Gamma Knife Unit Research Articles

SU‐EE‐A2‐05: A Planning System for Dynamic Gamma Knife Radiosurgery

Purpose: To develop a prototype planning system for dynamic Gamma Knife radiosurgery. Method and Materials: Dynamic Gamma Knife is a new concept for implementing Gamma Knife radiosurgery. It uses the spherical high dose volume created by the Gamma Knife unit is as a 3D “paintbrush”, and treatment planning becomes routing the “paintbrush” to “paint” a 3D tumor volume. We have recently finished developing an inverse planning system for dynamic Gamma Knife radiosurgery. Our planning system is a “turn‐key” solution, where the inverse planning problem is solved as a traveling salesman problem combined with constrained least square optimizations. Its input parameters include: (1) contoured anatomy and prescriptions, (2) desired delivery time, (3) APS (advanced patient positioning system) speed, and (4) amount of system memory available. The outputs include: (1) a control point sequence for the APS system together with shot configuration and beam‐on times at each control point, and (2) dose distributions and DVH plots for each structures. Results and Conclusion: We have tested simulated cases that included a spherical tumor, a C‐shaped tumor, and a C‐shaped tumor surrounding a spherical critical structure. The results of these tests showed that: (1) dynamic Gamma Knife radiosurgery is ideally suited for inverse planning, where high quality radiosurgery plans with tight isodose distributions and sharp DVH curves can be obtained in minutes of computation; (2) dynamic radiosurgery plans are more conformal and uniform than current plans and have significantly shorter delivery times; (3) dynamic Gamma Knife radiosurgery can maintain steep dose gradient (around 13% per mm) between the target volume and the surrounding critical structures; (4) with dynamic Gamma Knife radiosurgery, one can obtain a family of plans representing a tradeoff between the delivery time and the dose distributions, thus giving the clinician flexibility in choosing a plan based on the clinical situations.

SU-FF-T-540: Assessment of Clinical Response Factors of Acoustic Neuromas After Gamma Knife Treatment

Purpose The aim of this study is to associate dosimetric measures and other treatment related parameters with the clinical follow‐up results in order to examine factors that are related with the response of acoustic neuromas after Gamma Knife radiosurgery.Materials and Methods 25 patients, who were treated for acoustic neuromas with the Leksell Gamma Knife (model C) unit and an automated positioning system (APS), are examined. For the development of the treatment plans, the dedicated GammaPlan treatment planning system was used. The imaging information was based on CT and MRIimages using the stereotactic frame in position. Target response is expressed as a significant size reduction of the target volume (response group). For each patient, the treatment outcome (follow‐up of at least 2 years) and the dose distribution were available. Results The initial target volume in the response group was 5.5 cc (0.7–12.1 cc) and 5.0 cc (0.2–19.3 cc) in the non‐response group. The corresponding prescribed doses range between 11–12 Gy with a mean reference isodose of 49% (45–54%) and 11–14 Gy with isodose 47% (40–50%), respectively. The mean and minimum doses (D mean, D min) are 15.8 and 8.5 Gy against 16.4 and 8.1 Gy, respectively. The average Paddick conformity index was 0.8 (0.5–0.9) and 0.8 (0.7–0.9) in the two patient groups, respectively. The D min seems to differentiate well the two patient groups since in a ROC analysis it gives an area under the ROC curve of 0.69. A positive association of target response with D min was found (odds ratio = 4.3) at the cutoff dose of 8.7 Gy. Conclusions The probability of target response increases considerably for minimum doses above 8.7 Gy. Consequently, this dose can be considered as a primary threshold of response. The response group receives larger average minimum dose than the non‐response group.

