Stereotactic Radiosurgery System Research Articles

Evaluation of CyberKnife Radiosurgery for Recurrent Trigeminal Neuralgia.

Stereotactic radiosurgery (SRS) has evolved as an accepted treatment for medication resistant trigeminal neuralgia. Initial results are very good but follow-up over three to five years shows a gradual return of pain in up to 50% of treated patients, often requiring further treatment. The results with repeat SRS using the isocentric Gamma Knife (GK) (Elekta, Stockholm, Sweden), especially in patients having initially good results, are very similar to the outcomes after the initial treatment although there is an increased risk of residual facial numbness secondary to the additional radiation dose to the trigeminal nerve. However, after 2000, non-isocentric SRS systems began to be used for treating trigeminal neuralgia including the CyberKnife (CK) (Accuray, Sunnyvale, California) as well as various linear accelerator (LINAC) based systems. This report specifically examines a series of recurrent trigeminal cases treated by the same group of physicians with the CK system. Similar doses and locations on the trigeminal nerve and/or the root entry zone were used for both initial and repeat SRS treatment regardless of system used. Although there are numerous series reporting the use of GK for recurrent treatment for recurrent trigeminal neuralgia, there are no series reviewing the results and long-term effectiveness using CK for repeat SRS for recurrent trigeminal pain. We reviewed 23 cases that had initial treatment for trigeminal neuralgia either surgically or with SRS with either the GK or CK and then a later second procedure only with CK. The follow-up after the second CK SRS ranged from three to 13 years found that the results are very similar to the multiple reports in the literature describing second or third SRS treatments with the GK. Results of repeat radiosurgery treatment of recurrent trigeminal neuralgia appear to be independent of the system used and are primarily based on proper target and dose to the trigeminal nerve.

Design and implementation of a 3D-MR/CT geometric image distortion phantom/analysis system for stereotactic radiosurgery

The design, construction and application of a multimodality, 3D magnetic resonance/computed tomography (MR/CT) image distortion phantom and analysis system for stereotactic radiosurgery (SRS) is presented.The phantom is characterized by (1) a 1 × 1 × 1 (cm)3 MRI/CT-visible 3D-Cartesian grid; (2) 2002 grid vertices that are 3D-intersections of MR-/CT-visible ‘lines’ in all three orthogonal planes; (3) a 3D-grid that is MR-signal positive/CT-signal negative; (4) a vertex distribution sufficiently ‘dense’ to characterize geometrical parameters properly, and (5) a grid/vertex resolution consistent with SRS localization accuracy.When positioned correctly, successive 3D-vertex planes along any orthogonal axis of the phantom appear as 1 × 1 (cm)2-2D grids, whereas between vertex planes, images are defined by 1 × 1 (cm)2-2D arrays of signal points.Image distortion is evaluated using a centroid algorithm that automatically identifies the center of each 3D-intersection and then calculates the deviations dx, dy, dz and dr for each vertex point; the results are presented as a color-coded 2D or 3D distribution of deviations.The phantom components and 3D-grid are machined to sub-millimeter accuracy, making the device uniquely suited to SRS applications; as such, we present it here in a form adapted for use with a Leksell stereotactic frame.Imaging reproducibility was assessed via repeated phantom imaging across ten back-to-back scans; 80%–90% of the differences in vertex deviations dx, dy, dz and dr between successive 3 T MRI scans were found to be ⩽0.05 mm for both axial and coronal acquisitions, and over >95% of the differences were observed to be ⩽0.05 mm for repeated CT scans, clearly demonstrating excellent reproducibility.Applications of the 3D-phantom/analysis system are presented, using a 32-month time-course assessment of image distortion/gradient stability and statistical control chart for 1.5 T and 3 T GE TwinSpeed MRI systems.

The use of the cyberknife stereotactic radiosurgery system for the treatment of intracranial and extracranial residual or recurrent chordoma

