Gamma Knife Unit Research Articles

Characterization of the radiation field surrounding the Leksell Gamma Knife® and shielding applications

The aim of this study is to improve the characterization and modeling of the radiation field surrounding the Leksell Gamma Knife®-PerfexionTM. The improved characterization of the radiation field enables more accurate shielding calculations to be performed for the areas adjacent to the treatment room. With the aid of a high-purity germanium detector and a satellite dose rate meter, γ-ray spectra and ambient dose equivalent H*(10) data were acquired at various locations in the field of a Leksell Gamma Knife unit in a treatment room at Karolinska University Hospital, Sweden. These measurements were used to validate the results of the PEGASOS Monte Carlo simulation system with a PENELOPE kernel. The levels of the radiation that passes through the shielding of the machine (leakage radiation) are shown to be much lower than what is suggested by various bodies, e.g. the National Council on Radiation Protection and Measurements, to be used when calculating radiation shielding barriers. The results clearly indicate that Monte Carlo simulations may be used in structural shielding design calculations for γ rays from the Leksell Gamma Knife.

Gamma Knife Radiosurgery for Central Arteriovenous Malformations— A Single-Center Experience

Introduction: Arteriovenous malformations with basal ganglia, brainstem, and thalamic locations represent difficult vessel anomalies with critical locations. These central aneurysms are related with high morbidity and mortality rates. This retrospective study aims to examine the results of patients with basal ganglia, brainstem, and thalamic arteriovenous malformations remedied with gamma knife radiosurgery (GKS) at our clinic.
 Method: May 2005 - December 2020, the results of patients with GKS basal ganglia, thalamus, and brainstem located AVM were analyzed, in Gazi University Gamma Knife Unit.
 Results: May 2005 - December 2020, a total of 859 patients with intracranial AVM were treated at the Gamma Knife Unit of the Gazi University Neurosurgery Clinic. Seventy-three patients with basal ganglia, brainstem, and thalamic locations were seen. Of the total 73 patients included in our study, 14 patients (19.2%) in the basal ganglia, 13 patients (17.8) in the brain stem, and 46 patients (63%) in the thalamic region were located. The mean volume of AVMs is 4565,54 mm3. The patients who underwent embolization before gamma knife surgery treatment were found to be 19.2%. The mean of AVMs' obliteration time after SRS is 38.4 months. We detected that there wasn't a remarkable statistical rate relationship between AVMs volume, Spetzler Martin grading scale, nidus type, gender, and prior embolization (p = 0.05).
 Conclusions: GKS is an best choice treatment method used for all AVMs today. However, it has been accepted as the first choice treatment method, especially in central AVMs, which are risky in terms of mortality and morbidity, and the results are also quite good.

The Impact of Dose on Treatment Outcomes in Patients Treated for Brain Metastases With a Hypofractionated Technique on Gamma Knife