SU-FF-T-257: Gamma Knife Absolute Dose Rate Calibration Using the AAPM TG-51 Protocol and Comparison with the AAPM TG-21 Protocol

Purpose: In 1999 the American Association of Physicists in Medicine published a new calibration protocol for high energyphoton and electron dosimetry, the Task Group 51 protocol, which has, for the most part, replaced the former Task Group 21 protocol. However, the AAPM Task Group 21 protocol continues to be widely used for the absolute dose calibration of Gamma Knife units. The TG‐51 protocol is based on an absorbed dose calibration factor, whereas the TG‐21 protocol is based on an air kerma factor. The purpose of this study is to compare the TG‐51 and TG‐21 protocols for absorbed dose calibration of the Leksell Gamma Knife. Method and Materials: An ionization chamber was placed in the center of a spherical water phantom of 10 cm in diameter and measurements necessary for application of the TG‐51 protocol were taken. This data was used, following TG‐51 Worksheet A, to calculate the dose rate to water at 5 cm depth. A dose rate measurement was also performed following the TG‐21 protocol using the standard Elekta polystyrene phantom. Results: The dose rate measured using the TG‐21 protocol was 2.412 Gy/min at 8 cm depth and, using an exponential attenuation approximation (μ = 0.0063 mm−1)5 for the attenuation of the pencil beam radiation from the Gamma Knife, the dose rate measured at 5 cm using the TG‐51 protocol equates to a value of 2.435 Gy/min at 8 cm depth. Conclusion: The ratio of dose rate calculated using the TG‐51 protocol to that measured using the TG‐21 protocol was 1.01. The estimated overall uncertainty in each calibration result is less than 2%. Since the same ion chamber was used in both calibrations, the uncertainty in the comparison between the two calibration results is estimated to be 1%.

TU‐E‐213A‐01: Quality Assurance for the Leksell Gamma Knife Perfexion™

The Leksell Gamma Knife_ Perfexion_ is the latest gamma stereotactic radiosurgery (GSR) unit manufactured by Elekta (Elekta Instrument, AB, Sweden). Introduced in 2006, it is substantially different in design and operation from previous Gamma Knife (GK) models. The major differences are in the collimation system, the configuration of the radioactive 60Co sources, and in the patient positioning system. The Perfexion_ has one fixed tungsten collimator containing 576 different collimating channels, whereas older GK models have one primary collimator, located inside the unit, and 4 interchangeable secondary collimators that are mounted manually. In the Perfexion_, 192 60Co sources are situated on 8 moveable sectors (24 sources per sector), that slide over the collimator placing the sources over the planned collimating channels to deliver the desired radiation shot size. In older GK models, by contrast, 201 60Co sources are fixed in space and arranged in a pseudo‐hemispherical configuration. Finally, for the Perfexion™ the treatment table, which moves with sub‐millimeter accuracy in three orthogonal directions, is the patient positioning system. For older model GK units, patient positioning is accomplished by attaching the Leksell stereotactic frame to the secondary collimator via specialized hardware called trunnions or via the automatic‐patient‐positioner (APS).The specific goals of quality assurance (QA) for gamma stereotactic radiosurgery include determining the dose rate for the largest available collimator, measuring the relative output factors and beam profiles for each collimator size and comparing the results to those used in or calculated by the treatment‐planning program, and verifying that the radiation focal point coincides with the center of the patient positioning system. Although these QA tasks for GSR units have remained essentially unchanged over the years, new tools and protocols designed for linear accelerator QA have become available that can be applied to QA for GSR devices. With this in mind, a new AAPM Task Group (TG‐178) has recently been formed and charged with updating QA procedures for older GSR devices that have static sources, creating new QA protocols for GSR devices that are characterized by moving sources, and suggesting a protocol for ionization chamber calibration specific to GSR devices.This lecture will describe the operation of the Leksell Gamma Knife_ Perfexion_ and the unique features of QA for GSR units, with emphasis on the Perfexion_. Some of the questions faced by TG‐178 will be discussed, old QA procedures will be reviewed and possible new QA tests will be suggested.Learning Objectives:1. Review QA requirements for GSR units2. Understand the design differences between older GK models and the Perfexion_ and the implications of these differences on routine QA3. Become familiar with tools provided by the manufacturer to facilitate GK QA4. Understand the difficulties associated with dosimetric calibration of GSR units5. Review the use of radiochromic film for GK QA

