Stereotactic Localization Research Articles

Is Head Frame Still the Gold Standard of Fixation for Linac Stereotactic Radiosurgery?

Frameless stereotactic radiosurgery (SRS) has been introduced into the field as an alternative to frame based SRS for brain. However, the potential head movement within the mask and its impact on position variation are not fully studied. This study compared the difference of intra-treatment position variation between frameless and frame based SRS. Twenty-eight frame based patients (43 targets) and 19 frameless patients (37 targets) treated at our institution since last October were used in this study. Head frame and mask system were used as the fixation device, respectively. Patient was initially setup using stereotactic localization box. Patient positioning image guidance procedure was then performed and correction was made if initial setup error exceeded tolerance, which was 0.75 mm for translation and 1 degree for rotation. Three additional sets of verification x-rays were obtained pre, mid, and post treatment for all patients. The patient positioning software calculated residue shifts and rotations were recorded. The translational variation along each direction, and 3D variation among pre-, mid-, and post-treatment were calculated and compared between frame based and frameless groups. For frame based SRS, the mean and standard deviation of the variations along each direction are 0.18±0.16 mm anterior-posterior (AP), 0.22±0.18 mm superior-inferior (SI), and 0.13±0.08 mm left-right (LR), respectively. For frameless SRS, the mean and standard deviation of the variations are 0.27±0.19 mm AP, 0.32±0.19 mm SI, and 0.19±0.14 mm LR. The 3D variation of frame based SRS is 0.37±0.15 mm. It is less than that of frameless SRS, 0.51±0.19 mm (t-test p<0.001). The probability for a target being within 0.5 mm and 0.7 mm 3D variation is 81% and 100% respectively for the frame based SRS, and 46% and 86% respectively for the frameless SRS. Frame based SRS shows statistically significant less intra-treatment variation than frameless SRS. When selecting fixation method, this should be taken into consideration when targets are near critical structures or very small. However, the absolute variations are very small for both methods. For most SRS applications, the difference is not considered clinical significant. The largest variation is on the SI direction for frameless SRS, which may reflect the current clam shell mask design that leaves an opening on top of head gives the patient less restriction on this direction. The variation may be caused by the potential head movement within the mask or frame, mechanical uncertainty caused by couch rotation, and calculation uncertainty of patient positioning software. However, the latter two uncertainties can be factored out when comparing the results, since both groups are facing the same uncertainties and the only difference is the fixation method.

Investigation of 3-dimensional Optical Image Guided Frameless Stereotactic Radiosurgery Using Homegrown Volumetric Modulated Arc Therapy

Recently, powerful on-board imaging technologies have propelled SRS into the frameless era. Yet, all the mainstream on-board imaging systems resort to ionizing X-rays as imaging sources. Moreover, most SRS procedures are planned with IMRT, a modality that suffers from an elevated MU, a reduced MU-to-cGy coefficient, and a prolonged treatment time. To tackle these issues, we commissioned a novel frameless stereotactic system for intracranial SRS immobilization, a 3D optical surface imaging system for target localization and real-time patient monitoring, and a homegrown VMAT system for SRS planning. Here, we present our initial results of a feasibility study. The frameless stereotactic system includes carbon fiber base boards, a cranial stereotactic localizer frame, a support bridge, patient-specific mouthpiece assemblies, and a portable vacuum pump. The portable vacuum pump applies a negative pressure to the patient mouthpiece. The resulting vacuum suction seals the mouthpiece precisely into the palate of the mouth. VMAT-SRS plans were computed with a single 360° arc containing 180 equally-spaced beams, optimized incrementally using a differentiated optimization scheme. The target localization was accomplished with a video-based 3D optical surface imaging system, consisting of three 3D camera units and fast 3D image registration software. After the patient had been initially setup, the 3D optical surface imaging system acquired a verification skin surface of the patient. This verification image was registered to a reference skin surface created from the planning 3DCT in a user-defined region of interest. The registration software calculated three translational shifts (ΔVRT, ΔLNG, and ΔLAT), and three rotational shifts (ΔPITCH, ΔROLL, and ΔYAW) for patient re-positioning. The frameless system was found to be able to secure a sustained immobilization accuracy of ∼ ±1.0 mm and 1.0° for intracranial SRS as measured by CBCT and 3D real-time optical surface imaging. With this system, the patient could be simulated, planned, and treated on different days. VMAT-SRS plan optimization converged in 10 minutes for a PTV of ∼ 35 cm3 and 6 other critical structures. VMAT-SRS plans exhibited superior dose distribution to IMRT-SRS plans as measured by conformality and homogeneity indices. The target dose inhomogeneity was less than 17% compared to about 30% for conventional SRS plans. The doses to the critical structures for VMAT-SRS plans were less than or comparable to those for conventional SRS plans. With this new approach, the treatment time was reduced by more than one hour. Our initial data revealed that the synergistic combination of the frameless system, VMAT-SRS, and the 3D real-time optical surface imaging could be an optimal intracranial SRS sub-modality.

