The Use of Cone Beam Computed Tomography for Image Guided Gamma Knife Stereotactic Radiosurgery: Initial Clinical Evaluation

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.

Clinical Feasibility of Using Single-isocentre Non-coplanar Volumetric Modulated Arc Therapy Combined with Non-coplanar Cone Beam Computed Tomography in Hypofractionated Stereotactic Radiotherapy for Five or Fewer Multiple Intracranial Metastases

AIMS

To evaluate the clinical feasibility of single-isocentre non-coplanar volumetric modulated arc therapy (NC-VMAT) with non-coplanar cone beam computed tomography (NC-CBCT) in hypofractionated stereotactic radiotherapy (HSRT) for five or fewer multiple brain metastases.

MATERIALS AND METHODS

Ten patients with multiple brain metastases who underwent single-isocentre NC-VMAT HSRT with limited couch rotations (within ±45°) and NC-CBCT with a limited scanning range (150-200°) were included in the current analysis. Conventional single-isocentre coplanar VMAT (C-VMAT) plans were generated and compared with NC-VMAT plans. The intracranial response and toxicities of single-isocentre NC-VMAT HSRT were also evaluated.

RESULTS

Compared with C-VMAT, NC-VMAT generated better target conformity (P < 0.05), a lower gradient index (P < 0.05) and better normal brain tissue sparing, especially for volume ≥12 Gy, with a median reduction of 12.65 cm³. For 45° couch rotation, NC-CBCT produced sufficient image quality to differentiate bony anatomy, even with a 150° scanning range, which could be successfully used for patient set-up correction. After NC-CBCT, 57.1% of the measured non-coplanar set-up errors exceeded the threshold value. The median gamma passing rate of NC-VMAT was higher than that of C-VMAT plans (P < 0.05). The non-coplanar beam of NC-VMAT with NC-CBCT corrections exhibited superior gamma passing rate to that without NC-CBCT corrections. The intracranial objective response rate and disease control rate for all patients were 80% (8/10) and 100% (10/10), respectively, and the most common toxicities were headache (20%) and dizziness (20%).

CONCLUSION

NC-VMAT with limited couch rotation (within ±45°) combined with NC-CBCT with a limited scanning range (150-200°) markedly improves the plan quality and set-up accuracy in single-isocentre multiple-target HSRT.

Assessment of intra-fraction motion during frameless image guided Gamma Knife stereotactic radiosurgery

As frameless stereotactic radiosurgery increase in use, the aim of this study was to evaluate intra-fraction motion through cone-beam CT (CBCT) and high-definition motion management (HDMM) systems. Intra-fraction motion measured between localization, repeat localization and post-treatment CBCTs were correlated to intra-faction motion indicated by the HDMM files using the Pearson coefficient (r). A total of 302 plans were reviewed from 263 patients (114 male, 149 female); 216 pairs of localization-repeat localization, and 260 localization-post-treatment CBCTs were analyzed against HDMM logs. We found the magnitude of intra-fraction motion detected by the HDMM system were larger than the corresponding CBCT results.

Initial clinical experience of surface guided stereotactic radiation therapy with open-face mask immobilization for improving setup accuracy: a retrospective study

Purpose

To propose a specific surface guided stereotactic radiotherapy (SRT) treatment procedure with open-face mask immobilization and evaluate the initial clinical experience in improving setup accuracy.

Methods and materials

The treatment records of 48 SRT patients with head lesions were retrospectively analyzed. For each patient, head immobilization was achieved with a double-shell open-face mask. The anterior shell was left open to expose the forehead, nose, eyes and cheekbones. The exposed facial area was used as region-of-interest for surface tracking by AlignRT (VisionRT Inc, UK). The posterior shell provided a sturdy and personalized headrest. Patient initial setup was guided by 6DoF real-time deltas (RTD) using the reference surface obtained from the skin contour delineated on the planning CT images. The endpoint of initial setup was 1 mm in translational RTD and 1 degree in rotational RTD. CBCT guidance was performed to derive the initial setup errors, and couch shifts for setup correction were applied prior to treatment delivery. CBCT couch shifts, AlignRT RTD values, repositioning rate and setup time were analyzed.