Purpose: Brain metastases are the most common type of diagnosed central nervous system tumours, occurring in approximately 40% of all cancer patients. Historically, treatment consisted of whole brain radiotherapy (WBRT), however current practice for patients with limited intracranial disease, is to offer stereotactic radiosurgery (SRS). In 2016, our centre acquired a Gamma Knife (GK) unit, designed to treat brain metastases with a stereotactic approach, using an invasive frame based immobilization system. In 2017, a mask based frameless system was introduced, expanding indications for patients and allowing for the treatment of hypofractionated courses on this unit. Given the unique dosimetric characteristics of the GK, our research focused on patients treated for intact brain metastases, with hypofractionated doses, and aimed to report adverse radiation effects and local control. Methods: This was a retrospective study, including all patients with intact brain metastases, treated on the GK unit, with hypofractionated (5 fraction) treatments, between 2017-2020. Hypofractionated treatments are generally given to larger lesions (≥2cm) or those lesions situated in eloquent areas of the brain, such as the motor cortex or brainstem. Patients were excluded if the specific metastases received prior SRS or hypofractionated treatment, however those who received previous whole brain radiotherapy were included. Those without post -treatment imaging follow up were also excluded from the analyses. Competing risk methods were used to determine local control and symptomatic adverse radiation effect (ARE) rates per lesion and Kaplan Meier methods were used to determine survival estimates. The Common Terminology Criteria for Adverse events (CTCAE) v 5.0 was used to determine symptomatic ARE. Results: A total of 146 patients and 299 metastases were identified. The most frequent histologies were lung (38.5%), breast (31.4%) and melanoma (13.7%). 53.4% of patients had a KPS > 70% and the majority had multiple lesions treated at the time of their hypofractionated radiosurgery (62.3%). Only 18% had received prior WBRT. Prescribed doses ranged from 20-27.5Gy with the median being 27.5Gy in 5 fractions prescribed in 54.2% of cases. The median volume of the treated lesions was 2.54cc (range 0.004-24.92cc). Clinical follow-up ranged from 0.5 to 49.9 months, with the median being 13.8 months. The 1 year local control was 84.8%, and on multivariable analyses, 1 year local failure rates increased (8.3% (95% CI, 4.6-13.2) vs 23.5% (95%CI,16.6-31)) with decreasing prescribed doses (27.5Gy vs 20-25Gy) respectively. In terms of adverse radiation effects, there were 32 (10.7%) incidents identified, of which 14 (44%) were symptomatic. For symptomatic AREs, the 1 and 2 year rates were 1.7% (95% CI, 0.7-3.8) and 4.5% (95% CI, 2.4-7.7), respectively. Overall survival for this patient population was 12.7 months, with the 1 and 2 year rates being 51.3% (95% CI 42.6-59.2) and 36.8% (95%, CI 28.5-45.1), respectively. Conclusion: For patients with intact brain metastases, GK hypofractionated treatments are a safe and effective treatment modality. A 27.5Gy prescription dose in 5 fractions is associated with improved local control and a low risk of symptomatic ARE at one year.

Determination of Collimator Helmet Factors for Leksell Gamma Knife 4C Unit Using GAF Chromic EBT3 Film and ImageJ Software.

Background:Measurement of Collimator helmet factors (CHF) is an important quality assurance procedure to be performed on Leksell Gamma Knife unit at regular interval to make sure that the interchangeable collimator helmet fit into the source channels without any positional inaccuracy which leads to major treatment error. The primary aim of this study is to measure the CHFs for Elekta Leksell Gamma knife 4C helmets using GafChromic EBT3 film and Image J software. Methods:GafChromic EBT3 film, EPSON expression 10000 XL scanner and Image J analysis software was used for this study. The calibration curve of GafChromic EBT3 film was generated with known dose values for 14 mm collimator helmet using ImageJ software. The collimator helmet factor (CHF) for 4mm, 8mm and 14 mm collimator helmets were measured by normalizing dose rates of 4mm, 8mm and 14 mm to the dose rate of 18 mm collimator helmet using the previously generated calibration curve. The measured CHF was compared to Elekta reference value and previously published mean values. Results:The measured CHFs were 0.896, 0.958, and 0.986 for 4mm, 8mm and 14mm collimators respectively. The percentage difference obtained was 1.7 %, 0.21 %, 0.1 % between measured values and reference values. Conclusion:The measurement of CHFs in LGK 4C unit using GafChromic EBT3 film and ImageJ software is a reliable method to verify the manufacturer quoted CHFs in routine quality assurance procedures.

C-methionine-PET-guided Gamma Knife radiosurgery boost as adjuvant treatment for newly diagnosed glioblastomas

Background: The most common glial tumor is the glioblastoma, and the prognosis remains dismal despite a multimodal therapeutic approach. The role of radiosurgery for the treatment of glioblastomas has been evaluated in several studies with some benefit at the recurrent stage. We evaluate the results of the protocol administered at the Gamma Knife unit administering radiosurgery as a boost to metabolic active parts of the tumor after the patient had completed traditional external beam radiotherapy (XBRT) as part of the Stupp protocol for high-grade gliomas.Methods: This is a retrospective analysis of seven patients with newly diagnosed glioblastomas who were treated with Gamma Knife radiosurgery as a boost after receiving XBRT as part of the Stupp protocol. The target of radiation was determined according to the findings of the C-methionine PET scan in relation to magnetic resonance images. The primary end point of this study was to determine the progression-free survival (PFS) from the time of diagnosis.Results: The median age of patients was 48.8 years and the mean Karnofsky performance score was 92.8%. The median PFS was 12.4 months. No radiation adverse effects were documented.Conclusion: Stereotactic radiosurgery is safe to use in the upfront treatment for these patients and appears to have a beneficial role in improving the PFS. This beneficial role seems to be conditioned not only by the time the treatment is administered but also where the radiation dose is targeted to.

