Assessment of the accuracy and stability of frameless gamma knife radiosurgery

Evaluation of radiation detectors for the determination of field output factors in Leksell Gamma Knife dosimetry using 3D printed phantom inserts

The Leksell Gamma Knife® Perfexion™ and Icon™ have a unique geometry, containing 192 60Co sources with collimation for field sizes of 4 mm, 8 mm, and 16 mm. 4 mm and 8 mm collimated fields lack lateral charged particle equilibrium, so accurate field output factors are essential. This study performs field output factor measurements for the microDiamond, microSilicon, and RAZOR™ Nano detectors. 3D printed inserts for the spherical Solid Water® Phantom were fabricated for microDiamond detector, the microSilicon unshielded diode and the RAZOR™ Nano micro-ionisation chamber. Detectors were moved iteratively to identify the peak detector signal for each collimator, representing the effective point of measurement of the chamber. In addition, field output correction factors were calculated for each detector relative to vendor supplied Monte Carlo simulated field output factors and field output factors measured with a W2 scintillator. All field output factors where within 1.1 % for the 4 mm collimator and within 2.3 % for the 8 mm collimator. The 3D printed phantom inserts were suitable for routine measurements if the user identifies the effective point of measurement, and ensures a reproducible setup by marking the rotational alignment of the cylindrical print. Measurements with the microDiamond and microSilicon can be performed faster compared to the RAZOR™ Nano due to differences in the signal to noise ratio. All detectors are suitable for field output factor measurements for the Leksell Gamma Knife® Perfexion™ and Icon™.

Homogenous dose prescription in Gamma Knife Radiotherapy: Combining the best of both worlds

PURPOSE

Stereotactic radiosurgery with linear accelerators (LINACs) or Leksell Gamma Knife® (LGK, Elekta AB) is an established treatment option for intracranial tumors. When those are involving/abutting organs at risk (OAR), homogenous and normofractionated treatments outmatch single fraction deliveries. In such situations, it would be desirable to balance LINAC's homogeneity benefits with LGK's dose gradient attributes. In this study, we determined homogeneity and OAR sparing ranges using a non-clinical, homogenous prototype version of LGK Lightning.

METHODS

We retrospectively analyzed thirty fractionated LGK Icon in-house patients with acoustic neuromas, pituitary adenomas and meningiomas. Four treatment plans were generated (54 Gy,1.8 Gy/fx) per patient: one LINAC plan, one clinical Lightning plan ("LGK") and two prototype Lightning plans ("LGK Hom" and "LGK OAR"). We analyzed Dmean and D2% for different OAR, Gradient Index (GI), Paddick Conformity Index (PCI), Homogeneity Index (HI) and beam-on-time (BOT).

RESULTS

While the LINAC vs. Lightning plans (LGK Hom|LGK OAR|LGK) boast better homogeneity (median: 1.08 vs. 1.18|1.24|1.35) and shorter BOT (median: 137 s vs. 432 s|510 s|510 s), Lightning plans show improved GI (median: 6.68 vs. 3.86|3.50|3.19), similar PCI (median: 0.75 vs. 0.76|0.75|0.82) and significantly reduced OAR doses. For in-tumor OAR, LGK Hom and LINAC plans achieves similar OAR sparing with improved GI for LGK Hom.

CONCLUSIONS

This study is a preliminary attempt to combine the dosimetric advantages of LINAC and LGK treatment planning. We observed that LGK plan homogeneity can be improved toward LINAC standards while maintaining the LGK advantage of favorable OAR doses and GI. Additionally, in-tumor OAR hotspots can be considerably reduced.

Plan Assessment Metrics for Dose Painting in Stereotactic Radiosurgery

Purpose

As radiation therapy treatment precision increases with advancements in imaging and radiation delivery, dose painting treatment becomes increasingly feasible, where targets receive a nonuniform radiation dose. The high precision of stereotactic radiosurgery (SRS) makes it a good candidate for dose painting treatments, but no suitable metrics to assess dose painting SRS plans exist. Existing dose painting assessment metrics weigh target overdose and underdose equally but are unsuited for SRS plans, which typically avoid target underdose more. Current SRS metrics also prioritize reducing healthy tissue dose through selectivity and dose fall-off, and these metrics assume single prescriptions. We propose a set of metrics for dose painting SRS that would meet clinical needs and are calculated with nonuniform dose painting prescriptions.

Methods and Materials

Sample dose painting SRS prescriptions are first created from Gamma Knife SRS cases, apparent diffusion coefficient magnetic resonance images, and various image-to-prescription functions. Treatment plans are found through semi-infinite linear programming optimization and using clinically determined isocenters, then assessed with existing and proposed metrics. Modified versions of SRS metrics are proposed, including coverage, selectivity, conformity, efficiency, and gradient indices. Quality factor, a current dose painting metric, is applied both without changes and with modifications. A new metric, integral dose ratio, is proposed as a measure of target overdose.

Results

The merits of existing and modified metrics are demonstrated and discussed. A modified conformity index using mean or minimum prescription dose would be suitable for dose painting SRS with integral or maximum boost methods, respectively. Either modified efficiency index is a suitable replacement for the existing gradient index.