Dynamic gamma knife radiosurgery

Gamma knife has been the treatment of choice for various brain tumors and functional disorders. Current gamma knife radiosurgery is planned in a ‘ball-packing’ approach and delivered in a ‘step-and-shoot’ manner, i.e. it aims to ‘pack’ the different sized spherical high-dose volumes (called ‘shots’) into a tumor volume. We have developed a dynamic scheme for gamma knife radiosurgery based on the concept of ‘dose-painting’ to take advantage of the new robotic patient positioning system on the latest Gamma Knife C™ and Perfexion™ units. In our scheme, the spherical high dose volume created by the gamma knife unit will be viewed as a 3D spherical ‘paintbrush’, and treatment planning reduces to finding the best route of this ‘paintbrush’ to ‘paint’ a 3D tumor volume. Under our dose-painting concept, gamma knife radiosurgery becomes dynamic, where the patient moves continuously under the robotic positioning system. We have implemented a fully automatic dynamic gamma knife radiosurgery treatment planning system, where the inverse planning problem is solved as a traveling salesman problem combined with constrained least-square optimizations. We have also carried out experimental studies of dynamic gamma knife radiosurgery and showed the following. (1) Dynamic gamma knife radiosurgery is ideally suited for fully automatic inverse planning, where high quality radiosurgery plans can be obtained in minutes of computation. (2) Dynamic radiosurgery plans are more conformal than step-and-shoot plans and can maintain a steep dose gradient (around 13% per mm) between the target tumor volume and the surrounding critical structures. (3) It is possible to prescribe multiple isodose lines with dynamic gamma knife radiosurgery, so that the treatment can cover the periphery of the target volume while escalating the dose for high tumor burden regions. (4) With dynamic gamma knife radiosurgery, one can obtain a family of plans representing a tradeoff between the delivery time and the dose distributions, thus giving the clinician one more dimension of flexibility of choosing a plan based on the clinical situations.

Air kerma based dosimetry calibration for the Leksell Gamma Knife

No accepted official protocol exists for the dosimetry of the Leksell Gamma Knife (GK) stereotactic radiosurgery device. Establishment of a dosimetry protocol has been complicated by the unique partial-hemisphere arrangement of 201 individual 60Co beams simultaneously focused on the treatment volume and by the rigid geometry of the GK unit itself. This article proposes an air kerma based dosimetry protocol using either an in-air or in-acrylic phantom measurement to determine the absorbed dose rate of fields of the 18 mm helmet of a GK unit. A small-volume air ionization chamber was used to make measurements at the physical isocenter of three GK units. The absorbed dose rate to water was determined using a modified version of the AAPM Task Group 21 protocol designed for use with 60Co-based teletherapy machines. This experimentally determined absorbed dose rate was compared to the treatment planning system (TPS) absorbed dose rate. The TPS used with the GK unit is Leksell GammaPlan. The TPS absorbed dose rate at the time of treatment is the absorbed dose rate determined by the physicist at the time of machine commissioning decay corrected to the treatment date. The TPS absorbed dose rate is defined as absorbed dose rate to water at the isocenter of a water phantom with a radius of 8 cm. Measurements were performed on model B and C Gamma Knife units. The absorbed dose rate to water for the 18 mm helmet determined using air-kerma based calculations is consistently between 1.5% and 2.9% higher than the absorbed dose rate provided by the TPS. These air kerma based measurements allow GK dosimetry to be performed with an established dosimetry protocol and without complications arising from the use of and possible variations in solid phantom material. Measurements were also made with the same ionization chamber in a spherical acrylic phantom for comparison. This methodology will allow further development of calibration methods appropriate for the smaller fields of GK units to be compared to a well established standard.