Adoption of focused ultrasound thalamotomy for essential tremor: why so much fuss about FUS?

BackgroundFocused ultrasound (FUS) was approved as a new treatment modality for essential tremor (ET) in 2016. The goal of this study was to quantify FUS adoption for ET and understand...

Radiation Concerns for the Neuroanesthesiologists

AbstractWith the advent of minimally invasive neurosurgical techniques and rapid innovations in the field of neurointervention, there has been a sharp rise in diagnostic and therapeutic modalities requiring radiation exposure. Neuroanesthesiologists are currently involved in various procedures inside as well as outside the operating room (OR) like intensive care units, interventional suites, and gamma knife units. The ambit expands from short-lasting diagnostic scans to lengthy therapeutic procedures performed under fluoroscopic guidance. Hence, a modern-day neuroanesthesiologist has to bear the brunt of the radiation exposure in both inside and outside the OR. However, obliviousness and nonadherence to the relevant radiation safety measures are still prevalent. Radiation protection and safety are topics that need to be discussed with new vigor in the light of current practice.

Application of polymer dose gels for millimeter scale target/tumor pre-treatment immitation using gamma knife facility

Polymer gels are widely used for absorbed dose verification in radiotherapy, providing information about 3D dose distribution in the irradiated volume as well as serving as a simulation phantom, for the observation of radiation induced structural changes, related to gel’s polymerization upon irradiation. Due to high spatial resolution and unable sensitivity through gel’s composition, dosimeter gels is a powerful and precise tool for verification of individual treatment plans especially when high treatment doses are prescribed.The aim of this work was to modify and characterize gel dosimeters that are capable of millimetre scale target/tumour imitation and high dose assessment in small irradiated volume, for their further applications in clinical dosimetry and verification of millimetre scale target treatment procedures using Gamma knife unit.

Gamma knife surgery in the treatment of intracanalicular vestibular schwannomas.

According to the literature, gamma knife surgery (GKS) is a promising method for intracanalicular vestibular schwannoma (IVS) management, providing excellent tumor growth control rates (91%-100%) and good hearing preservation rates (41%-76%), but this evidence originates primarily from a small series of patients. The aim of this study was to present the outcomes of GKS in the largest group of patients with IVS studied to date, with particular emphasis on the long-term outcomes of treatment. The study included 136 consecutive patients with unilateral IVS, who underwent GKS in 2011-2015. Mean age of the patients was 54±12.6years. All patients were operated on with a 192-source cobalt-60 gamma knife unit. All patients had complete follow-up documentation and the mean duration of the follow-up was 52±13.8months (6-83months). Neurological status (facial and trigeminal nerve), hearing and instability/dizziness presence were determined prior to GKS, immediately after the procedure, and during the follow-up visits. Tumor growth control was obtained in 124/136 (~91.2%) patients. Hearing improvement was observed in 32/136 (23.5%) patients, and there was a distinct cluster of 9 patients (6.6%) regaining serviceable hearing after GKS, whereas in 36 patients (26.5%) was stable. Four patients developed facial nerve dysfunction, including 3 periodic hemifacial spasm and 1 partial paresis, which resolved spontaneously within 12months of GKS. None of the operated patients showed new, debilitating neurological deficits, including trigeminal sensory disturbances or hydrocephalus. GKS is a highly effective treatment for IVS, associated with low morbidity and good tumor growth control.

100 Years since the birth of Ladislau Steiner. Creativity of Neurosurgery.

Ladislau Steiner (1920–2013) was a Romanian neurosurgeon, born in the historic and picturesque region of Făgăraş. He was educated by some of the best doctors and professors in Romania, during the communist regime. After his escape through the communist regime, in 1961, at 41 years old, he started his neurosurgical and radiosurgical career at Karolinska Institute, in Stockholm, under the renown Herbert Olivecrona and Lars Leksell. He worked here for 25 years, until he retired in 1987 as head of 1st and 2nd Departments of Neurosurgery in the institute’s affiliated clinic Sophiahemmet Hospital. He is most known in Sweden as the first to introduce microsurgical techniques in neurosurgery, but internationally he is known as “the unofficial emissary of Gamma Knife Surgery”. After his retirement, he continued his practice at University of Virginia, USA, for another 23 years and another two years at International Neurosciences Institute, Hannover, Germany, being a Professor of Neurosurgery and Radiology of Gamma Knife Surgery. Thanks to his efforts, Romanian neurosurgery had an accelerated progress after the fall of communism, in 1989. Also, thanks to him, Romania was the first Southeast European country with a Gamma Knife unit. For his efforts, he was appointed Honorary President of the Romanian Society of Neurosurgery. We pay tribute to a great Romanian neurosurgeon who managed to touch the lives of thousands of patients and doctors from almost all countries in the world, in time when the world was divided between east and west, communism and freedom.