Conclusions

The proposed modified SRS metrics are appropriate measures of plan quality for dose painting SRS plans and have the advantage of giving equal values as the original SRS metrics when applied to single-prescription plans.

Combined helical tomotherapy and Gamma Knife stereotactic radiosurgery for high-grade recurrent orbital meningioma: a case report

Orbital meningioma is a rare type of orbital tumor with high invasiveness and recurrence rates, making it extremely challenging to treat. Due to the special location of the disease, surgery often cannot completely remove the tumor, requiring postoperative radiation therapy. Here, we report a case of an elderly male patient with right-sided proptosis, visual impairment, and diplopia. Imaging diagnosis revealed a space-occupying lesion in the extraconal space of the right orbit. Pathological and immunohistochemical examination of the resected tumor confirmed it as a grade 3 anaplastic meningioma. Two months after surgery, the patient complained of right eye swelling and a magnetic resonance imaging (MRI) scan showed a recurrence of the tumor. The patient received helical tomotherapy (TOMO) in the postoperative tumor bed and high-risk areas within the orbit with a total dose of 48Gy. However, there was no significant improvement in the patient's right eye swelling, and the size of the recurrent lesion showed no significant change on imaging. Gamma knife multifractionated stereotactic radiosurgery (MF-SRS) was then given to the recurrent lesion with 50% prescription dose 13.5Gy/3f, once every other day. An imaging diagnosis performed 45 days later showed that the tumor had disappeared completely. The patient's vision remained unchanged, but diplopia was significantly relieved after MF-SRS. We propose a new hybrid treatment model for recurrent orbital meningioma, where conventional radiation therapy ensures local control of high-risk areas around the postoperative cavity, and MF-SRS maximizes the radiation dose to recurrent lesion areas while protecting surrounding tissues and organs.

On the stability of Leksell Vantage stereotactic head frame fixation in Gamma Knife radiosurgery: a study based on cone-beam computed tomography imaging and the High Definition Motion Management system

OBJECTIVE

The authors' objective was to investigate the stability of the newly introduced Vantage stereotactic frame fixation in single-fraction Gamma Knife radiosurgery.

METHODS

A total of 255 patients were included in this work and treated with the Vantage frame and Leksell Gamma Knife (LGK) Icon equipped with cone-beam computed tomography (CBCT) imaging. After the frame was mounted on the patient's head, a CT scan was acquired. After the patient was positioned on the couch of LGK, CBCT was acquired to verify the target position before treatment delivery. A second CBCT examination was acquired after treatment delivery to assess intrafractional motion. During treatment delivery, the High Definition Motion Management (HDMM) system was enabled to track a marker on the nose tip of the patient as a surrogate of intracranial motion. The stability of the Vantage frame was deconstructed into two parts: 1) motion between CT and the first CBCT prior to treatment delivery (CT-CBCT1), and 2) motion between CBCT procedures during treatment delivery (CBCT1-CBCT2). Transformation between CT and CBCT1 was given by Leksell GammaPlan, whereas transformation between CBCT1 and CBCT2 required mathematical processing of the transformation and coregistration matrices in the source files.

RESULTS

The average CT-CBCT1 displacement vector was 0.31 mm, with a range as great as 1.09 mm, and 89% and 97% of cases were within 0.5 mm and 0.7 mm, respectively. The CBCT1-CBCT2 displacement vectors averaged at 0.09 mm, with 97% of cases being within 0.2 mm. Spatial shift in the posterior direction was evident, with 94% of cases demonstrating this trend and averaging 0.05 mm. This was attributed to increased pressure on the posterior fixation pins. The HDMM displacement vectors presented larger values with an average of 0.4 mm and a range as great as 1.6 mm, and 98% of cases were within 1.0 mm. The correlation between CBCT1-CBCT2 and HDMM displacements was weak, which was attributed to the high stability of the Vantage frame and consequently small target displacements coupled with the sensitivity of HDMM to face mimics.

CONCLUSIONS

This work demonstrated that the Vantage frame possess the same degree of submillimeter stability as the well-established Leksell Coordinate Frame G (G-frame). Displacements between CT and CBCT1 were 3 times higher than between CBCT1 and CBCT2. A suggested HDMM threshold of 1.2 mm ensures a target accuracy within 0.2 mm in Vantage frame treatments.

Does Size Matter? On the Role of Stereotactic Radiosurgery for Large Vestibular Schwannomas as Seen in an Institutional Experience of Gamma Knife Radiosurgery for High-Grade Tumors

OBJECTIVE

Management of large vestibular schwannoma (VS) is controversial. Surgery has historically been the treatment of choice, but emerging literature suggests that definitive stereotactic radiosurgery is feasible. We report our institutional experience of control and morbidity outcomes treating Koos grade 3-4 VS with Gamma Knife radiosurgery (GKRS).