Fatal case of intracerebral hemorrhage during gamma knife treatment for metastases

This case report presents a patient suffering acute fatal intracranial–intratumoral hemorrhage during a gamma knife treatment session. Acute hemorrhage during a radiosurgery session is extremely rare and a plausible cause for this case is discussed along with a literature review of previously reported incidents. The patient was a 71-year-old male presenting with three large intracranial lesions and an underlying primary renal cell carcinoma malignancy. Because of a severe kyphotic deformity resulting from ankylosing spondylitis, the patient was placed in a moderate Trendelenburg position to allow his head to fit into the gamma knife unit during the radiosurgery session. The two left-sided lesions were to be treated with 20 Gy to the 50% isodose line, and the right-sided lesion with 16 Gy to the 40% isodose line. Anesthesia was available throughout the treatment session to aid with pain control. The gamma knife treatment was aborted because the patient suffered a generalized seizure while in the unit. Immediate head CT of the patient revealed large acute hemorrhages into all three intracranial masses. This proved to be a fatal complication. It is likely that this positioning contributed to the hemorrhage. The clinical history of this patient is provided as well as a review of the literature on acute intracranial hemorrhage associated with radiosurgical therapy.

SU-GG-T-224: Air Kerma Based Dosimetry Calibration for the Elekta Gamma Knife

Purpose: To present an air kerma based dosimetry methodology using either an in-air or in-acrylic phantom measurement to determine the dose rate of fields collimated by the 18mm helmet of a Leksell Gamma Knife (GK) unit. Method and Materials: This work used a small volume ionization chamber in either an in-air jig or a PMMA phantom to measure the dose rate provided by the 18mm helmet of the GK unit. The chamber was investigated with respect to angular response as well as size in the radiation field. Monte Carlo calculations were performed to determine accurate coefficients and corrections to enable the use of a modified and updated air-kerma based protocol with the unique and rigid geometry of the GK unit. In-air and in-PMMA measurements were made at three GK institutions and the dose rate was determined using the new protocol. Results: Chamber and field dimension studies showed that the ionization chamber in use was appropriate for the 18mm field of the GK unit. A chamber orientation study showed the optimal orientation of the chamber was along the patient axis of the GK unit. Measurements made with the PMMA phantom and in-air result in equivalent dose rates to within 0.35%. The dose rates determined using these methods are consistently higher than the dose rates provided by the treatment planning system. This difference ranges from 1.6% to 2.4% high. The in-air method is repeatable throughout time to within 0.2%. Conclusion: Although dosimetry protocols based on air-kerma calibrations have been appropriately replaced in many applications by dose-to-water calibrations, in the case of the GK unit, an air-kerma based calibration provides an elegant and direct method to calculate the dose rate from the 18mm GK helmet.

TU-FF-A2-05: 3D Dose Distribution of the Elekta Gamma Knife

Purpose: To design and implement a novel three‐dimensional (3D) radiochromic film phantom for use with the Gamma Knife (GK). Use of radiochromic film and a flat bed white light scanner will allow greater resolution than gel‐based 3D dosimeters. In addition, this method does not require sophisticated imaging tools such as CT or MRI machines. Method and Materials: A spherical Virtual Water phantom was constructed to hold up to a 4.7cm thick stack of radiochromic film made of film slices 5cm in diameter. This stack size is large enough to capture the entire dose distribution resulting from the GK fields. The resolution capable with this phantom depends on the scanner in use and the thickness of the radiochromic film, which in this case allows a resolution of up to 0.004×0.003×0.2mm3. Monte Carlo methods show that the presence of a thick stack of film does not significantly disturb the measured dose distribution. Results: Using this film phantom it is possible to obtain a three dimensional representation of the dose distribution of a GK unit. Due to the size of the films and the thickness of the stack, all the dose distribution information can be obtained in one exposure. This technique allows a higher scanning resolution to be achieved than is commonly obtained with geldosimetry, which is on the order of 1×1×2mm3. Conclusion: Fully characterizing any radiation field requires knowledge of the dose distribution as well as the absolute value of the dose. This film phantom provides a method for mapping the dose distribution of a three dimensional radiation field with excellent resolution.