Enhancing Radiotherapeutic Effect With Nanoparticle-Mediated Radiosensitizer Delivery Guided By Focused Gamma Rays In Lewis Lung Carcinoma-Bearing Mouse Brain Tumor Models.

BackgroundTargeting radiosensitizer-incorporated nanoparticles to a tumor could allow for less normal tissue toxicity with more efficient drug release, thus improving the efficacy and safety of radiation treatment. The aim of this study was to improve tumor-specific delivery and bioavailability of a nanoparticle-mediated radiosensitizer in mouse brain tumor models.MethodsA pH-sensitive nanoparticle, chitoPEGAcHIS, was conjugated to recombinant peptide HVGGSSV that could bind to tax-interaction protein 1 (TIP-1) as a radiation-inducible receptor. Then the c-Jun N-terminal kinase (JNK) inhibitor, SP600125 was incorporated into this copolymer to fabricate a HVGGSSV-chitoPEGAcHIS-SP600125 (HVSP-NP) nanoradiosensitizer. In vitro and in vivo radiation treatment were performed using a Gamma Knife unit. The tumor targetability of HVSP-NP was estimated by optical bioluminescence. Synergistic therapeutic effects of radiation treatment and HVSP-NP were investigated in Lewis lung carcinoma (LLC) cell-bearing mouse brain tumor models.ResultsThe SP600125 JNK inhibitor effectively reduced DNA damage repair to irradiated LLC cells. A pH sensitivity assay indicated that HVSP-NP swelled at acidic pH and increased in diameter, and its release rate gradually increased. Optical bioluminescence assay showed that radiation induced TIP-1 expression in mouse brain tumor and that the nanoradiosensitizer selectively targeted irradiated tumors. Radiation treatment with HVSP-NP induced greater apoptosis and significantly inhibited tumor growth compared to radiation alone.ConclusionAs a novel nanoradiosensitizer, HVSP-NP was found to be able to selectively target irradiated tumors and significantly increase tumor growth delay in LLC-bearing mouse brain tumor models. This research shows that delivering a pH-sensitive nanoradiosensitizer to a brain tumor in which TIP-1 is induced by radiation can result in improved radiosensitizer-release in an acidic microenvironment of tumor tissue and in created synergistic effects in radiation treatment.

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.

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. 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. 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. 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.

Study the effect of source to diaphragm distance (SDD) on beam penumbra width of Gamma Knife machine model 4C using Monte Carlo simulation

Introduction: Stereotactic radiosurgery (SRS) is one of the conformal radiotherapy methods and Gamma Knife is a non-invasive SRS technique to treat intracranial lesions with 201 Co- 60 sources without need to surgery and bleeding. Beam penumbra phenomenon leads to irradiation outside the field and normal tissues. Therefore, considering it is momentous to attain the acceptable treatment plan. Materials and Methods: In this study, Gamma Knife unit was modeled using EGSnrc/BEAMnrc Monte Carlo code. SDD (Source to Diaphragm Distance) parameter that is one of the affecting factors on physical penumbra width was studied. For this purpose, SDD was increased for 4, 8, 14 and 18 mm collimator sizes in five steps and each step as much as one centimeter. Single beam profiles were calculated using EGSnrc/DOSXYZnrc code at isocentre depth in a cubic Plexiglas phantom. MATLAB and EXCEL software were used to plot profiles and measure physical penumbra width . Results: Based on the results, physical penumbra width (90%-50%) does not show a regular trend with increasing SDD. This may be attributed to scatter radiation which is due to the low distance between the Gamma Knife collimator end and phantom surface. Best results were observed for 4 and 8 mm collimators in 1 cm increase in which penumbra width decreased from 0.785 and 0.790 mm to 0.749 and 0.736 mm for 4 and 8 mm collimator sizes, respectively. Conclusion: It is recommended to increase SDD of Gamma Knife 4C around 1 centimeter to reduce penumbra width for small tumor sizes located near the critical nerves.