METHODS

An institutional review board-approved database compiled outcomes of Koos grade 3-4 VS treated by GKRS from March 2014 to January 2021 with >6 months' follow-up. Baseline symptoms per Common Terminology Criteria for Adverse Events definitions were recorded. Control rates, toxicities, and post-treatment volumetric changes were analyzed. Aggregate impairment scores (AIs) were defined by the sum of relevant Common Terminology Criteria for Adverse Events grades to categorize symptomatic burdens. Baseline and post-treatment AIs were tested for association with definitive versus adjuvant strategies.

RESULTS

In total, 34 patients with Koos grade 3-4 VS were identified, 19 treated with definitive GKRS (GKRS-D) and 15 with adjuvant GKRS (GKRS-A). Median follow-up was 34.2 months for GKRS-D and 48.8 months for GKRS-A. Patients who received GKRS-A had greater AIs at presentation (3.73 vs. 2.11, P = 0.017). Irrespective of treatment approach, tumor control rates were 100% without instances of brainstem necrosis or shunt placement. Six of 19 patients who received GKRS-D had improved post-treatment AI, and 63% of patients who received GKRS-D and 66% of patients who received GKRS-A had tumor shrinkage >20%.

CONCLUSIONS

In well-selected patients with Koos grade 3-4 VS, definitive stereotactic radiosurgery may be an appropriate strategy with excellent control and minimal toxicity. Our data suggest that the need for surgical decompression should be considered based on pretreatment symptom burden rather than tumor size.

Optimal mask fixation method for frameless radiosurgery with Leksell Gamma Knife IconTM

The Leksell Gamma Knife (LGK) Icon^TM is used for mask-based and frame-based fixation. The mask fixation provides a noninvasive method. However, an optimal mask fixation method is yet to be established. We evaluated the characteristics of three mask fixation methods (Plain, Folded, and Wide) for the LGK Icon^TM . Force-sensitive resistor sensors were attached to the forehead, supraorbital, zygoma, mandible, and occipital bone of the phantom, and digital humidity and temperature sensors were attached to both temporal lobes. Cone-beam computed tomography (CBCT) and high-definition motion management (HDMM) for each mask fixation method were used to evaluate the phantom motion during the initial application. Subsequently, the mask was removed and reapplied on the second (1st reapplication) and third days (2nd reapplication). In the initial application, forces acting on most portions of the phantom were stabilized within 1.5 h. The largest force acted on the occipital bone for the Plain and Wide methods and on the mandible for the Folded method. The temperature rapidly approaches the initial temperature, whereas the humidity gradually approached the initial humidity in all fixation methods. The Folded method exhibited a significantly lower translation along the Y-axis of the Leksell coordinate system, and rotations along all axes were under 0.5°. The HDMM values remained at 0.1 mm for all fixation methods. In the reapplications, the force acting on the occipital bone was significantly greater than that during the initial application for all mask fixation methods; the temperature and humidity remained unchanged. All mask fixation methods in the 1st reapplication were not significantly different from those in the 2nd reapplication. The Folded method is recommended as an optimal mask fixation for patients who require tight fixation; the Wide method can be considered if patient comfort is a priority.

ISRS Technical Guidelines for Stereotactic Radiosurgery: Treatment of Small Brain Metastases (≤1 cm in Diameter)

PURPOSE

The objective of this literature review was to develop International Stereotactic Radiosurgery Society (ISRS) consensus technical guidelines for the treatment of small, ≤1 cm in maximal diameter, intracranial metastases with stereotactic radiosurgery. Although different stereotactic radiosurgery technologies are available, most of them have similar treatment workflows and common technical challenges that are described.

METHODS AND MATERIALS

A systematic review of the literature published between 2009 and 2020 was performed in Pubmed using the Preferred Reporting Items for Systematic Review and Meta-analyses (PRISMA) methodology. The search terms were limited to those related to radiosurgery of brain metastases and to publications in the English language.

RESULTS

From 484 collected abstract 37 articles were included into the detailed review and bibliographic analysis. An additional 44 papers were identified as relevant from a search of the references. The 81 papers, including additional 7 international guidelines, were deemed relevant to at least one of five areas that were considered paramount for this report. These areas of technical focus have been employed to structure these guidelines: imaging specifications, target volume delineation and localization practices, use of margins, treatment planning techniques, and patient positioning.

CONCLUSION

This systematic review has demonstrated that Stereotactic Radiosurgery (SRS) for small (1 cm) brain metastases can be safely performed on both Gamma Knife (GK) and CyberKnife (CK) as well as on modern LINACs, specifically tailored for radiosurgical procedures, However, considerable expertise and resources are required for a program based on the latest evidence for best practice.

Retrospective Analysis of Treatment Workflow in Frame-Based and Frameless Gamma Knife Radiosurgery

Objective

To improve the efficiency of frame-based and frameless Gamma Knife® Icon™ (GKI) treatments by analyzing the workflows of both treatment approaches and identifying steps that lead to prolonged patient in-clinic or treatment time.