Results

The absolute values of median (maximal) CBCT couch shifts were 0.4 (1.3) mm in VRT, 0.1 (2.5) mm in LNG, 0.2 (1.6) mm in LAT, 0.1(1.2) degree in YAW, 0.2 (1.4) degree in PITCH and 0.1(1.3) degree in ROLL. The couch shifts and AlignRT RTD values exhibited highly agreement except in VRT and PITCH (p value < 0.01), of which the differences were as small as negligible. We did not find any case of patient repositioning that was due to out-of-tolerance setup errors, i.e., 3 mm and 2 degree. The surface guided setup time ranged from 52 to 174 s, and the mean and median time was 97.72 s and 94 s respectively.

Conclusions

The proposed surface guided SRT procedure with open-face mask immobilization is a step forward in improving patient comfort and positioning accuracy in the same process. Minimized initial setup errors and repositioning rate had been achieved with reasonably efficiency for routine clinical practice.

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.

Brain stereotactic radiosurgery using MR-guided online adaptive planning for daily setup variation: An end-to-end test

Online magnetic resonance (MR)‐guided radiotherapy is expected to benefit brain stereotactic radiosurgery (SRS) due to superior soft tissue contrast and capability of daily adaptive planning. The purpose of this study was to investigate daily adaptive plan quality with setup variations and to perform an end‐to‐end test for brain SRS with multiple metastases treated with a 1.5‐Tesla MR‐Linac (MRL). The RTsafe PseudoPatient Prime brain phantom was used with a delineation insert that includes two predefined structures mimicking gadolinium contrast‐enhanced brain lesions. Daily adaptive plans were generated using six preset and six random setup variations. Two adaptive plans per daily MR image were generated using the adapt‐to‐position (ATP) and adapt‐to‐shape (ATS) workflows. An adaptive patient plan was generated on a diagnostic MR image with simulated translational and rotational daily setup variation and was compared with the reference plan. All adaptive plans were compared with the reference plan using the target coverage, Paddick conformity index, gradient index (GI), Brain V12 or V20, optimization time and total monitor units. Target doses were measured as an end‐to‐end test with two ionization chambers inserted into the phantom. With preset translational variations, V12 from the ATS plan was 17% lower than that of the ATP plan. With a larger daily setup variation, GI and V12 of the ATS plan were 10% and 16% lower than those of the ATP plan, respectively. Compared to the ATP plans, the plan quality index of the ATS plans was more consistent with the reference plan, and within 5% in both phantom and patient plans. The differences between the measured and planned target doses were within 1% for both treatment workflows. Treating brain SRS using an MRL is feasible and could achieve satisfactory dosimetric goals. Setup uncertainties could be accounted for using online plan adaptation. The ATS workflow achieved better dosimetric results than the ATP workflow at the cost of longer optimization time.

Assessment of intra-fraction motion during automated linac-based SRS treatment delivery with an open face mask system

PURPOSE/OBJECTIVE

To evaluate intra-fraction target shift during automated mono-isocentric linac-based stereotactic radiosurgery with open-face mask system and optical real-time tracking.

MATERIALS/METHODS

Ninety-five patients were treated using automated linac-based stereotactic radiosurgery in 1-5 fractions with single isocenter for a total of 195 fractions. During treatment, patient positioning was tracked real-time with optical surface guidance and immobilized with a rigid open-face mask. Patients were re-positioned if optical surface guidance error exceeded 1 mm magnitude or 1°. Translational and rotational intra-fractional changes were determined by post-treatment CBCT matched to the planning CT. Target specific error was calculated by translation and rotation matrices applied to isocenter and target spatial coordinates.

RESULTS

For 132 fractions with isocenter within a single target, the median shift magnitude was 0.40 mm with a maximum shift of 1.17 mm. A total of 398 targets treated for plans having multiple or single targets that lied outside isocenter, resulted in a median shift magnitude of 0.46 mm, with median translational shifts of 0.20 mm and 0.20° rotational shifts. A 1 mm PTV margin was insufficient in 18% of targets at a distance greater than 6 cm away from isocenter, but sufficient for 96% of targets within 6 cm.

CONCLUSIONS

The findings of this study support 1 mm PTV expansion due to intra-fraction motion to ensure target coverage for plans with isocenter placement less than 6 cm away from the targets.

Combined effect of dose gradient and rotational error on prescribed dose coverage for single isocenter multiple brain metastases in frameless stereotactic radiotherapy

BACKGROUND

To evaluate the combined effect of rotational error and dose gradient on target dose coverage in frameless stereotactic radiotherapy.