Patient Motion Analysis of First 50 Frameless Fixation Cases with Leksell Gamma Knife ICON

A new generation of Gamma Knife®, ICON™, provides either frame-based or frameless patient fixation for high accuracy radiation surgery treatment. With the frameless patient fixation, a High Definition Motion Management (HDMM) system monitors the patients motion and controls the Gamma Knife beam delivery with a user defined tolerance of acceptable motion. Patient motion is recorded into a logfile. This study analyzed the logfile to derive the extents of the patient motion in 3-dimensional space. The infrared camera of the HDMM system tracks the relative position of the patient marker, attached to the patient’s nose, with respect to four reference markers fixed on the mask adapter that locks to the unit. The ICON produces several log files that record events and messages from the operations of Gamma Knife unit. The log files report the patient marker position when the motion deviated from a previous position by 0.2 mm. The log files were exported to a Universal Serial Bus (USB) memory stick and the patient motion was extracted with an in-house software that statistically analyzed the patient motion in terms of average positions and standard deviation over several “beam-on” times. First 50 treatments with the frameless patient fixation have been analyzed and presented. There were 50 treatments analyzed with the following sites; sizable brain metastasis targets were 27, meningioma were 21, and 2 GBM treatments were included. Overall treatment time was 37.6 ± 20.1 min (range, 10.4 to 107.7 min) and age of patients was 66.7 ± 10.4 years old. Net beam-on time was 20.8±10.2 min. Elapsed beam time including beam paused time due to motion out of the user defined tolerance was 27.3±18.7 min, and elapsed treatment time including patient loading and unloading time was 37.6±20.1 min. The total mean displacement in three dimensions and its standard deviation was 0.33±0.27, 0.25±0.19, and 0.43±0.35 mm in x, y, and z, respectively. The maximum patient motion was 3.95, 3.71, and 6.87 mm (1st generation ICON automatically removed patients when out of tolerance, it did not retract sources) and mean range was 1.19, 1.08, and 1.91 mm in x, y, and z, respectively. Patient recorded motion demonstrated motion in superior-inferior direction. Patient motions with first 50 frameless fixation cases with Leksell Gamma Knife® ICON™ have been statistically analyzed using log files. Results of analysis confirms that the HDMM of the ICON successfully manage the patient motion within desired tolerance. It is expected that the results will be used to reproduce the dose delivery under consideration of patient motions.

Intracranial stereotactic radiosurgery.

Stereotactic radiosurgery (SRS) is the use of a single high dose of radiation, stereotactically directed to an intracranial region of interest, in order to create a lesion or obliterate a preexisting one. This technology has evolved over the years into the use of multiple radiation sources oriented at a variety of angles, thus permitting the creation of various treatment target shapes. This allows for non-open surgical treatment of intracranial pathologies, which significantly decreases the risk of morbidity. The destruction of pathological tissue following radiosurgery is a stepwise process that involves a number of different stages, beginning with the necrotic stage, followed by the resorption stage, and concluding with the glial scar formation stage. There are currently a number of different delivery methods of SRS, including linear accelerators, Gamma Knife units, and charged particle methods (Bragg-peak and plateau-beam). Various intracranial lesions exhibit different responses to radiosurgery; however, most lesions of appropriate size tend to respond favorably. Radiosurgery is used today in the treatment of brain metastases, meningiomas, vestibular schwannomas, sellar and suprasellar lesions, and arteriovenous malformations. SRS is widely used to treat functional conditions, such as trigeminal neuralgia and intractable tremor. The treatment of intracranial lesions with radiosurgery can result in undesirable effects on the adjacent normal brain, resulting in adverse radiation effects. The distinction between tumor progression and adverse radiation effects can be challenging but is aided by various imaging modalities. Treatment options for this condition include observation, corticosteroids, pentoxifylline and vitamin E, bevacizumab, laser-interstitial thermal therapy, and surgical resection.

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.

Magnetic resonance imaging characteristics and the prediction of outcome of vestibular schwannomas following Gamma Knife radiosurgery.