Methods

The treatment processes of 57 GKI patients, 16 frame-based and 41 frameless cases were recorded and analyzed. For frame-based treatments, time points were recorded for various steps in the process, including check-in, magnetic resonance imaging (MRI) completion, plan approval, and treatment start/end times. The time required for completing each step was calculated and investigated. For frameless treatments, the actual and planned treatment times were compared to evaluate the patient tolerance of the treatment. In addition, the time spent on room cleaning and preparation between treatments was also recorded and analyzed.

Results

For frame-based cases, the average in-clinic time was 6.3 hours (ranging from 4 to 8.7 hours). The average time from patient check-in to plan approval was 4.2 hours (ranging from 2.8 to 5.5 hours), during which the frame was placed, stereotactic reference MRI images were taken, target volumes were contoured, and the treatment plan was developed and second-checked. For patients immobilized with a mask, treatment pauses triggered by the intra-fractional motion monitoring system resulted in a significantly longer actual treatment time than the planned time. In 50 (or 55%) of the 91 frameless treatments, the patient on-table time was longer than the planned treatment time by more than 10 minutes, and in 19 (or 21%) of the treatments the time difference was larger than 20 minutes. Major treatment interruptions, defined as pauses leading to a longer than 10-minute delay, were more commonly encountered in patients with a planned treatment time longer than 40 minutes, which accounted for 64% of the recorded major interruptions.

Conclusion

For frame-based cases, the multiple pretreatment steps (from patient check-in to plan approval) in the workflow were time-consuming and resulted in prolonged patient in-clinic time. These pretreatment steps may be shortened by performing some of these steps before the treatment day, e.g., pre-planning the treatment using diagnostic MRI scans acquired a few days earlier. For frameless patients, we found that a longer planned treatment time is associated with a higher chance of treatment interruption. For patients with a long treatment time, a planned break or consideration of fractionated treatments (i.e., 3 to 5 fractionated stereotactic radiosurgery) may optimize the workflow and improve patient satisfaction.

Zero Setup Margin Mask versus Frame Immobilization during Gamma Knife® Icon™ Stereotactic Radiosurgery for Brain Metastases

We compared the clinical outcomes of BM treated with mask immobilization with zero-SM (i.e., zero-PTV) to standard zero-SM frame immobilization SRS. Consecutive patients with BM, 0.5−2.0 cm in maximal diameter, treated with single-fraction SRS (22−24 Gy) during March 2019−February 2021 were included. Univariable and multivariable analysis were performed using the Kaplan−Meier method and Cox proportional hazards regression. A total of 150 patients with 453 BM met inclusion criteria. A total of 129 (28.5%) lesions were treated with a zero-SM mask immobilization and 324 (71.5%) with zero-SM frame immobilization. Frame immobilization treatments were associated with a higher proportion of gastrointestinal and fewer breast-cancer metastases (p = 0.024), and a higher number of treated lesions per SRS course (median 7 vs. 3; p < 0.001). With a median follow up of 15 months, there was no difference in FFLF between the mask and frame immobilization groups on univariable (p = 0.29) or multivariable analysis (p = 0.518). Actuarial FFLF at 1 year was 90.5% for mask and 92% for frame immobilization (p = 0.272). Radiation necrosis rates at 1 year were 12.5% for mask and 4.1% for frame immobilization (p = 0.502). For BM 0.5−2.0 cm in maximal diameter treated with single-fraction SRS using 22−24 Gy, mask immobilization with zero SM produces comparable clinical outcomes to frame immobilization. The initial findings support omitting a SM when using mask immobilization with this treatment approach on a Gamma Knife® Icon™.

Total workflow uncertainty of frameless radiosurgery with the Gamma Knife Icon cone-beam computed tomography

Objective

Frameless treatment with the Gamma Knife Icon is still relatively new as a treatment option. As a result, additional confidence/knowledge about the uncertainty that exists within each portion of the treatment workflow could be gained especially regarding steps that have not been previously studied in the literature.

Methods

The Icon base delivery device (Perfexion) uncertainty is quantified and validated. The novel portions of the Icon such as mask immobilization, cone‐beam computed tomography image guidance, and the intrafraction motion management methods are studied specifically and to a greater extent to determine a total workflow uncertainty of frameless treatment with the Icon.

Results

The uncertainty of each treatment workflow step has been identified with the total workflow uncertainty being identified in this work as 1.3 mm with a standard deviation of 0.51 mm.

Conclusion

The total uncertainty of frameless treatment with the Icon has been evaluated and this data may indicate the need for setup margin in this setting with data that could be used by other institutions to calculate needed setup margin per their preferred recipe after validation of this data in their context.

Gamma Knife evolving instrumentation

There have been advances in both the hardware and software used in GKNS. The first major change in hardware had been Gamma Knife PERFEXION which introduced in 2006 had given more space for treatment, and removed the need for helmets, facilitating the treatment of complex conditions. Gamma Knife ICON was commissioned first in 2017. This has two important changes. It is based on the PERFEXION model, but it is constructed to permit frameless treatments. It also has an attached CBCT apparatus which may be used to define the stereotactic space. The Gamma Knife software has also improved in two important respects. The speedy calculations available to modern computer power has enabled improvements in the accuracy of the determination of intracranial radiation absorption between source and target. The other improvement has been the introduction of inverse treatment planning which continues to be under development.