WE‐A‐BRCD‐01: Stereotactic Radiosurgery: State of the Art Technology and Implementation

Since its introduction nearly 60 years ago, stereotactic radiosurgery has become the standard of care for the noninvasive destruction of intracranial tissues or lesions that may be inaccessible or unsuitable for open surgery. Today, modern stereotactic radiosurgery is practiced using advanced image guided treatment planning and specialized delivery systems including micro‐ MLC equipped linacs, CyberKnife, and Gamma Knife machines. Stereotactic radisourgery delivers a large dose to a precisely defined volume in a short time, and as such requires the utmost attention to precision and quality assurance. Also critical is the meticulous design of treatment processes that eliminate the possibility of potentially disastrous errors. In this presentation we review the fundamental aspects of stereotactic targeting and delivery, the technologies for stereotactic localization and treatment of cranial targets, and the quality assurance aspects associated with establishing and maintaining a clinical radiosurgery program. Examples of radiosurgery cases will be presented from the best practice sites utilizing Gamma Knife, CyberKnife, and linac delivery systems, followed by an expert panel discussion of quality measures for treatment planning and delivery.Learning Objectives:1. Differentiate how radiation is delivered for Gamma Knife, CyberKnife and Linac‐based (conventional and robotic) stereotactic radiosurgery.2. Define the treatment planning parameters, imaging requirements and workflow for Gamma Knife, CyberKnife and Linac‐based stereotactic radiosurgery.3. Discuss measures for assuring accuracy in stereotactic localization and dose delivery for Gamma Knife, CyberKnife and Linac‐based stereotactic radiosurgery.4. Discuss uncertainties and limitations associated with Gamma Knife, CyberKnife and Linac‐based stereotactic radiosurgery

SU-E-J-78: Enhance Patient Positioning Accuracy with Optically Guided Frameless Target Localization Technique for Intracranial Radiation Therapy.

To make use of the optical guided frameless stereotactic target localization technique to detect the interfraction and intrafraction patient positioning errors and correct the patient position for fractional intracranial therapy, therefore, to enhance the patient positioning accuracy. Four patients who underwent fractionated intracranial intensity modulated radiation therapy (IMRT) were studied. For each patient, in addition to the thermoplastic head mask, frameless array and bite-block were used for patient positioning. Optical guided frameless planning was utilized to define the coordinates of the isocenter during the treatment planning procedure. In each treatment fraction, the patient was first set up by matching the room lasers to the BB markers attached on the mask. The optical guided frameless system then captured the patient position and reported displacements of the current patient position to the planned patient position. These displacements were called interfraction errors. The treatment plan was designed to have up to 8 non-coplanar fields. Dose delivery of the non-coplanar fields required couch rotation which introduced isocenter displacements called intrafraction errors. Population statistics of interfraction and intrafraction errors (4 patient, 110 fractions) were calculated. The optical guided frameless system monitored the patient positioning errors and provided guidance for correction prior to the dose delivery. Among all patients and radiation treatment fractions, the overall detected interfraction error was 3.6±1.3 mm (mean ±SD) and the intrafraction error was 1.4±0.8 mm. Both types of errors were online corrected with the guidance of optical guided frameless system. Conventional laser guided thermoplastic mask patient position for intracranial therapy has interfraction and intrafraction errors. The optical guided frameless target localization technique allows clinicians to dectect and correct these errors online, therefore, to enhance the patient positioning accuracy for intracranial radiation therapy.