Dosimetric and radiobiological evaluation of dose distribution perturbation due to head heterogeneities for Linac and Gamma Knife stereotactic radiotherapy

Introduction. In SRT/SRS, dedicated treatment planning systems are used for the calculation of the dose distribution. The majority of these systems utilize the standard TMR/OAR formalism for dose calculation as well as they usually neglect any perturbation due to head heterogeneities. The aim of this study is to examine the errors due to head heterogeneities for both absolute and relative dose distributions in stereotactic radiotherapy. Materials and methods. Dosimetric measurements in phantoms have been made for linac stereotactic irradiation. CT-based phantoms have been used for Monte Carlo simulations for both linac-based stereotactic system and Gamma Knife unit. Absolute and relative dose distributions have been compared between homogeneous and heterogeneous media. DVH and TCP results are presented for all cases. Results. The maximum absolute dose difference at the isocenter was 2.2% and 6.9% for the linac and Gamma Knife respectively. The impact of heterogeneity in the target DVH was minor for the linac technique whereas considerable difference was observed for the Gamma Knife treatment. This was reflected also to the radiobiological evaluation, where the maximum TCP difference for the linac system was 2.7% and for the Gamma Knife was 4%. Discussion and conclusions. The errors rising from the existence of head heterogeneities are not negligible especially for the Gamma Knife which uses lower energy beams. The errors of the absolute dose calculation could be easily eliminated by implementing a simple heterogeneity correction algorithm at the TPS. Nevertheless, the errors for not taking into account the lateral electron transport would require a more sophisticated approach and even direct Monte Carlo calculation.

PITUITARY ADENOMAS TREATED WITH GAMMA KNIFE RADIOSURGERY

To analyze pituitary adenoma volume changes after gamma knife radiosurgery (GKRS) in patients with 3 years of follow-up and to investigate factors that might affect these changes. Between January 1997 and March 2004, a total of 1930 patients were treated in the Gamma Knife Unit of the Marmara University Department of Neurosurgery in Istanbul, Turkey. Three hundred sixty of these patients had pituitary adenomas (PAs). This prospectively designed clinical study documents the radiological-volumetric analysis for the first 100 of these patients with PAs who had a minimum of 3 years of follow-up and met the study requirements. Each tumor was assessed with serial magnetic resonance imaging scans after radiosurgery; at each time point, adenoma volume was expressed as a percentage of the tumor's initial volume. Volume changes were investigated relative to margin dose, the cavernous sinus infiltration, and endocrinological type of adenoma. At the end of the first year after GKRS, the PA volumes had decreased to approximately 90% of the initial volume on average. The corresponding approximate averages for the ends of Years 2 and 3 were 80 and 70% of the initial volume, respectively. At 3 years after GKRS, the PAs in the group with a peripheral dose of less than 17 Gy were reduced to approximately 80% of the initial volume on average. In contrast, the tumors in the patients with marginal doses of 21 to 23 Gy were reduced to approximately 60% of the initial volume at this stage. The adenomas treated with the highest marginal doses (>27 Gy) showed the earliest volume decreases after GKRS (6-9 mo after the procedure). Cavernous sinus noninfiltrating adenomas showed greater volume decreases after GKRS; on average, these masses were reduced to approximately 50% of their initial volume at 3 years. In contrast, the PAs that had infiltrated the cavernous sinus had only dropped to approximately 80% of their initial volume at this stage. The growth hormone-secreting PAs showed the maximum volume decrease with GKRS. On average, these lesions were approximately 60% of their initial volume at the 3-year stage. The nonfunctioning tumors and the prolactin-secreting adenomas showed similar volume changes over time. On average, these tumors had dropped to approximately 75 and 70% of the initial volume, respectively, by 3 years after GKRS. Gamma knife radiosurgery halts the growth of pituitary adenomas. Cavernous sinus extension and margin dose are the most important determinants of adenoma volume after this type of therapy.