Mask-Based versus Frame-Based Gamma Knife ICON Radiosurgery in Brain Metastases: A Prospective Randomized Trial

BACKGROUND

Radiosurgery is performed with a diversity of instruments relying usually either on a stereotactic frame or a mask for patient head fixation. Comfort and safety efficacy of the 2 systems have never been rigorously evaluated and compared.

MATERIAL AND METHOD

Between February 2016 and January 2017, 58 patients presenting with nonsmall cell lung cancer brain metastases have been treated by Gamma Knife radiosurgery (GKS) with random use of a frame or a mask for fixation were included patients older than 18, with <5 brain metastases (at the exclusion of brainstem and optic pathway's locations) and no earlier history of radiotherapy. The primary outcome measure was the pain scale assessment (PSA) at the beginning of the GKS procedure.

RESULTS

The PSA at the beginning of the GKS procedure was not different between the 2 groups. The PSA at the day before GKS, before magnetic resonance imaging, just after frame application, and the day after radiosurgery (departure) has shown no difference between the 2 groups. At the end of the radiosurgery itself (just after frame or mask removal) and 1 h after, the mean pain scale was higher in patients treated with the frame (p < 0.05 and p < 0.001, respectively) but 2 patients were not able to tolerate the mask discomfort and had to be treated with frame. Tumor control and morbidity probability were demonstrated to be no difference between the 2 groups in this population of patients with BM not in highly functional area. The median of the extra dose to the body due to the cone-beam computed tomography was 7.5 mGy with a maximum of 35 mGy in patients treated with a mask fixation (null in the others treated with frame). Mask fixation was associated to longer treatment time although the beam on time was not different between the 2 groups.

CONCLUSION

In selected patients, with brain oligo-metastases out of critical location, single-dose mask-based GKS can be done with a comfort and a safety efficacy comparable to frame-based GKS. There seems to be no clear patient data that confirm the value of the mask system with regards to comfort.

Personalized treatment gating thresholds in frameless stereotactic radiosurgery using predictions of dosimetric fidelity and treatment interruption

PURPOSE

Gamma Knife Icon (GKI) enables a user-defined gating threshold for intrafraction motion during stereotactic radiosurgery (SRS). An optimal threshold would ensure dosimetric fidelity of the planned distribution and minimize treatment time extension by gating. A prediction of motion characteristics for a patient based on a retrospective database of motion traces could be beneficial to evaluating the choice of gating threshold. A short acquisition of motion may help to define a personalized threshold that balances dosimetric accuracy and treatment length. This study aims to evaluate the performance of a prediction of motion and the resultant dosimetric consequences for a range of motion gating thresholds.

METHODS

A database of 2552 motion traces (776 patients) was analyzed using previously published methods to characterize patient intrafraction motion on the GKI. For a selection of six fractionated SRS patient cases (two patients with single brain metastasis, four vestibular schwannomas), a 10-min sample of motion was used to classify motion and identify traces in the database with similar metrics. The similar motion traces were used to perform a predictive reconstruction of the selected patient's delivered dose for a range of motion thresholds. The remaining fractions were reconstructed and compared to that predicted. From the six cases, 26 fractions were used to predict the number of interruptions (n = 26), change in target coverage (n = 26), and change in brainstem maximum dose (vestibular cases only, n = 20). The difference between mean predicted and reconstructed values was compared for accuracy.

RESULTS

The difference between mean prediction and reconstructed values was 0.32 ± 0.38% in target coverage, 2.36 ± 5.06 interruptions, and 0.15 ± 0.24 Gy for the brainstem maximum dose. Sixty-seven of the 72 predictions (26 coverage, 26 interruptions, and 20 brainstem maximum dose) were within one standard deviation of the predicted mean.

CONCLUSIONS

Large databases of motion traces were used to characterize patient performance and predict motion performance. Dosimetric deterioration due to motion and extension of treatment duration can be predicted in some cases using only a short acquisition of motion and the treatment plan. This reconstruction may provide benefit in generating a patient-specific motion threshold.

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.

A quality assurance tool for daily checks of the gamma knife high definition motion management system

PURPOSE

This note describes the performance of a quality assurance (QA) tool built for daily checks of the Gamma Knife's high definition motion management (HDMM) system.

METHODS

The tool is a three-dimensional (3D)-printed platform with a raised corner in the center. A reflector post is placed at the corner and the HDMM tool is zeroed to this position. Gage blocks produce very accurate gaps between the post and corner and the HDMM system's readout is compared to the gage block thickness. The HDMM system and tool were tested for noise, stability, reproducibility, linearity, accuracy and overall setup times plus ease of use.

RESULTS

The QA tool performed with accuracies better than 0.1 mm. The setup and use of this tool take less than two minutes making it a suitable tool for daily use.

CONCLUSION

This QA tool is a cost-effective solution that provides a fast and easy confirmation of the HDMM accuracy, making it suitable for daily QA checks of the HDMM system.