METHODS

Three spherical targets of different diameters (1, 1.5, and 2 cm) were drawn and placed equidistantly on the same axial brain computed tomography (CT) images. To test the different isocenter-target distances, 2.5- and 5-cm configurations were prepared. Volumetric modulated arc therapy plans were created for different dose gradients from the target, in which the dose gradients were modified using the maximum dose inside the target. To simulate the rotational error, CT images and targets were rotated in two ways by 0.5°, 1°, and 2°, in which one rotation was in the axial plane and the other was in three dimensions. The initial optimized plan parameters were copied to the rotated CT sets, and the doses were recalculated. The coverage degradation after rotation was analyzed according to the target dislocation and 12-Gy volume.

RESULTS

A shallower dose gradient reduced the loss of target coverage under target dislocation, and the effect was clearer for small targets. For example, the coverage of the 1-cm target under 1-mm dislocation increased from 93 to 95% by increasing the Paddick gradient index from 5.0 to 7.9. At the same time, the widely accepted necrosis indicator, the 12-Gy volume, increased from 1.2 to 3.5 cm³, which remained in the tolerable range. From the differential dose volume histogram (DVH) analysis, the shallower dose gradient ensured that the dose-deficient under-covered target volume received a higher dose similar to that in the prescription.

CONCLUSIONS

For frameless stereotactic brain radiotherapy, the gradient, alongside the margin addition, can be adjusted as an ancillary parameter for small targets to increase target coverage or at least limit coverage reduction in conditions with probable positioning error.

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.

Evaluation of intrafractional head motion for intracranial stereotactic radiosurgery with a thermoplastic frameless mask and ceiling-floor-mounted image guidance device

PURPOSE

To evaluate intrafractional head motion (IFM) in patients who underwent intracranial stereotactic radiosurgery with the ExacTrac X-ray system (ETX) and a frameless mask.

METHODS

A total of 143 patients who completed a pre-treatment examination for IFM were eligible for this study. The frameless mask type B R408 (Klarity Medical & Equipment Co., Ltd., Guangzhou, China), which covers the back of the head, and the entire face, was used for patient immobilization. After the initial 6D correction and first X-ray verification (IFM₁), X-ray verification was performed every 3 min for a duration of 15 min. The IFM_p (2 ≤ p ≤ 6) was calculated as the positional difference from IFM₁. In addition, the inter-phase IFM (IP-IFM) and IFM_m were calculated. The IP-IFM was defined as |IFM_p - IFM_p-1|, and IFM_m as the difference between the values after all patients were asked to move their heads intentionally with the frameless mask on.

RESULTS

Both translational IFM_p and IP-IFM exceeded 1 mm for a single patient, whereas, for all patients, the translational IFM_m values were kept to within 1 mm in all directions. The proportions of the rotational IFM_p, IP-IFM, and IFM_m values within 0.5° were greater than 94.4%, 98.6%, and 90.2% for all of the rotational axes, respectively.

CONCLUSIONS

A frameless mask achieved highly accurate patient positioning in combination with ETX and a 6°-of-freedom robotic couch; however, a deviation over 1 mm and 0.5° was observed with low frequency. Therefore, X-ray verification and correction are required during treatment.

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.

Predictors of Treatment Interruption During Frameless Gamma Knife Icon Stereotactic Radiosurgery

Purpose

The Gamma Knife (GK) Icon allows for the delivery of stereotactic radiosurgery using a thermoplastic mask in combination with intrafraction motion monitoring using high definition motion management. The system pauses treatment if the magnitude of motion in all directions exceeds 1 to 1.5 mm, causing a break in treatment and prolongation of the session. We reviewed the records of patients treated in a frameless manner on our GK Icon system to determine predictors for treatment interruption.

Methods and Materials

We reviewed the records of patients treated between May 2019 and May 2020 on the GK Icon using a frameless technique for brain metastases, gliomas, schwannomas, and meningiomas. We recorded treatment time as noted in the plan document, actual treatment delivery time, and any pauses in treatment. We tabulated baseline characteristics including age, gender, diagnosis, performance status, and shifts at time of treatment. We used a receiver operating curve analysis to determine a timepoint corresponding with treatment interruption. We then conducted a logistic regression analysis to generate odds ratios for likelihood of treatment.