Objectives: Chordomas are relatively rare primary malignant bone tumors that arise from remnants of the notochord along the craniospinal axis. They tend to be locally invasive around the clival or sacro-coccygeal regions and critical neurological structures are often involved. Complete surgical excision is rarely possible and post-resection radiation enhances local control of the disease. The CyberKnife is a frameless imageguided LINAC radiosurgical system that allows treatment of both intracranial and extracranial lesion. We review here the methodology of the system and present our experience of treating skull base and spinal chordomas. Methods: Between 1994 and 2008, 22 patients with a histological diagnosis of a chordoma underwent stereotactic radiosurgery (SRS) at Stanford University Hospitals. 17 patients underwent frameless SRS at the Stanford CyberKnife Center. A retrospective and prospective review of these patients was undertaken by obtaining clinical and radiological information from an institutional review board-approved prospective database. Results: There were 11 males and 6 females with an average age of 42 years (range 10-73). 12 lesions were in the clivus, 3 in the cervical spine below C2, 1 lumbar and 1 sacral region. All but one patient had prior surgery. 9 had undergone one resection, 5 had two and 2 had three resections. Three patients had prior radiation or proton beam therapy. The average tumor volume treated was 10.9 cm3 (range 1.9-31.6 cm3), with a mean radiation dose of 29.3 Gy (range 18-50 Gy), and a mean maximum intratumoral dose of 41.0 Gy (range 24.1-67.6 Gy). Patients were followed up for an average of 4 years. Tumor control was achieved in 12 out of the 17 cases (70.6%). 5 of the 12 patients showed tumor size reduction. However, 5 out of the 17 cases showed progression with one developing a new lesion at a lower cervical vertebra despite good local control. None of the patients with tumor control developed new neurological deficits, but 4 of the 5 cases with tumor progression showed worsening neurological deficits. There was no radiation associated complications. Conclusions: Good tumor control rates can be achieved with no significant morbidity with CyberKnife based stereotactic radiosurgery. Cases with poor control tended to have complexmultiple surgical resections. Longer follow-up in larger series is required to adequately compare this technique with other forms of radiation therapy.

Refinement of MLC modeling improves commercial QA dosimetry system for SRS and SBRT patient-specific QA.

Mobius 3D (M3D) provides a volumetric dose verification of the treatment planning system's calculated dose using an independent beam model and a collapsed cone convolution superposition algorithm. However, there is a lack of investigation into M3D's accuracy and effectiveness for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT) quality assurance (QA). Here, we collaborated with the vendor to develop a revised M3D beam model for SRS/SBRT cases treated with a 6X flattening filter-free (FFF) beam and high-definition multiple leaf collimator (HDMLC) on an Edge linear accelerator. Eighty SRS/SBRT cases, planned with AAA dose algorithm and validated with Gafchromic film, were compared to M3D dose calculations using 3D gamma analysis with 2%/2 mm gamma criteria and a 10% threshold. A revised beam model was developed by refining the HD-MLC model in M3D to improve small field dose calculation accuracy and beam profile agreement. All cases were reanalyzed using the revised beam model. The impact of heterogeneity corrections for lung cases was investigated by applying lung density overrides to five cases. For the standard and revised beam models, respectively, the mean gamma passing rates were 94.6% [standard deviation (SD): 6.1%] and 98.0% [SD: 1.7%] (for the overall patient), 88.2% [SD: 17.3%] and 93.8% [SD: 6.8%] (for the brain PTV), 71.4% [SD: 18.4%] and 81.5% [SD: 14.3%] (for the lung PTV), 83.3% [SD: 16.7%] and 67.9% [SD: 23.0%] (for the spine PTV), and 78.6% [SD: 14.0%] and 86.8% [SD: 12.5%] (for the PTV of all other sites). The lung PTV mean gamma passing rates improved from 74.1% [SD: 7.5%] to 89.3% [SD: 7.2%] with the lung density overridden. The revised beam model achieved an output factor within 3% of plastic scintillator measurements for 2 × 2 cm2 MLC field size, but larger discrepancies are still seen for smaller field sizes which necessitate further improvement of the beam model. Special attention needs to be paid to small field dosimetry, MLC modeling, and inhomogeneity corrections in the beam model for SRS/SBRT QA. The improvements noted in this study, and further collaborations between clinical physicists and the vendor to refine the M3D beam model could enable M3D to become a premier SRS/SBRT QA tool.

To frame or not to frame? Cone-beam CT-based analysis of head immobilization devices specific to linac-based stereotactic radiosurgery and radiotherapy.