3D-printed headrest for frameless Gamma Knife radiosurgery: Design and validation

Purpose

Frameless Gamma Knife stereotactic radiosurgery (SRS) uses a moldable headrest with a thermoplastic mask for patient immobilization. An efficacious headrest is time consuming and difficult to fabricate due to the expertise required to mold the headrest within machine geometrical limitations. The purpose of this study was to design and validate a three‐dimensional (3D)‐printed headrest for frameless Gamma Knife SRS that can overcome these difficulties.

Materials and methods

A headrest 3D model designed to fit within the frameless adapter was 3D printed. Dosimetric properties of the 3D‐printed headrest and a standard‐of‐care moldable headrest were compared by delivering a Gamma Knife treatment to an anthropomorphic head phantom fitted with an ionization chamber and radiochromic film. Ionization measurements were compared to assess headrest attenuation and a gamma index was calculated to compare the film dose distributions. A volunteer study was conducted to assess the immobilization efficacy of the 3D‐printed headrest compared to the moldable headrest. Five volunteers had their head motion tracked by a surface tracking system while immobilized in each headrest for 20 min. The recorded motion data were used to calculate the average volunteer movement and a paired t‐test was performed.

Results

The ionization chamber readings were within 0.55% for the 3D‐printed and moldable headrests, and the calculated gamma index showed 98.6% of points within dose difference of 2% and 2 mm distance to agreement for the film measurement. These results demonstrate that the headrests were dosimetrically equivalent within the experimental uncertainties. Average motion (±standard deviation) of the volunteers while immobilized was 1.41 ± 0.43 mm and 1.36 ± 0.51 mm for the 3D‐printed and moldable headrests, respectively. The average observed volunteer motion between headrests was not statistically different, based on a P‐value of 0.466.

Conclusions

We designed and validated a 3D‐printed headrest for immobilizing patients undergoing frameless Gamma Knife SRS.

Prospective assessment of mask versus frame fixation during Gamma Knife treatment for brain metastases

PURPOSE

The newest generation of the Leksell Gamma Knife (GK) allows frame based as well as frameless treatment. We here report outcomes of a prospective non-randomized study on mask fixation (MF) versus frame fixation (FF) for GK treatment of brain metastases.

METHODS

The decision for FF or MF was made on a case-by-case basis. Factors considered were patients' preference, proximity of critical structures, V₁₂ and treatment time. Either stereotactic radiosurgery or fractionated stereotactic radiotherapy in up to 3 fractions was performed. For MF, a PTV margin of 1 mm was added. Follow-up included quarterly MRI scans. The primary outcome was local control. Secondary endpoints were progression-free survival (PFS), overall survival (OS) and the incidence of radionecrosis.

RESULTS

A total of 197 lesions (169 FF and 28 MF) were treated in 76 patients (59 FF and 17 MF). 187 lesions were treated with SRS and 10 with FSRT. Median dose was 22 Gy in both groups and median follow-up was 9.3 months. There was a higher local failure rate (HR: 3.69; 95%CI: 1.13-12.0; p = 0.03) with 11 local failures in the FF and none in the MF cohort. No differences were observed between the groups for OS (median: n.r. vs. 16.9 months; HR:1.00; 95%CI: 0.41-2.46; p = 0.999) and PFS (median: 6.9 vs. 8.4 months; HR: 0.92; 95%CI: 0.47-1.79; p = 0.800). Three cases of radionecrosis occurred with FF but none with MF (p = 0.67).

CONCLUSIONS

Gamma Knife treatment with MF does not result in worse outcome or increased rates of radionecrosis in this non-randomized study.

Dosimetric comparison of fractionated radiosurgery plans using frameless Gamma Knife ICON and CyberKnife systems with linear accelerator-based radiosurgery plans for multiple large brain metastases

OBJECTIVE

For patients with multiple large brain metastases with at least 1 target volume larger than 10 cm3, multifractionated stereotactic radiosurgery (MF-SRS) has commonly been delivered with a linear accelerator (LINAC). Recent advances of Gamma Knife (GK) units with kilovolt cone-beam CT and CyberKnife (CK) units with multileaf collimators also make them attractive choices. The purpose of this study was to compare the dosimetry of MF-SRS plans deliverable on GK, CK, and LINAC and to discuss related clinical issues.

METHODS

Ten patients with 2 or more large brain metastases who had been treated with MF-SRS on LINAC were identified. The median planning target volume was 18.31 cm3 (mean 21.31 cm3, range 3.42-49.97 cm3), and the median prescribed dose was 27.0 Gy (mean 26.7 Gy, range 21-30 Gy), administered in 3 to 5 fractions. Clinical LINAC treatment plans were generated using inverse planning with intensity modulation on a Pinnacle treatment planning system (version 9.10) for the Varian TrueBeam STx system. GK and CK planning were retrospectively performed using Leksell GammaPlan version 10.1 and Accuray Precision version 1.1.0.0 for the CK M6 system. Tumor coverage, Paddick conformity index (CI), gradient index (GI), and normal brain tissue receiving 4, 12, and 20 Gy were used to compare plan quality. Net beam-on time and approximate planning time were also collected for all cases.