Results

We identified 150 patients meeting inclusion criteria. The majority (82%) were patients with brain metastases. The median age was 63 and the median dose was 27 Gy (16-30 Gy) in 3 fractions (1-5 fractions). The median treatment time was 23 minutes (4-108 minutes). Sixty-nine patients (46%) had at least 1 pause in treatment (range, 1-7). Receiver operating curve analysis revealed treatment time >19 minutes and rotation >0.47 degrees to be associated with interruption. Multivariable logistic regression revealed rotation >0.47 degrees and treatment time >19 minutes as predictive of interruption.

Conclusions

For patients with rotations exceeding 0.47 degrees or an extended treatment time, physicians should expect treatment interruptions, consider fractionation to lessen table time, or use a frame-based approach.

Patient perspectives on frame versus mask immobilization for gamma knife stereotactic radiosurgery

PURPOSE

To assess patient experiences and perspectives following Gamma Knife (GK) stereotactic radiosurgery (SRS) using frame versus mask immobilization.

METHODS

Patients who received GK-SRS using both frame and mask immobilization were included in this study. One-on-one semi-structured interviews, led by a third-party expert, were used to gain insight into the patient experience. To reduce memory bias of either immobilization device, patients underwent the interview at their follow-up appointment. Initial assessment of patient transcriptions was completed by one study staff; a second member reviewed transcripts for thematic saturation. All interviews were independently coded for themes to minimize interpretation bias.

RESULTS

Fifteen patients were consented; 12 were successfully interviewed (3 lost due to deteriorating health status). Interviews ranged from 30 to 60 min in duration. The most common patient concern regarding the frame was pain (9 patients), while the primary concerns with the mask system were the ability to remain still (6 patients) and claustrophobia (4 patients). Eleven patients chose the mask as their preferred choice in terms of their overall experience. Two themes emerged during the interviews that spoke to patient satisfaction with each process: unexpected pain with frame placement; and tightness experienced while wearing the mask during treatment.

CONCLUSIONS

From the patient perspective there was overwhelming agreement that the mask was the preferred choice for GK-SRS. The patient experience could be improved by enhanced education to better prepare patients on what to expect during the frame placement and mask treatment processes.

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.

The Application of the Virtual Environment for Radiotherapy Training to Strengthen IGRT Education

The use of simulation to enhance the quality of preclinical teaching and learning in radiation therapy is increasing. This article discusses the use of the Virtual Environment for Radiotherapy Training (VERT) in supporting teaching on image-guided radiation therapy (IGRT) and image matching concepts. The authors review the capabilities of VERT and discuss how it is currently applied in undergraduate radiation therapy teaching. The integration of IGRT theory with hands-on image matching practice using VERT simulation in educational environments has many potential benefits. These include the potential to strengthen the students' knowledge and skills in online-image acquisition and review of planar two-dimensional images and cone beam computed tomography images. It is anticipated that learner engagement will improve as well as refine analytical skills and confident practice in critical assessment of IGRT images. The authors encourage the utilization of technology that provides students with hands-on skills so they are better prepared for clinical environments.

Dosimetric effect of rotational setup errors in stereotactic radiosurgery with HyperArc for single and multiple brain metastases

Purpose

In stereotactic radiosurgery (SRS) with single‐isocentric treatments for brain metastases, rotational setup errors may cause considerable dosimetric effects. We assessed the dosimetric effects on HyperArc plans for single and multiple metastases.

Methods

For 29 patients (1–8 brain metastases), HyperArc plans with a prescription dose of 20–24 Gy for a dose that covers 95% (D_95%) of the planning target volume (PTV) were retrospectively generated (Ref‐plan). Subsequently, the computed tomography (CT) used for the Ref‐plan and cone‐beam CT acquired during treatments (Rot‐CT) were registered. The HyperArc plans involving rotational setup errors (Rot‐plan) were generated by re‐calculating doses based on the Rot‐CT. The dosimetric parameters between the two plans were compared.

Results

The dosimetric parameters [D_99%, D_95%, D_1%, homogeneity index, and conformity index (CI)] for the single‐metastasis cases were comparable (P > 0.05), whereas the D_95% for each PTV of the Rot‐plan decreased 10.8% on average, and the CI of the Rot‐plan was also significantly lower than that of the Ref‐plan (Ref‐plan vs Rot‐plan, 0.93 ± 0.02 vs 0.75 ± 0.14, P < 0.01) for the multiple‐metastases cases. In addition, for the multiple‐metastases cases, the Rot‐plan resulted in significantly higher V_10Gy (P = 0.01), V_12Gy (P = 0.02), V_14Gy (P = 0.02), and V_16Gy (P < 0.01) than those in the Ref‐plan.