PurposeNoninvasive frameless systems are increasingly being utilized for head immobilization in stereotactic radiosurgery (SRS). Knowing the head positioning reproducibility of frameless systems and their respective ability to limit intrafractional head motion is important in order to safely perform SRS. The purpose of this study was to evaluate and compare the intrafractional head motion of an invasive frame and a series of frameless systems for single fraction SRS and fractionated/hypofractionated stereotactic radiotherapy (FSRT/HF‐SRT).MethodsThe noninvasive PinPoint system was used on 15 HF‐SRT and 21 SRS patients. Intrafractional motion for these patients was compared to 15 SRS patients immobilized with Cosman‐Roberts‐Wells (CRW) frame, and a FSRT population that respectively included 23, 32, and 15 patients immobilized using Gill‐Thomas‐Cosman (GTC) frame, Uniframe, and Orfit. All HF‐SRT and FSRT patients were treated using intensity‐modulated radiation therapy on a linear accelerator equipped with cone‐beam CT (CBCT) and a robotic couch. SRS patients were treated using gantry‐mounted stereotactic cones. The CBCT image‐guidance protocol included initial setup, pretreatment and post‐treatment verification images. The residual error determined from the post‐treatment CBCT was used as a surrogate for intrafractional head motion during treatment.ResultsThe mean intrafractional motion over all fractions with PinPoint was 0.62 ± 0.33 mm and 0.45 ± 0.33 mm, respectively, for the HF‐SRT and SRS cohort of patients (P‐value = 0.266). For CRW, GTC, Orfit, and Uniframe, the mean intrafractional motions were 0.30 ± 0.21 mm, 0.54 ± 0.76 mm, 0.73 ± 0.49 mm, and 0.76 ± 0.51 mm, respectively. For CRW, PinPoint, GTC, Orfit, and Uniframe, intrafractional motion exceeded 1.5 mm in 0%, 0%, 5%, 6%, and 8% of all fractions treated, respectively.ConclusionsThe noninvasive PinPoint system and the invasive CRW frame stringently limit cranial intrafractional motion, while the latter provides superior immobilization. Based on the results of this study, our clinical practice for malignant tumors has evolved to apply an invasive CRW frame only for metastases in eloquent locations to minimize normal tissue exposure.

Towards frameless maskless SRS through real-time 6DoF robotic motion compensation

Stereotactic radiosurgery (SRS) uses precise dose placement to treat conditions of the CNS. Frame-based SRS uses a metal head ring fixed to the patient’s skull to provide high treatment accuracy, but patient comfort and clinical workflow may suffer. Frameless SRS, while potentially more convenient, may increase uncertainty of treatment accuracy and be physiologically confining to some patients. By incorporating highly precise robotics and advanced software algorithms into frameless treatments, we present a novel frameless and maskless SRS system where a robot provides real-time 6DoF head motion stabilization allowing positional accuracies to match or exceed those of traditional frame-based SRS. A 6DoF parallel kinematics robot was developed and integrated with a real-time infrared camera in a closed loop configuration. A novel compensation algorithm was developed based on an iterative closest-path correction approach. The robotic SRS system was tested on six volunteers, whose motion was monitored and compensated for in real-time over 15 min simulated treatments. The system’s effectiveness in maintaining the target’s 6DoF position within preset thresholds was determined by comparing volunteer head motion with and without compensation. Comparing corrected and uncorrected motion, the 6DoF robotic system showed an overall improvement factor of 21 in terms of maintaining target position within 0.5 mm and 0.5 degree thresholds. Although the system’s effectiveness varied among the volunteers examined, for all volunteers tested the target position remained within the preset tolerances 99.0% of the time when robotic stabilization was used, compared to 4.7% without robotic stabilization. The pre-clinical robotic SRS compensation system was found to be effective at responding to sub-millimeter and sub-degree cranial motions for all volunteers examined. The system’s success with volunteers has demonstrated its capability for implementation with frameless and maskless SRS treatments, potentially able to achieve the same or better treatment accuracies compared to traditional frame-based approaches.

Dependence of volume calculation and margin growth accuracy on treatment planning systems for stereotactic radiosurgery.

Uncertainties in radiotherapy target structures are partly dependent on differences between volume calculation and margin growing methods in treatment planning systems (TPS). These uncertainties are exacerbated with very small structures such as those common in stereotactic radiosurgery. Data from a national commissioning programme for SRS was used to assess variation in reported volumes for six benchmark cases, including malignant and benign indications. Reported volumes were compared both with and without any margins added according to local practice. 137 plans were submitted, with a total of 311 structures and covering seven TPS. For volumes < 1 cm3 agreement was within 0.05 cm3, and for volumes > 1 cm3 agreement was within 5%. Systematic differences were seen between TPS, partly because of different methods for calculating the end slice volume. About one third of structures had a margin added, of 1-2 mm. Most TPS over-grew the volumes, compared to the approximation of a perfect sphere, especially Pinnacle and Eclipse. Differences between volume calculation methods may lead to 5-10% variation in reported volumes from different TPS. This should be taken into account when comparing multicentre studies, and it is recommended that a minimum volume of 0.05 cm3 be used for any near-point doses to allow more consistent comparisons. When margins are added to small structures, there may be up to 40% difference to nominal margin size. Such differences are still small compared to interobserver variation in delineation. Advances in knowledge: This study quantifies the potential uncertainties in clinical volume calculation and margin growth with small radiosurgical targets.