Implementation of Monte Carlo Simulations for the Gamma Knife System

Currently the Gamma Knife system is accompanied with a treatment planning system, Leksell GammaPlan (LGP) which is a standard, computer-based treatment planning system for Gamma Knife radiosurgery. In LGP, the dose calculation algorithm does not consider the scatter dose contributions and the inhomogeneity effect due to the skull and air cavities. To improve the dose calculation accuracy, Monte Carlo simulations have been implemented for the Gamma Knife planning system. In this work, the 201 Cobalt-60 sources in the Gamma Knife unit are considered to have the same activity. Each Cobalt-60 source is contained in a cylindric stainless steel capsule. The particle phase space information is stored in four beam data files, which are collected in the inner sides of the 4 treatment helmets, after the Cobalt beam passes through the stationary and helmet collimators. Patient geometries are rebuilt from patient CT data. Twenty two Patients are included in the Monte Carlo simulation for this study. The dose is calculated using Monte Carlo in both homogenous and inhomogeneous geometries with identical beam parameters. To investigate the attenuation effect of the skull bone the dose in a 16cm diameter spherical QA phantom is measured with and without a 1.5mm Lead-covering and also simulated using Monte Carlo. The dose ratios with and without the 1.5mm Lead-covering are 89.8% based on measurements and 89.2% according to Monte Carlo for a 18mm-collimator Helmet. For patient geometries, the Monte Carlo results show that although the relative isodose lines remain almost the same with and without inhomogeneity corrections, the difference in the absolute dose is clinically significant. The average inhomogeneity correction is (3.9 ± 0.90) % for the 22 patients investigated. These results suggest that the inhomogeneity effect should be considered in the dose calculation for Gamma Knife treatment planning.

Radiation shielding for gamma stereotactic radiosurgery units

Shielding calculations for gamma stereotactic radiosurgery units are complicated by the fact that the radiation is highly anisotropic. Shielding design for these devices is unique. Although manufacturers will answer questions about the data that they provide for shielding evaluation, they will not perform calculations for customers. More than 237 such units are now installed in centers worldwide. Centers installing a gamma radiosurgery unit find themselves in the position of having to either invent or reinvent a method for performing shielding design.This paper introduces a rigorous and conservative method for barrier design for gamma stereotactic radiosurgery treatment rooms. This method should be useful to centers planning either to install a new unit or to replace an existing unit. The method described here is consistent with the principles outlined in Report No. 151 from the U.S. National Council on Radiation Protection and Measurements. In as little as 1 hour, a simple electronic spreadsheet can be set up, which will provide radiation levels on planes parallel to the barriers and 0.3 m outside the barriers.PACS numbers: 87.53.Ly, 87.56By, 87.52Tr

TH‐D‐AUD‐01: A Dynamic Dose Delivery Model for Gamma Knife Radiosurgery

Purpose: To develop a dynamic dose delivery model for Gamma Knife radiosurgery by taking advantage of the robotic patient positioning system in the latest Gamma Knife model. Methods and Material Gamma Knife has been the treatment of choice for many brain tumors and functional disorders. Current Gamma Knife radiosurgery is a ball‐packing approach, whose goal is to “pack” the different sized spherical high‐dose volumes into a target tumor volume. We have developed a dynamic Gamma Knife treatment scheme based on the concept of “dose‐painting” to take advantage of the new robotic patient positioning system on the latest Gamma Knife unit. In our scheme, the spherical high dose volume created by the Gamma Knife unit will be viewed as a 3D spherical “paintbrush,” and treatment planning reduces to finding the best route of the “paintbrush” to “paint” a 3D tumor volume. Under our dose‐painting concept, Gamma Knife radiosurgery becomes dynamic, where the patient is moving continuously under the robotic positioning system. Planning is modeled as a variation of the traveling salesman problem. Results: A theoretical study of dynamic Gamma Knife radiosurgery scheme has been carried out for a C‐shaped tumor volume. Dose distributions are calculated using an established analytical dose calculation engine. The dose distribution of the dynamic plan is calculated by fine interpolation from equally placed and equally weighted shots. The study showed that the new dynamic scheme can provide equal or superior dose distributions with significantly (> 40%) shortened treatment time. Conclusion: We have proposed and tested a dynamic delivery scheme for Gamma Knife radiosurgery using robotic patient positioning on the latest Gamma Knife model. A prototype study has been carried out. The results showed that our dynamic scheme provides equal or better dose distributions than current Gamma Knife method but with a significantly shortened treatment time.