Conclusion

The rotational setup errors for multiple brain metastases cases caused non‐negligible underdosage for PTV and significant increases of V_10Gy to V_16Gy in SRS with HyperArc.

Frameless Image-Guided Radiosurgery for Multiple Brain Metastasis Using VMAT: A Review and an Institutional Experience

We undertook a structured review of stereotactic radiosurgery (SRS) using linear particle accelerator (linac) equipment, focusing on volumetric modulated arc therapy (VMAT) technology, and frameless image-guided radiotherapy (IGRT), for the treatment of brain metastases. We analyzed the role of linac SRS and its clinical applications, exploring stereotactic localization. Historically, there was a shift from fixed frames to frameless approaches, moving toward less invasive treatments. Thus, we reviewed the concepts of VMAT for multiple-target applications, comparing its dosimetric and technical features to those of other available techniques. We evaluated relevant technical issues and discussed the planning parameters that have gained worldwide acceptance to date. Thus, we reviewed the current literature on the clinical aspects of SRS, especially its main indications and how the advantages of VMAT may achieve clinical benefits in such scenarios. Finally, we reported our institutional results on IGRT-VMAT for SRS treatments for patients with multiple brain metastases.

Frameless Stereotactic Radiosurgery on the Gamma Knife Icon: Early Experience From 100 Patients

BACKGROUND

The Gamma Knife (GK) Icon (Elekta AB) uses a cone-beam computed tomography (CBCT) scanner and an infrared camera system to support the delivery of frameless stereotactic radiosurgery (SRS). There are limited data on patients treated with frameless GK radiosurgery (GKRS).

OBJECTIVE

To describe the early experience, process, technical details, and short-term outcomes with frameless GKRS at our institution.

METHODS

We reviewed our patient selection and described the workflow in detail, including image acquisition, treatment planning, mask-based immobilization, stereotactic CBCT localization, registration, treatment, and intrafraction monitoring. Because of the short interval of follow-up, we provide crude rates of local control.

RESULTS

Data from 100 patients are reported. Median age is 67 yr old. 56 patients were treated definitively, 21 postoperatively, and 23 had salvage GKRS for recurrence after surgery. Forty-two patients had brain metastases, 26 meningiomas, 16 vestibular schwannomas, 9 high-grade gliomas, and 7 other histologies. Median doses to metastases were 20 Gy in 1 fraction (range: 14-21), 24 Gy in 3 fractions (range: 19.5-27), and 25 Gy in 5 fractions (range: 25-30 Gy). Thirteen patients underwent repeat SRS to the same area. Median treatment time was 17.7 min (range: 5.8-61.7). We found an improvement in our workflow and a greater number of patients eligible for GKRS because of the ability to fractionate treatments.

CONCLUSION

We report a large cohort of consecutive patients treated with frameless GKRS. We look forward to studies with longer follow-up to provide valuable data on clinical outcomes and to further our understanding of the radiobiology of hypofractionation in the brain.

Dosimetric comparisons of different hypofractionated stereotactic radiotherapy techniques in treating intracranial tumors > 3 cm in longest diameter

OBJECTIVE

The authors sought to compare the dosimetric quality of hypofractionated stereotactic radiosurgery in treating sizeable brain tumors across the following treatment platforms: GammaKnife (GK) Icon, CyberKnife (CK) G4, volumetric modulated arc therapy (VMAT) on the Varian TrueBeam STx, double scattering proton therapy (DSPT) on the Mevion S250, and intensity modulated proton therapy (IMPT) on the Varian ProBeam.

METHODS

In this retrospective study, stereotactic radiotherapy treatment plans were generated for 10 patients with sizeable brain tumors (> 3 cm in longest diameter) who had been treated with VMAT. Six treatment plans, 20-30 Gy in 5 fractions, were generated for each patient using the same constraints for each of the following radiosurgical methods: 1) GK, 2) CK, 3) coplanar arc VMAT (VMAT-C), 4) noncoplanar arc VMAT (VMAT-NC), 5) DSPT, and 6) IMPT. The coverage; conformity index; gradient index (GI); homogeneity index; mean and maximum point dose of organs at risk; total dose volume (V) in Gy to the normal brain for 2 Gy (V2), 5 Gy (V5), and 12 Gy (V12); and integral dose were compared across all platforms.