A Dosimetric Study of a Novel Rotating Gamma Ray System for Stereotactic Body Radiation Therapy

Stereotactic radiosurgery (SRS) unites with current state of art techniques have provided excellent conformity, precision, and versatility for small tumor lesions. Arguably the best known systems are the Gamma ray systems like GammaKnife and the linac based robotic systems like CyberKnife. However, GammaKnife systems are limited to only treating intracranial targets. In this study, we investigate a novel rotating gamma ray system for SRS of intracranial targets and stereotactic body radiation therapy (SBRT) of extracranial body sites as compared to the CyberKnife machine. The RT Pro planning system (version 1.00.4557, Prowess, Concord, CA) was used to generate treatment plans for a prototype CybeRay system for 12 patients, who were previously treated on the CyberKnife. The new CybeRay system (OUR United RT Group., Xian, China) has a rotation gantry containing 16 Co-60 sources focusing at the isocenter. The prototype implemented in RT Pro included 4 different changeable collimators while an improved version included 7 collimators. The treatment can also swing 35° in the superior direction. CybeRay is equipped with an electronic portal imaging device, a cone-beam CT system, and a 6 degrees of freedom couch. Treatment plans from both treatment systems were compared based on isodose distributions, dose volume histograms, conformity index (CI), and the estimated treatment time. CybeRay and CyberKnife results were compared using Wilcoxon signed-rank tests. The planned target volume (PTV) ranged from 2.7cc to 59.5cc. The estimated treatment delivery time for CybeRay plans ranged from 3.7-60.2 minutes as compared to 13-67minutes with Cyberknife (p=0.05). The mean value of CI was 1.23 for Cyberknife as compared to 1.27 for CybeRay (p=0.2). On the other hand, CybeRay was superior with regards to peripheral doses appearing as excessive isodose lines extending to distal normal tissues. To quantify the difference between the two systems, a ring structure (D5cm) was drawn at 5cm distant to the PTV. The maximum dose received by D5cm averaged over 12 patients was 7Gy for CybeRay and 13.4Gy for CyberKnife (p=0.0024). A newly designed rotating gamma ray machine was evaluated through a retrospective dosimetric study using patients previously treated on CyberKnife system. The new machine exhibited high precision stereotaxy and accurate target localization with promising dosimetric capabilities for stereotactic radiation therapy of extra-cranial lesions.Abstract 3565Vol (cc)Dose Gy/fxtreat timeCID5cmCRCKCRCKCRCKAdrenal2736/343.26421.291.188.718.911.850/360.2531.31.236.224.117.745/356.2521.161.2211.415.1Liver4.460/321.2341.191.194.220.44550/528.1381.091.0912.720.3pancreas7.527/311.6441.211.192.85.549.110/131.6381.381.141.53.110.210/145.5331.581.571.33.6Breast25.114/73.69131.211.211112.259.514/74.94191.211.366.28.8Lung12.648/425501.291.188.615.82.754/317.2671.371.259.213.1Mean29401.271.23713.4 Open table in a new tab

Truebeam Radiosurgery for the Treatment of Trigeminal Neuralgia: Preliminary Results at a Single Institution.

IntroductionRadiosurgery is now an established method of satisfactory pain control in patients with trigeminal neuralgia (TN). The Varian Truebeam STx (Varian Medical Systems, Palo Alto, CA) linear accelerator (LINAC) system is an arc-based, frameless stereotactic radiosurgery system used for the treatment of TN. To our knowledge, there has been only one published series of patient histories that documents the use of a frameless LINAC system for the treatment of TN. We describe the treatment parameters, patient outcomes, and complications associated with the treatment of TN.MethodsAll patients treated with the Truebeam system for TN between 2012 and 2015, with at least a six-month follow-up, were identified. A dose of 90 Gy was delivered to the isocenter using a 0.5 cm diameter cone. The cisternal segment of the trigeminal nerve was placed at the location of the LINAC isocenter using an ExacTrac™ (Brainlab, Munich, Germany) image guidance system. The radiosurgical dose, Barrow Neurologic Institute pain score (BNI PS), symptom recurrence, magnetic resonance imaging (MRI) radiographic changes, and other complications, including Barrow Neurologic Institute facial numbness score (BNI FN), were analyzed.ResultsA total of 18 patients—15 women and 3 men—with a mean age of 58 years (median: 59 years; range: 22-84 years) were treated at our institution. Fourteen patients (78%) had a BNI PS of IIIb or better, which was considered successful treatment. Twelve patients had excellent (BNI PS I) pain relief and two patients had good (BNI PS II-IIIB; recurrence after one year) pain relief. The pain of four patients recurred after a mean of 10 months.ConclusionTruebeam radiosurgery can provide effective and safe treatment for patients suffering from TN. The efficacy appears similar to other frame- and frameless-based systems