SU‐FF‐T‐167: Dose Perturbation From Head Frame at Gamma Knife Stereotactic Radiosurgery

Purpose: To investigate the possible dose perturbation from head frame which is attached to patient skull to provide precise location of target in brain and is not considered during dose calculation in Gamma Knife SRS. Method and Materials: An EGS4 based Monte Carlo simulation was applied to Gamma Knife SRS. In the simulation, the geometries of the stationary collimator and the helmet collimator were reconstructed exactly according to Gamma Knife Model C. The materials of the collimators were selected as close to the original materials as possible. All particles were traced through the stationary collimator and the helmet collimator until they pass the inner surface of the helmet or were absorbed. Patient's geometries were rebuilt from CT data. The dose was calculated using Monte Carlo simulation with and without presence of head frame using identical beam parameters. DVH was compared in a small volume around the iso‐center. Results: For patient's data, Monte Carlo results showed that although the 50% to 90% isodose lines remained almost the same with and without the presence of head frame, the maximum distance between two 10% isodose lines could be as far as 1.5 mm. The dose with head frame was about 1.3% lower than the dose without head frame. The dose difference varied from 1% to 2% among five patients. Conclusion: The head frame may have significant impact on peripheral dose distribution as well as absolute dose. Since Automatic Positioning System is applied on Gamma Knife unit, average shot number for each patient has been increased to produce a conformal dose distribution to match the irregular tumor volume. Regular position check can't exclude the possibility that some shots are affected by the head frame. The dose perturbation from head frame should be considered in dose calculation.

SU‐FF‐T‐247: Implementation of MRI‐Based Monte Carlo Simulation for Gamma Knife SRS

Purpose: To verify the dosimetry accuracy for MRI‐based planning of Gamma Knife Stereotactic Radiosurgery. Method and Materials: In Monte Carlo simulation, Gamma Knife unit geometry was reconstructed exactly as the original unit. Materials were selected as close to the actual one as possible. Patients were scanned on both a CT unit and a 1.5 T MRI scanner. Simulations were performed for these patients using homogeneous geometry based on CT and MRI, and heterogeneous geometry built based on CT numbers or different densities for MR contoured skull structures. The homogenous density was chosen as 1.0g/cc and the density of skull was chosen in the range 1.5 – 2.0 g/cc. Isodose distributions and DVHs were used in comparison. Results: The dose in the homogeneous geometries based on CT was about 3.2% higher than the dose in heterogeneous geometry based on CT. The difference in the DVH between homogeneous MRI and heterogeneous CT geometry was also around 3.3%. After applying heterogeneity correction to the skull for MRI, the difference was reduced to less than 2%. The 90%, 50% and 10% isodose lines matched each other very well between homogeneous CT and MRI geometries and heterogeneous CT and MRI geometries. Conclusion: A useful CT and MRI based Monte Carlo simulation has been developed for Gamma Knife dose verification. For many Gamma Knife centers, MRI is the only choice for Gamma Knife planning. Our results show that MRI‐based Monte Carlo planning for Gamma Knife is feasible after applying proper skull heterogeneity correction.

Monte Carlo simulations to optimize experimental dosimetry of narrow beams used in Gamma Knife radio-surgery

The Leksell Gamma Knife is a stereotactic radio-surgery unit for the treatment of small volumes (on the order of 25 mm 3) that employs a hemispherical configuration of 201 60Co sources and appropriate configurations of collimation to form beams of 4, 8, 14 and 18 mm nominal diameter at the Unit Center Point (UCP). Although Monte Carlo (MC) simulation is well suited for narrow-beam dosimetry, experimental dosimetry is required at least for acceptance testing and quality assurance purposes. Besides other drawbacks of conventional point dosimeters, the main problems associated with narrow-beam dosimetry in stereotactic applications are accurate positioning and volume averaging. In this work, MCNPX and EGSnrc MC simulation dosimetry results for a Gamma Knife unit are benchmarked through their comparison to treatment planning software calculations based on radio-chromic film measurements. Then, MC dosimetry results are utilized to optimize the only three-dimensional experimental dosimetry method available; the polymer gel-Magnetic Resonance Imaging (MRI) method. MC results are used to select the spatial resolution in the imaging session of the irradiated gels and validate a mathematical tool for the localization of the UCP in the three-dimensional experimental dosimetry data acquired. Experimental results are compared with corresponding MC calculations and shown capable to provide accurate dosimetry, free of volume averaging and positioning uncertainties.