RESULTS

Among the 6 techniques, GK consistently produced a sharper dose falloff despite a greater central target dose. GK gave the lowest GI, with a mean of 2.7 ± 0.1, followed by CK (2.9 ± 0.1), VMAT-NC (3.1 ± 0.3), and VMAT-C (3.5 ± 0.3). The highest mean GIs for the proton beam treatments were 3.8 ± 0.4 for DSPT and 3.9 ± 0.4 for IMPT. The GK consistently targeted the lowest normal brain volume, delivering 5 to 12 Gy when treating relatively smaller- to intermediate-sized lesions (less than 15-20 cm3). Yet, the differences across the 6 modalities relative to GK decreased with the increase of target volume. In particular, the proton treatments delivered the lowest V5 to the normal brain when the target size was over 15-20 cm3 and also produced the lowest integral dose to the normal brain regardless of the target size.

CONCLUSIONS

This study provides an insightful understanding of dosimetric quality from both photon and proton treatment across the most advanced stereotactic radiotherapy platforms.

Assessment of motion error for frame-based and noninvasive mask-based fixation using the Leksell Gamma Knife Icon radiosurgery system

OBJECTIVE

The Leksell Gamma Knife Icon (GK Icon) radiosurgery system can utilize cone-beam computed tomography (CBCT) to evaluate motion error. This study compares the accuracy of frame-based and frameless mask-based fixation using the Icon system.

METHODS

A retrospective cohort study was conducted to evaluate patients who had undergone radiosurgery with the GK Icon system between June and December 2017. Patients were immobilized in either a stereotactic head frame or a noninvasive thermoplastic mask with stereotactic infrared (IR) camera monitoring. Setup error was defined as displacement of the skull in the stereotactic space upon setup as noted on pretreatment CBCT compared to its position in the stereotactic space defined by planning MRI for frame patients and defined as skull displacement on planning CBCT compared to its position on pretreatment CBCT for mask patients. For frame patients, the intrafractionation motion was measured by comparing pretreatment and posttreatment CBCT. For mask patients, the intrafractionation motion was evaluated by comparing pretreatment CBCT and additional CBCT obtained during the treatment. The translational and rotational errors were recorded.

RESULTS

Data were collected from 77 patients undergoing SRS with the GK Icon. Sixty-four patients underwent frame fixation, with pre- and posttreatment CBCT studies obtained. Thirteen patients were treated using mask fixation to deliver a total of 33 treatment fractions. Mean setup and intrafraction translational and rotation errors were small for both fixation systems, within 1 mm and 1° in all axes. Yet mask fixation demonstrated significantly larger intrafraction errors than frame fixation. Also, there was greater variability in both setup and intrafraction errors for mask fixation than for frame fixation in all translational and rotational directions. Whether the GK treatment was for metastasis or nonmetastasis did not influence motion uncertainties between the two fixation types. Additionally, monitoring IR-based intrafraction motion for mask fixation—i.e., the number of treatment stoppages due to reaching the IR displacement threshold—correlated with increasing treatment time.

CONCLUSIONS

Compared to frame-based fixation, mask-based fixation demonstrated larger motion variations. The variability in motion error associated with mask fixation must be taken into account when planning for small lesions or lesions near critical structures.

Stereotactic Shifts During Frame-Based Image-Guided Stereotactic Radiosurgery: Clinical Measurements

PURPOSE

To determine the magnitude and reason for discrepancies between frame- and cone beam computed tomography (CBCT)-determined stereotactic coordinates, we reviewed frame-based Gamma Knife radiosurgery procedures in which CBCT was performed before treatment.

METHODS AND MATERIALS

Clinical and treatment documentation was reviewed for 150 frame placements for which stereotactic coordinates were defined via both frame and fiducials on computed tomography imaging and CBCT. Treatment planning system-reported rotational and translational differences and standard deviations (SDs) between frame-based and CBCT-based stereotactic coordinates were recorded. Potential clinical predictors for increased differences were collected. Multiple linear regressions were performed to evaluate for associations with increased translations and rotations.

