT's ability to detect those offsets was determined.

Results

Cone beam computed tomography defines stereotactic space well within the established limits of the mechanical alignment system. The CBCT to CBCT registration can detect offsets accurately to within 0.1 mm and 0.5°. In all cases, some disagreement existed between fiducial localization and that of CBCT which in some cases was small, but also was as high as 0.43 mm in the phantom domain and as much as 1.54 mm in actual patients.

Conclusion

Cone beam computed tomography demonstrates consistent accuracy in defining stereotactic space. Since both localization methods do not agree with each other consistently, the more reliable method must be identified. Cone beam computed tomography can accurately determine offsets occurring within stereotactic space that would be nondiscernible utilizing the fiducial method and seems to be more reliable. Using CBCT localization offers the opportunity to streamline workflow both from a patient and clinic perspective and also shows patient position immediately prior to treatment.

Relationship between pin type and depth of skull penetration during frame placement for Gamma Knife radiosurgery

PURPOSE

Our retrospective study addresses the largely unknown effect of fixation pin type on depth of skull penetration during Gamma Knife frame placement.

METHODS

Absolute and relative depths were compared for 404 pins of aluminum and titanium in 101 patients who underwent Gamma Knife frame placement at 0.4 Nm (3.5 lb-in) torque.

RESULTS

Effect of pin type was significant: penetration was less in aluminum than titanium pins, for absolute (0.27 mm vs. 0.83 mm) and relative (5.16% vs. 15.53%) depths (p < 0.001 each). Fifty percent of aluminum pins did not measurably penetrate the outer table of the skull whereas 95% of titanium pins penetrated into cancellous bone.

CONCLUSIONS

Although these pin penetration differences did not have a clear clinical correlate, we now recommend titanium pins for their consistent, deeper penetration, which may result in greater frame stability favorable for most clinical applications. Future studies may define optimal torque for aluminum pins.