OBJECTIVE Gamma Knife surgery (GKS) is a promising treatment modality for patients with vestibular schwannomas (VSs), but a small percentage of patients have persistent postradiosurgical tumor growth. The aim of this study was to determine the clinical and quantitative MRI features of VS as predictors of long-term tumor control after GKS. METHODS The authors performed a retrospective study of all patients with VS treated with GKS using the Leksell Gamma Knife Unit between 2005 and 2013 at their institution. A total of 187 patients who had a minimum of 24 months of clinical and radiological assessment after radiosurgery were included in this study. Those who underwent a craniotomy with tumor removal before and after GKS were excluded. Study patients comprised 85 (45.5%) males and 102 (54.5%) females, with a median age of 52.2 years (range 20.4-82.3 years). Tumor volumes, enhancing patterns, and apparent diffusion coefficient (ADC) values were measured by region of interest (ROI) analysis of the whole tumor by serial MRI before and after GKS. RESULTS The median follow-up period was 60.8 months (range 24-128.9 months), and the median treated tumor volume was 3.54 cm3 (0.1-16.2 cm3). At last follow-up, imaging studies indicated that 150 tumors (80.2%) showed decreased tumor volume, 20 (10.7%) had stabilized, and 17 (9.1%) continued to grow following radiosurgery. The postradiosurgical outcome was not significantly correlated with pretreatment volumes or postradiosurgical enhancing patterns. Tumors that showed regression within the initial 12 months following radiosurgery were more likely to have a larger volume reduction ratio at last follow-up than those that did not (volume reduction ratio 55% vs 23.6%, respectively; p < 0.001). Compared with solid VSs, cystic VSs were more likely to regress or stabilize in the initial postradiosurgical 6-12-month period and during extended follow-up. Cystic VSs exhibited a greater volume reduction ratio at last follow-up (cystic vs solid: 67.6% ± 24.1% vs 31.8% ± 51.9%; p < 0.001). The mean preradiosurgical maximum ADC (ADCmax) values of all VSs were significantly higher for those with tumor regression or stabilization at last follow-up compared with those with progression (2.391 vs 1.826 × 10-3 mm2/sec; p = 0.010). CONCLUSIONS Loss of central enhancement after radiosurgery was a common phenomenon, but it did not correlate with tumor volume outcome. Preradiosurgical MRI features including cystic components and ADCmax values can be helpful as predictors of treatment outcome.

SU‐F‐T‐553: Sector Output Uniformity of the Perfexion Gamma Knife

Purpose:To study the uniformity of single‐sector outputs from the 3 collimators of the Perfexion Gamma Knife (PGK) unit and investigate the potential impact on Gamma Knife treatment planning.Methods:An A16 ion chamber and a SFD diode detector were used to measure both the output factors and the single‐sector outputs from a PGK unit. The detectors were placed in custom inserts in the center of a spherical solid water phantom and connected to a Max 4000 electrometer during the measurements. For single‐sector outputs, a treatment plan with a prescription dose of 1 Gy at the focal point was generated for each sector of the 16 mm, 8 mm, and 4 mm collimators and was exported to the console computer for dose delivery. The procedure was repeated 3 times for each sector and an average value was taken as the sector output.Results:The differences between the output factors from the diode measurements and the clinical values for the 8 mm and 4 mm collimators were −0.4% and −2.1% respectively. The corresponding values for the A16 ion chamber were −3.5% and −17.1%. The maximum differences among the 8 single‐sector outputs for the 16 mm, 8 mm, and 4 mm collimators were 4%, 3.7%, and 4.6% respectively. Variations of the sector outputs (defined as the standard deviation‐to‐mean ratio) among the 8 single‐sector outputs for the 16 mm, 8 mm, and 4 mm collimators were measured at 1.16%, 1.32%, and 1.81%. Measurements using the ion chamber were similar to those acquired with the diode for the 16 mm and 8 mm collimators.Conclusion:Single‐sector outputs for the 8 sectors of the 16 mm, 8 mm, and 4 mm collimators of the PGK differed by less than 5% and are within the specified range of planning accuracy.