Design of a modulated orthovoltage stereotactic radiosurgery system

To achieve stereotactic radiosurgery (SRS) dose distributions with sharp gradients using orthovoltage energy fluence modulation with inverse planning optimization techniques. A pencil beam model was used to calculate dose distributions from an orthovoltage unit at 250kVp. Kernels for the model were derived using Monte Carlo methods. A Genetic Algorithm search heuristic was used to optimize the spatial distribution of added tungsten filtration to achieve dose distributions with sharp dose gradients. Optimizations were performed for depths of 2.5, 5.0, and 7.5cm, with cone sizes of 5, 6, 8, and 10mm. In addition to the beam profiles, 4π isocentric irradiation geometries were modeled to examine dose at 0.07mm depth, a representative skin depth, for the low energy beams. Profiles from 4π irradiations of a constant target volume, assuming maximally conformal coverage, were compared. Finally, dose deposition in bone compared to tissue in this energy range was examined. Based on the results of the optimization, circularly symmetric tungsten filters were designed to modulate the orthovoltage beam across the apertures of SRS cone collimators. For each depth and cone size combination examined, the beam flatness and 80-20% and 90-10% penumbrae were calculated for both standard, open cone-collimated beams as well as for optimized, filtered beams. For all configurations tested, the modulated beam profiles had decreased penumbra widths and flatness statistics at depth. Profiles for the optimized, filtered orthovoltage beams also offered decreases in these metrics compared to measured linear accelerator cone-based SRS profiles. The dose at 0.07mm depth in the 4π isocentric irradiation geometries was higher for the modulated beams compared to unmodulated beams; however, the modulated dose at 0.07mm depth remained <0.025% of the central, maximum dose. The 4π profiles irradiating a constant target volume showed improved statistics for the modulated, filtered distribution compared to the standard, open cone-collimated distribution. Simulations of tissue and bone confirmed previously published results that a higher energy beam (≥200keV) would be preferable, but the 250kVp beam was chosen for this work because it is available for future measurements. A methodology has been described that may be used to optimize the spatial distribution of added filtration material in an orthovoltage SRS beam to result in dose distributions with decreased flatness and penumbra statistics compared to standard open cones. This work provides the mathematical foundation for a novel, orthovoltage energy fluence-modulated SRS system.

Orthogonal image pairs coupled with OSMS for noncoplanar beam angle, intracranial, single-isocenter, SRS treatments with multiple targets on the Varian Edge radiosurgery system

PurposeTo characterize the accuracy of noncoplanar image guided radiation therapy with the Varian Edge radiosurgery system for intracranial stereotactic radiosurgery (SRS) treatments by assessing the accuracy of kV/kV orthogonal pair registration with Optical Surface Monitoring System (OSMS) monitoring relative to cone beam computed tomography (CT). Methods and materialsA Computerized Imaging Reference System head phantom and Encompass SRS Immobilization System were used to determine collision-free space for orthogonal image pairs (kV/kV) for couch rotations (CRs) of 45°, 30°, 15°, 345°, 330°, and 315°. Couch-induced shifts were measured using kV/kV orthogonal image pairs, OSMS, and cone beam CT. The kV/kV image pairs and OSMS localization accuracy was also assessed with respect to cone beam CT. ResultsMean orthogonal image pair differences for 315°, 330°, 345°, 15°, 30°, and 45° CRs were ≤±0.60 mm and ±0.37°. OSMS localization accuracy was ≤±0.25 mm and ±0.20°. Correspondingly, kV/kV localization accuracy was ≤±0.30 mm and ±0.5°. Shift differences for various image pairs at all CRs were ≤±1.10 mm and ±0.7°. Cone beam CT deviation was 0.10 mm and 0.00° without patient motion or CR. ConclusionBased on our study, CR-induced shifts with the Varian Edge radiosurgery system will not produce noticeable dosimetric effects for SRS treatments. Thus, replacing cone beam CT with orthogonal kV/kV pairs coupled with OSMS at the treatment couch angle could reduce the number of cone beam CT scans that are acquired during a standard SRS treatment while providing an accurate and safe treatment with negligible dosimetric effects on the treatment plan.