Prediction of intracranial edema after radiosurgery of meningiomas

This study was focused on the development of models with which to predict the occurrence of intracranial edema after Gamma Knife surgery (GKS) of meningiomas, based on clinical and imaging data collected in a large group of patients. Data in 368 patients with 381 meningiomas treated using the Leksell Gamma Knife unit were analyzed. Follow up of more than 24 months was available in 331 patients (90%); this time period ranged from 24 to 120 months (median 51 months). The actuarial tumor control rate was 97.9% at 5 years. Perilesional edema after GKS was radiologically confirmed in 51 patients (15.4%) and 32 of them (9.7%) were symptomatic; symptoms were temporary in 23 (6.9%) and permanent in nine (2.7%). Ten different factors were proposed as potential predictors for the occurrence of the intracranial edema after GKS: patient's sex, patient's age, previous surgery, edema before GKS treatment, lobulated margin of meningioma, heterogeneous appearance of the tumor, tumor volume, tumor location, maximum dose to the tumor, and dose to the tumor margin. To identify factors having influence on edema occurrence, univariate and multivariate statistical analyses were performed. There was a significant difference in the incidence of edema for different patient age groups and a significantly higher incidence of edema occurrence in patients in whom no surgical procedure was performed before GKS, those with edema present before GKS, those with a tumor volume larger than 10 cm3, those in whom the tumor was located in the anterior fossa, those in whom the maximum dose to the tumor was higher than 30 Gy, and for different tumor margin doses. A binary logistic regression multifactorial prediction model was used to identify the following significant factors to predict of edema occurrence after GKS: previous surgery, edema before the treatment, tumor volume, tumor location, and tumor margin dose. Based on these models estimates of the occurrence of edema after the GKS can be made, and consequently treatment parameters can be adjusted to reduce the occurrence of edema. These results may provide grounds for additional patient care such as more frequent follow up or possibly administration of steroids.

2803

2803

Fast, three-dimensional, MR Imaging for polymer gel dosimetric applications involving high dose and steep dose gradients

Polymer gels constitute water equivalent integrating detectors, which, combined with magnetic resonance imaging (MRI), can provide accurate three dimensional (3D) dose distributions in contemporary radiotherapy applications where the small field dimensions and steep dose gradients induce limitations to conventional dosimeters. One of the main obstacles for adapting the method for routine use in the clinical setting is the cost effectiveness of the MRI readout method. Currently, optimized Carr-Purcell-Meiboom-Gill (CPMG) multiple spin echo imaging pulse sequences are commonly used which however result in long imaging times. This work evaluates the efficiency of 3D, dual-echo, k-space segmented turbo spin echo (TSE) scanning sequences for accurate dosimetry with sub-millimetre spatial resolution in strenuous radiation therapy applications. PABIG polymer gel dosimeters were irradiated with an 192Ir High Dose Rate brachytherapy source, the 4 mm and 8 mm collimator helmets of a gamma knife unit and a custom made x-knife collimator of 1 cm diameter. Profile and dose distribution measurements using TSE are benchmarked against corresponding findings obtained by the commonly used, but time consuming, CPMG sequence as well as treatment planning calculations, Monte Carlo (MC) simulations and film measurements. The implementation of a high Turbo factor was found to provide comparable accuracy, allowing a 64-fold MRI scan acceleration compared to conventional multi-echo sequences. The availability of TSE sequences in typical MRI installations greatly facilitates the introduction of polymer gel dosimetry in the clinical environment as a practicable tool for the determination of full 3D dose distributions in contemporary radiotherapy applications.