TH‐A‐BRC‐00: New Task Groups for External Beam QA: An Overview

AAPM TG‐135U1 QA for Robotic Radiosurgery ‐ Sonja Dieterich Since the publication of AAPM TG‐135 in 2011, the technology of robotic radiosurgery has rapidly developed. AAPM TG‐135U1 will provide recommendations on the clinical practice for using the IRIS collimator, fiducial‐less real‐time motion tracking, and Monte Carlo based treatment planning. In addition, it will summarize currently available literature about uncertainties. Learning Objectives: Understand the progression of technology since the first TG publication Learn which new QA procedures should be implemented for new technologies Be familiar with updates to clinical practice guidelinesAAPM TG‐178 Gamma Stereotactic Radiosurgery Dosimetry and Quality Assurance ‐ Steven GoetschPurpose:AAPM Task Group 178 Gamma Stereotactic Radiosurgery Dosimetry and Quality Assurance was formed in August, 2008. The Task Group has 12 medical physicists, two physicians and two consultants.Methods:A round robin dosimetry intercomparison of proposed ionization chambers, electrometer and dosimetry phantoms was conducted over a 15 month period in 2011 and 2012 (Med Phys 42, 11, Nov, 2015). The data obtained at 9 institutions (with ten different Elekta Gamma Knife units) was analyzed by the lead author using several protocols.Results:The most consistent results were obtained using the Elekta ABS 16cm diameter phantom, with the TG‐51 protocol modified as recommended by Alfonso et al (Med Phys 35, 11, Nov 2008). A key white paper (Med Phys, in press) sponsored by Elekta Corporation, was used to obtain correction factors for the ionization chambers and phantoms used in this intercomparison. Consistent results were obtained for both Elekta Gamma Knife Model 4C and Gamma Knife Perfexion units as measured with each of two miniature ionization chambers.Conclusion:The full report gives clinical history and background of gamma stereotactic radiosurgery, clinical examples and history, quality assurance recommendations and outline of possible dosimetry protocols. The report will be reviewed by the AAPM Working Group on Recommendations for Radiotherapy External Beam Quality Assurance and then by the AAPM Science Council before publication in Medical Physics Survey of possible calibration protocols for calibration of Gamma Stereotactic Radiosurgery (GSR) devices Overview of modern Quality Assurance techniques for GSRAAPM TG‐218 Tolerance Levels and Methodologies for IMRT Verification QA ‐ Moyed MiftenPatient‐specific IMRT QA measurement is a process designed to identify discrepancies between calculated and delivered doses. Error tolerance limits are not well‐defined or consistently applied across centers. The AAPM TG‐218 report has been prepared to improve the understanding and consistency of this process by providing recommendations for methodologies and tolerance limits in patient‐specific IMRT QA. Learning Objectives: Review measurement methods and methodologies for absolute dose verification Provide recommendations on delivery methods, data interpretation, the use of analysis routines and choice of tolerance limits for IMRT QASonja Dieterich has a research agreement with Sun Nuclear Inc. Steven Goetsch is a part‐time consultant for Elekta.

SU‐F‐P‐15: Report On AAPM TG 178 Gamma Knife Dosimetry and Quality Assurance

Purpose:AAPM Task Group 178 Gamma Stereotactic Radiosurgery Dosimetry and Quality Assurance was formed in August, 2008. The Task Group has 12 medical physicists, two physicians and two consultants.Methods:A round robin dosimetry intercomparison of proposed ionization chambers, electrometer and dosimetry phantoms was conducted over a 15 month period in 2011 and 2012 (Med Phys 42, 11, Nov, 2015). The data obtained at 9 institutions (with ten different Elekta Gamma Knife units) was analyzed by the lead author using several protocols.Results:The most consistent results were obtained using the Elekta ABS 16cm diameter phantom, with the TG‐51 protocol modified as recommended by Alfonso et al (Med Phys 35, 11, Nov 2008). A key white paper (Med Phys, in press) sponsored by Elekta Corporation, was used to obtain correction factors for the ionization chambers and phantoms used in this intercomparison. Consistent results were obtained for both Elekta Gamma Knife Model 4C and Gamma Knife Perfexion units as measured with each of two miniature ionization chambersConclusion:The full TG 178 report gives clinical history and background of gamma stereotactic radiosurgery, clinical examples and history, quality assurance recommendations and outline of possible dosimetry protocols. The report will be reviewed by the AAPM Working Group on Recommendations for Radiotherapy External Beam Quality Assurance and then by the AAPM Science Council before publication in Medical Physics.Consultant to Elekta, Inc