The spatial accuracy of two frameless, linear accelerator-based systems for single-isocenter, multitarget cranial radiosurgery.

Single‐isocenter, multitarget cranial stereotactic radiosurgery (SRS) is more efficient than using an isocenter for each target, but spatial positioning uncertainties can be magnified at locations away from the isocenter. This study reports on the spatial accuracy of two frameless, linac‐based SRS systems for multitarget, single‐isocenter SRS as a function of distance from the isocenter. One system uses the ExacTrac platform for image guidance and the other localizes with cone beam computed tomography (CBCT). For each platform, a phantom with 12 target BBs distributed up to 13.8 cm from the isocenter was aligned starting from five different initial offsets and then imaged with the treatment beam at seven different gantry and couch angles. The distribution of the resulting positioning errors demonstrated the value of adding a 1‐mm PTV margin for targets up to about 7–8 cm from the isocenter. For distances 10 cm or more, the CBCT‐based alignment remained within 1.1 mm while the ExacTrac‐based alignment differed by up to 2.2 mm.

Stereotactic radiosurgery for brain metastasis from gynecological malignancies.

Brain metastases are relatively uncommon in gynecological malignancies, and there is limited available data on their management. The present study reports the outcomes of patients with brain metastasis from gynecological malignancies who were treated with stereotactic radiosurgery (SRS). Patients with brain metastasis from a gynecological primary site were treated with SRS using the Cyberknife™ frameless SRS system. Primary lesions were treated with a single fraction of 16–22 Gy. A total of 3 resection cavities were treated with 8 Gy 3 times, meaning a total of 24 Gy, and 1 recurrent lesion was re-irradiated with 5 Gy 5 times, meaning a total of 25 Gy. All patients were followed up with regular magnetic resonance imaging and clinical examinations 1 month after treatment and every 2 months thereafter. A total of 20 lesions in 8 patients were included in this study; 1 patient presented with metastatic endometrial cancer and the remaining 7 presented with metastatic ovarian cancer. The median age was 61 years (range, 48–78 years). All patients had received systemic therapy prior to developing brain metastasis. A total of 3 patients underwent surgical resection and 1 patient was administered re-irradiation for recurrence. There were 3 local failures in 2 patients. The actuarial 1-, 2- and 3-year local control rates were 91, 91 and 76%, respectively. The median overall survival time was 29 months. No SRS-associated toxicities or neurological mortalities were observed. In conclusion, brain metastasis from gynecological malignancies is uncommon, however, SRS is a safe and effective treatment modality for local control as a primary or adjuvant treatment in patients with this disease.

Radiation Therapy for the Management of Brain Metastases.

Brain metastases are the most common malignant intracranial tumors and carry a poor prognosis. The management of brain metastases may include a variety of treatment modalities including surgical resection, radiation therapy, and/or systemic therapy. The traditional treatment for brain metastasis involved whole brain irradiation. However, improved systemic control of primary cancers has led to longer survival for some groups of patients and there is increasing need to consider the late effects of radiation to the entire brain. With advances in imaging and radiation treatment planning and delivery stereotactic radiosurgery has become more frequently utilized and may be delivered through Gamma Knife Stereotactic Radiosurgery or linear accelerator-based systems. Furthermore, experience in treating thousands of patients on clinical trials has led to diagnosis-specific prognostic assessment systems that help guide our approach to the management of this common clinical scenario. This review provides an overview of the literature supporting radiotherapy for brain metastasis and an update on current radiotherapeutic options that is tailored for the nonradiation oncologist.

Comparison of dosimetric characteristics between stationary and rotational gamma ray stereotactic radiosurgery systems based on Monte Carlo simulation