RESULTS

The absolute mean of the measured pitch, yaw, and roll shifts was 0.14 degrees (range -0.71-0.63 degrees, SD 0.19 degrees), 0.16 degrees (range -0.50 to 0.83 degrees, SD 0.21 degrees), and 0.12 degrees (range 0.37-0.51 degrees, SD 0.15 degrees), respectively. The absolute mean of the measured shifts in the left-right, anteroposterior, and superior-inferior direction was 0.29 mm (range -1.29 to 0.82 mm, SD 0.35 mm), 0.24 mm (range -0.59 to 0.33 mm, SD 0.19 mm), and 0.24 mm (range -0.69 to 0.91 mm, SD 0.27 mm), respectively. Three cases (2.0%) exceeded 1 mm in translational difference, all in the left-right direction (1.05, 1.13, and 1.29 mm). Lower Karnofsky Performance Scale status was associated with greater translational differences (vector magnitude, P = .023) and rotation (pitch, P = .044; yaw, P = .002). Usage of longer total pin length (sum of all 4 fixation pin lengths) was associated with increased rotation but not with translation (P < .001 and P = .56, respectively).

CONCLUSIONS

CBCT imaging in this cohort of frame-based cases suggests that the discrepancy in stereotactic coordinates is less than 1 mm or degree in most cases. Low Karnofsky Performance Scale status and longer total pin length correlate with larger differences between frame-defined and CBCT-defined stereotactic coordinates.

Assessment of image co-registration accuracy for frameless gamma knife surgery

Image co-registration is used in frameless gamma knife radiosurgery (GKSRS) to assign a stereotactic coordinate system and verify patient setup before irradiation. The accuracy of co-registration with cone beam computed tomography (CBCT) images of a Gamma Knife Icon^TM (GK Icon) was assessed, and the effects of the region of co-registration (ROC) were studied. CBCT-to-CBCT co-registration is used for patient setup verification, and its accuracy was examined by co-registering CBCT images taken at various configurations with a reference CBCT series. The accuracy of stereotactic coordinate assignment was investigated by co-registering stereotactic CT images with CBCT images taken at various configurations. An anthropomorphic phantom was used, and the coordinates of fifteen landmarks inside the phantom were measured. The co-registration accuracy between stereotactic magnetic resonance (MR) and CBCT images was evaluated using images from forty-one patients. The positions of the anterior and posterior commissures were measured in both a fiducial marker-based system and a co-registered system. To assess the effects of MR image distortions, co-registration was performed with four different ranges, and the accuracy of the results was compared. Co-registration between CBCT images gave a mean three-dimensional deviation of 0.2 ± 0.1 mm. The co-registration of stereotactic CT images with CBCT images produced a mean deviation of 0.5 ± 0.2 mm. The co-registration of MR images with CBCT images resulted in the smallest three-dimensional difference (0.8 ± 0.3 mm) when a co-registration region covering the skull base area was applied. The image co-registration errors in frameless GKSRS were similar to the imaging errors of frame-based GKSRS. The lower portion of the patient’s head, including the base of the skull, is recommended for the ROC.

Evaluation of stability of stereotactic space defined by cone-beam CT for the Leksell Gamma Knife Icon

The Gamma Knife Icon comes with an integrated cone‐beam CT (CBCT) for image‐guided stereotactic treatment deliveries. The CBCT can be used for defining the Leksell stereotactic space using imaging without the need for the traditional invasive frame system, and this allows also for frameless thermoplastic mask stereotactic treatments (single or fractionated) with the Gamma Knife unit. In this study, we used an in‐house built marker tool to evaluate the stability of the CBCT‐based stereotactic space and its agreement with the standard frame‐based stereotactic space. We imaged the tool with a CT indicator box using our CT‐simulator at the beginning, middle, and end of the study period (6 weeks) for determining the frame‐based stereotactic space. The tool was also scanned with the Icon's CBCT on a daily basis throughout the study period, and the CBCT images were used for determining the CBCT‐based stereotactic space. The coordinates of each marker were determined in each CT and CBCT scan using the Leksell GammaPlan treatment planning software. The magnitudes of vector difference between the means of each marker in frame‐based and CBCT‐based stereotactic space ranged from 0.21 to 0.33 mm, indicating good agreement of CBCT‐based and frame‐based stereotactic space definition. Scanning 4‐month later showed good prolonged stability of the CBCT‐based stereotactic space definition.