RESULTS

Plans from all 3 modalities satisfied clinical requirements in target coverage and normal tissue sparing. The mean CI was comparable (0.79, 0.78, and 0.76) for the GK, CK, and LINAC plans. The mean GI was 3.1 for both the GK and the CK plans, whereas the mean GI of the LINAC plans was 4.1. The lower GI of the GK and CK plans would have resulted in significantly lower normal brain volumes receiving a medium or high dose. On average, GK and CK plans spared the normal brain volume receiving at least 12 Gy and 20 Gy by approximately 20% in comparison with the LINAC plans. However, the mean beam-on time of GK (∼ 64 minutes assuming a dose rate of 2.5 Gy/minute) plans was significantly longer than that of CK (∼ 31 minutes) or LINAC (∼ 4 minutes) plans.

CONCLUSIONS

All 3 modalities are capable of treating multiple large brain lesions with MF-SRS. GK has the most flexible workflow and excellent dosimetry, but could be limited by the treatment time. CK has dosimetry comparable to that of GK with a consistent treatment time of approximately 30 minutes. LINAC has a much shorter treatment time, but residual rotational error could be a concern.

Evaluation of a new secondary dose calculation software for Gamma Knife radiosurgery

Current available secondary dose calculation software for Gamma Knife radiosurgery falls short in situations where the target is shallow in depth or when the patient is positioned with a gamma angle other than 90°. In this work, we evaluate a new secondary calculation software which utilizes an innovative method to handle nonstandard gamma angles and image thresholding to render the skull for dose calculation. 800 treatment targets previously treated with our GammaKnife Icon system were imported from our treatment planning system (GammaPlan 11.0.3) and a secondary dose calculation was conducted. The agreement between the new calculations and the TPS were recorded and compared to the original secondary dose calculation agreement with the TPS using a Wilcoxon Signed Rank Test. Further comparisons using a Mann‐Whitney test were made for targets treated at a 90° gamma angle against those treated with either a 70 or 110 gamma angle for both the new and commercial secondary dose calculation systems. Correlations between dose deviations from the treatment planning system against average target depth were evaluated using a Kendall’s Tau correlation test for both programs. The Wilcoxon Signed Rank Test indicated a significant difference in the agreement between the two secondary calculations and the TPS, with a P‐value < 0.0001. With respect to patients treated at nonstandard gamma angles, the new software was largely independent of patient setup, while the commercial software showed a significant dependence (P‐value < 0.0001). The new secondary dose calculation software showed a moderate correlation with calculation depth, while the commercial software showed a weak correlation (Tau = −.322 and Tau = −.217 respectively). Overall, the new secondary software has better agreement with the TPS than the commercially available secondary calculation software over a range of diverse treatment geometries.

Geometric and dosimetric effects of image co-registration workflows for Gamma Knife frameless radiosurgery

The Gamma Knife® Icon™ CBCT facilitates frameless radiosurgery. In the vendor-recommended workflow, MRI is co-registered directly to CBCT for planning. Alternatively, MRI is co-registered to a diagnostic CT, which is then co-registered to CBCT. Our objective is to evaluate if this additional CT is necessary for more accurate registrations. Nine small spherical targets were generated onto 14 patient data-sets. Single-shot treatment plans were created. Geometric and dosimetric differences between the two workflows were determined. Mean target displacement was 0.5±0.3mm; average PTV coverage loss was 4.3±5.0%. For 19 clinical targets in 14 patients, the mean displacement and coverage change was 0.6±0.4mm and 1.3±1.6%. Eleven surrogate landmarks were contoured on a phantom MRI and registered to the CBCT using both workflows. The registration uncertainty was 0.50±0.65mm and 0.32±0.47mm for the MRI-CT-CBCT and MRI-CBCT respectively. As neither workflow was significantly more accurate, the additional CT is unnecessary for most cases.

Real-Time Infrared Motion Tracking Analysis for Patients Treated With Gated Frameless Image Guided Stereotactic Radiosurgery

PURPOSE

The transition from frame-based brain stereotactic radiosurgery (SRS) to frameless delivery is supported by real-time intrafraction monitoring to ensure accurate delivery. The purpose of this study is to characterize these real-time motion traces in a large cohort of patients treated with frameless gated brain SRS and to develop patient-specific predictions of tolerance violations.

METHODS AND MATERIALS

SRS patients treated on the Gamma Knife Icon were immobilized using a device-specific thermoplastic head mask. A motion marker was fixed to the patient's nose, with gating and cone beam computed tomography (CBCT)-based corrections to the treatment at excursions from baseline exceeding 1.5 mm. The traces of 1446 fractions were analyzed according to magnitude (932 unique treatment plans for 462 unique individual patients), directional distribution of displacement, and stability. A neural network model was developed to predict interruptions based on a subset of trace data.