Stationary and rotational source are two types of source configuration used in commercial Gamma Ray Stereotactic Radiosurgery Systems (GRSRSs). However, it is unclear which one is better in dosimetric performance for clinical use. In this study, precise dose distributions in high resolution of the single source channel of two GRSRSs (the Elekta Leksell Gamma Knife® Model 4C (LGK 4C) for stationary GRSRS and the OUR XGD for rotational GRSRS) were generated by Monte Carlo simulation. Because of the geometrical symmetry, the overall dose distribution is generated by combining the transformed dose distributions of the single source channel. Output factors and penumbra widths were calculated and compared. The differences of output factors between two GRSRSs are minor. The penumbra widths of dose profile from all sources of the LGK 4C are 2.2-1.3-1.4 mm, 3.5-1.6-1.7 mm, 5.8-2.2-2.4 mm, and 7.2-2.6-3.0 mm (in XY-(+Z)-(−Z) form) for the secondary collimator of 4 mm, 8 mm, 14 mm, and 18 mm, respectively and those of the OUR XGD are 3.1-2.3-2.4 mm, 4.0-2.4-2.5 mm, 5.8-2.6-3.0 mm, and 7.2-3.0-3.6 mm for the secondary collimator of 5 mm, 10 mm, 15 mm, and 20 mm, respectively. The dose profiles in Z direction are not symmetrical, because all sources are distributed in +Z direction. Secondly, since the sources are located in higher latitude in the OUR XGD (14°–43°) comparing with that in the LGK 4C (6°–36°), the penumbra widths in Z direction of the OUR XGD are larger than those of the LGK 4C. Thirdly, although the penumbra widths of dose profiles from the single source channel of the LGK 4C are smaller than those of the OUR XGD, with rotational source configuration the OUR XGD has smaller penumbra width of dose profile from all sources in X–Y plane when the secondary collimator is more than 10 mm.

TU‐H‐BRC‐05: Stereotactic Radiosurgery Optimized with Orthovoltage Beams

Purpose:To achieve improved stereotactic radiosurgery (SRS) dose distributions using orthovoltage energy fluence modulation with inverse planning optimization techniques.Methods:A pencil beam model was used to calculate dose distributions from the institution's orthovoltage unit at 250 kVp. Kernels for the model were derived using Monte Carlo methods as well as measurements with radiochromic film. The orthovoltage photon spectra, modulated by varying thicknesses of attenuating material, were approximated using open‐source software. A genetic algorithm search heuristic routine was used to optimize added tungsten filtration thicknesses to approach rectangular function dose distributions at depth. Optimizations were performed for depths of 2.5, 5.0, and 7.5 cm, with cone sizes of 8, 10, and 12 mm.Results:Circularly‐symmetric tungsten filters were designed based on the results of the optimization, to modulate the orthovoltage beam across the aperture of an SRS cone collimator. For each depth and cone size combination examined, the beam flatness and 80–20% and 90–10% penumbrae were calculated for both standard, open cone‐collimated beams as well as for the optimized, filtered beams. For all configurations tested, the modulated beams were able to achieve improved penumbra widths and flatness statistics at depth, with flatness improving between 33 and 52%, and penumbrae improving between 18 and 25% for the modulated beams compared to the unmodulated beams.Conclusion:A methodology has been described that may be used to optimize the spatial distribution of added filtration material in an orthovoltage SRS beam to result in dose distributions at depth with improved flatness and penumbrae compared to standard open cones. This work provides the mathematical foundation for a novel, orthovoltage energy fluence‐modulated SRS system.

SU-F-T-576: Characterization of Two Dimensional Liquid Filled Detector Array(SRS 1000) in High Precision Cyberknife Robotic Radiosurgery System

Purpose: The aim of the study is to characterize a two dimensional liquid filled detector array SRS 1000 for routine QA in Cyberknife Robotic Radiosurgery system. Methods: SRS 1000 consists of 977 liquid filled ionization chambers and is designed to be used in small field SRS/SBRT techniques. The detector array has got two different spacial resolutions. Till field size of 5.5×5.5 cm the spacial resolution is 2.5mm (center to center) and after that till field size of 11 × 11 cm the spacial resolution is 5mm. The size of the detector is 2.3 × 2.3 0.5 mm with a volume of .003 cc. The CyberKnife Robotic Radiosurgery System is a frameless stereotactic radiosurgery system in which a LINAC is mounted on a robotic manipulator to deliver beams with a high sub millimeter accuracy. The SRS 1000's MU linearity, stability, reproducibility in Cyberknife Robotic Radiosurgery system was measured and investigated. The output factors for fixed and IRIS collimators for all available collimators (5mm till 60 mm) was measured and compared with the measurement done with PTW pin-point ionization chamber. Results: The MU linearity was measured from 2 MU till 1000 MU for doserates in the range of 700cGy/min – 780 cGy/min and compared with the measurement done with pin point chamber The MU linearity was with in 3%. The detector arrays stability and reproducibility was excellent and was withinin 0.5% The measured output factors showed an agreement of better than 2% when compared with the measurements with pinpoint chamber for both fixed and IRIS collimators with all available field sizes. Conclusion: We have characterised PTW 1000 SRS as a precise and accurate measurement tool for routine QA of Cyberknife Robotic radiosurgery system.

Technical Note: Dose gradients and prescription isodose in orthovoltage stereotactic radiosurgery.