RESULTS

The average displacement of the marker in the first fraction of all patients was 0.62 ± 0.25 mm with beam CBCT corrections, which would otherwise be modeled at 0.96 ± 0.96 mm without intrafraction motion correction (P < .0001). Twenty-nine percent of fractions delivered were interrupted, of which the Z-axis (superoinferior) motion was the largest contributor to excursion. Baseline corrections significantly compensated for the magnitude of motion in all 3 dimensions (P < .01). The motion relative to the first acquired CBCT was on average seen to consistently increase with treatment time, with the minimum P value occurring at 61.3 minutes. The neural network prediction model was able to predict treatment interruptions with 84% sensitivity on the first 5-minute sample of the trace.

CONCLUSIONS

Corrections to marker position significantly decreased marker excursions in all 3 axes compared with a single CBCT alignment. Patient-specific modeling may aid in the optimization of cases selected for frameless radiosurgery to increase the accuracy of planned delivery.

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.

Performance of the cone beam computed tomography-based patient positioning system on the Gamma Knife Icon™

PURPOSE

Cone beam computed tomography (CBCT) imaging has been implemented on the Leksell Gamma Knife^® Icon™ for assessing patient positioning in mask-based Gamma Knife radiosurgery. The purpose of this study was to evaluate the performance of the CBCT-based patient positioning system as a tool for frameless Gamma Knife radiosurgery.

METHODS

Daily quality assurance (QA) CBCT precision test results from a 12-month period were analyzed for the geometric accuracy and the stability of the imager. The performance of the image acquisition module and the image registration algorithm was evaluated using an anthropomorphic head phantom (CIRS Inc., Norfolk, VA) and a XYZR axis manual positioning stage (TOAUTO Inc., Guangdong, China). The head phantom was fixed on a mask adaptor and manually translated in the X, Y, Z directions or rotated around the X, Y, Z axes in the range of ±10 mm or ±10º. A CBCT scan was performed after each manual position setup followed by an image registration to the reference scan. To assess the overall setup uncertainties in fractionated treatment, two cylindrical Presage phantoms (Heuris Inc., Skillman, NJ) of 15 cm diameter and 10 cm height were irradiated with identical prescription dose and shot placement following standard mask-based treatment workflow according to two different fraction schedules: a single fraction treatment of 7.5 Gy and a 5-fraction treatment with 1.5 Gy per fraction.

RESULTS

The averaged vector deviations of the four marks from their preset values are 0.087, 0.085, 0.095, and 0.079 mm from the 212 daily QA tests. The averaged displacements in the X, Y, Z coordinates and the pitch, yaw, roll angles from the image registration tests are 0.23, 0.27, 0.14, 0.32º, 0.19º, 0.31º from the manual setup. The corresponding maximum differences are 0.41, 0.33, 0.29 mm, 0.45º, 0.31º, and 0.43º, respectively. Compared to the treatment plan using the 2% & 1 mm criteria, the averaged 2D Gamma passing rate is 98.25% for the measured dose distribution from the Presage phantom with 1-fraction irradiation and 95.12% for the 5-fraction irradiation. The averaged Gamma passing rates are 99.53% and 98.16% for the 1-fraction and 5-fraction irradiations using the 2% & 2 mm criteria.

CONCLUSIONS

The CBCT imager and the image registration algorithm can reproduce phantom position with <0.5 mm/0.5º uncertainty. A systematic contribution from the interfraction phantom repositioning procedure was observed in the Gamma analysis over the irradiated volumes of two end-to-end test phantoms.

Simultaneous optimization of isocenter locations and sector duration in radiosurgery

Stereotactic radiosurgery is an effective technique to treat brain tumors for which several inverse planning methods may be appropriate. We propose an integer programming model to simultaneous sector duration and isocenter optimization (SDIO) problem for Leksell Gamma Knife^® Icon^TM (Elekta, Stockholm, Sweden) to tractably incorporate treatment time. We devise a Benders decomposition scheme to solve the SDIO problem to optimality. The performances of our approaches are assessed using anonymized data from eight previously treated cases, and obtained treatment plans are compared against each other and against the clinical plans. The plans generated by our SDIO model all meet or exceed clinical guidelines while demonstrating high conformity.

A high-precision, geometric and registration accuracy full-system test method for adaptive SRS, demonstrated on Gamma Knife® Icon™

A novel full-system test (FST) phantom and method have been developed to demonstrate and quality assure the geometric accuracy of image co-registration and overall shot delivery in the context of SRS using Gamma Knife® Icon™. The method uses Vernier scale bars to achieve sub-voxel precision co-registration measurements and pin-located radiochromic films to determine overall shot delivery precision. Validation tests demonstrated that artificially applied registration errors of < 0.15 mm could be accurately detected and quantified. Cross-validation of full-system test results with the manufacturer standard focal precision test demonstrated that both approaches measure similar focal precision errors, to within < 0.1 mm, and that registration and focal precision components of the full-system geometric error can be successfully decoupled using our Vernier FST approach. CBCT co-registration errors were shown to be of comparable magnitude to the focal precision errors, demonstrating that CBCT registration based in-mask treatments can achieve sub-voxel geometric accuracy, rivalling traditional frame-based immobilisation. This full-system geometric test method and phantom design concept is in principle applicable to any SRS technique involving image co-registration.