The purpose of this work is to examine the trade-off between prescription isodose and dose gradients in orthovoltage stereotactic radiosurgery. Point energy deposition kernels (EDKs) describing photon and electron transport were calculated using Monte Carlo methods. EDKs were generated from 10 to 250 keV, in 10 keV increments. The EDKs were converted to pencil beam kernels and used to calculate dose profiles through isocenter from a 4π isotropic delivery from all angles of circularly collimated beams. Monoenergetic beams and an orthovoltage polyenergetic spectrum were analyzed. The dose gradient index (DGI) is the ratio of the 50% prescription isodose volume to the 100% prescription isodose volume and represents a metric by which dose gradients in stereotactic radiosurgery (SRS) may be evaluated. Using the 4π dose profiles calculated using pencil beam kernels, the relationship between DGI and prescription isodose was examined for circular cones ranging from 4 to 18 mm in diameter and monoenergetic photon beams with energies ranging from 20 to 250 keV. Values were found to exist for prescription isodose that optimize DGI. The relationship between DGI and prescription isodose was found to be dependent on both field size and energy. Examining this trade-off is an important consideration for designing optimal SRS systems.

Cardiac Radiosurgery for the Treatment of Atrial Fibrillation

An estimated 4 million Americans currently have atrial fibrillation (AF), the most common form of cardiac arrhythmia. The annual growth rate of AF is approximately 9.2% and advanced statistical analyses project that the prevalence of AF will be 12.1 million in the United States by 2030. The current therapeutic options to treat AF put certain patient groups at a high risk for a serious bleeding event, stroke or thromboembolism, myocardial infarction, and death. The most effective treatment for AF—catheter ablation—has been widely reported to require repeat operations and only leaves 59.4% of patients AF-free after five years. A majority of the reported complications and increased risks for catheter ablation occur because the procedure is invasive. Thus, there is a need for a novel, non-invasive therapeutic option to treat AF that can potentially expand the treatable patient population, increase patient safety, and improve clinical outcomes. CyberHeart Inc. has been exploring the efficacy and potential of cardiac radiosurgery as an option to treat heart conditions such as AF. With its proprietary CardioPlanTM software, the CyberHeart System utilizes the Cyberknife stereotactic radiosurgery platform to non-invasively ablate the heart. Early studies have displayed that the CyberHeart System is highly accurate at delivering radiation to cardiac targets and effective at minimizing radiation exposure to healthy tissue that is in close proximity to the targeted anatomic region. The focus of this review is to present the current standard of treatment for AF, identify the specific patient population that is at a high risk for the current treatment modalities, and address the potential for the CyberHeart Stereotactic Radiosurgery System to noninvasively ablate cardiac tissue and return sinus rhythm to patients with AF.

Five-fraction stereotactic radiosurgery (SRS) for single inoperable high-risk non-small cell lung cancer (NSCLC) brain metastases.

BackgroundAchieving durable local control while limiting normal tissue toxicity with definitive radiation therapy in the management of high-risk brain metastases remains a radiobiological challenge. The objective of this study was to examine the local control and toxicity of a 5-fraction stereotactic radiosurgical approach for treatment of patients with inoperable single high-risk NSCLC brain metastases.MethodsThis retrospective analysis examines 20 patients who were deemed to have “high-risk” brain metastases. High-risk tumors were defined as those with a maximum diameter greater than 2 cm and/or those located within an eloquent cortex. Patients were evaluated by a neurosurgeon prior to treatment and determined to be inoperable due to tumor or patient characteristics. Patients were treated using the CyberKnife® SRS system in 5 fractions to a total dose of 30 Gy, 35 Gy, or 40 Gy.ResultsTwenty patients with a median age of 65.5 years were treated from April 2010 to August 2014 in 5 fractions to a median total dose of 35 Gy. At a median follow up of 11.3 months local tumor control was observed in 18 of 20 metastases (90 %). Both local failures were observed in patients receiving a lower dose of 30 Gy. Median pre-treatment dexamethasone dose was 10 mg/day and median post-treatment nadir dose was 0 mg/day. Salvage intracranial therapy was required in 45 % of patients. Symptomatic radionecrosis was observed in 4 of 20 patients (20 %), two of which were treated to 40 Gy and the remainder to 35 Gy. Kaplan-Meier 1-year, 2-year, and median survival were calculated to be 45 %, 20 %, and 13.2 months, respectively.ConclusionsFive-fraction SRS to a total dose of 35 Gy appears to be a safe and effective management strategy for single high-risk NSCLC brain metastases, while a total dose of 40 Gy leads to an excess risk of neurotoxicity.