A 3-dimensional digital atlas of the ascending sensory and the descending motor systems in the pigeon brain

Pigeons are classic animal models for learning, memory, and cognition. The majority of the current understanding about avian neurobiology outside of the domain of the song system has been established using pigeons. Since MRI represents an increasingly relevant tool for comparative neuroscience, a 3-dimensional MRI-based atlas of the pigeon brain becomes essential. Using multiple imaging protocols, we delineated diverse ascending sensory and descending motor systems as well as the hippocampal formation. This pigeon brain atlas can easily be used to determine the stereotactic location of identified neural structures at any angle of the head. In addition, the atlas is useful to find the optimal angle of sectioning for slice experiments, stereotactic injections and electrophysiological recordings. This pigeon brain atlas is freely available for the scientific community.

WE‐B‐224‐01: Cranial SRS/SRT

In the 50‐plus years since it was first introduced, stereotactic radiosurgery, high‐dose irradiation of cranial neoplasms delivered in a single fraction, has become a standard of care in the treatment brain tumors, vascular malformations, functional disorders, and pain. Modern radiosurgery can be performed non‐invasively and on an outpatient basis, yet with an extremely high degree of accuracy. Within the past ten years, the field of radiosurgery has seen numerous technological enhancements, including: the development of dedicated devices for stereotactic delivery, the use of relocatable frames to facilitate fractionated delivery, the development of image guided and “frameless” approaches, and the application to extracranial tumor sites. Each of these developments is accompanied by its own challenges in assuring targeting and dosimetric accuracy. In this presentation we review the fundamentals aspects of stereotactic targeting and delivery, the technologies for stereotactic localization and treatment of cranial targets, and the quality assurance aspects associated with establishing and maintaining a clinical radiosurgery program. Specifically, the presentation will:1. Differentiate how radiation is delivered for Gamma Knife and Linac‐based (conventional and robotic) stereotactic radiosurgery.2. Define the treatment planning parameters for Gamma Knife and Linac‐based stereotactic radiosurgery.3. Discuss measures for assuring accuracy in stereotactic localization and dose delivery for Gamma Knife and Linac‐based stereotactic radiosurgery.

SU‐E‐J‐175: An Analysis of Localization Methods for SRS Using CBCT

Purpose: To evaluate multiple methods of localization for stereotactic radiosurgery (SRS) for spatial accuracy and time required for each procedure. Methods: Multiple phantoms were imaged using a helical diagnostic computed tomography (CT) scanner (Toshiba™ Aquilion) and a linear accelerator mounted cone‐beam computed tomography (CBCT). Scans (for both machines) were acquired with a matrix of 512×512 at a field of view of 340 mm. A Varian Medical Systems™ absolute position phantom, in‐house created uniformity phantom, LUCY™ phantom, and Catphan phantom were each imaged to compare spatial homogeneity, linearity, spatial resolution, contrast, and uniformity. A final test of localization accuracy was completed by localizing a phantom using three different Methods: headring localization using helical CT, headring localization using CBCT, and “anatomical” localization using on‐board CBCT. Results: Average spatial localization errors from absolute phantom spheres were similar for the helical CT scanner (∼0.37 mm) compared with the on‐board CBCT (∼0.35 mm). R2 values for a line fit to the spatial position of the rods in the uniformity phantom scanned with CBCT were &gt; 0.99. Spatial resolution of CBCT and CT were found to be similar using the CTP528 insert of the Catphan phantom (0.071 line pairs/cm). Spatial accuracy of localization by headring for CT and CBCT were similar but “anatomical” localization with CBCT may be less consistent. Conclusions: The spatial accuracy and linearity of CBCT may be adequate for stereotactic localization using a headring for SRS. Loss of low contrast resolution in CBCT may affect localization and position by CBCT alone. Headring localization using CBCT results in overall time savings without significant loss of spatial information. Localization by CBCT alone may result in increased spatial inaccuracies due to loss of contrast and potential MV radiation and on‐board imager isocenter mismatches.

Multiple intracranial capillary hemangiomas and transient cerebrovascular insufficiency

Hemangiomas constitute a heterogeneous group of benign vascular proliferations of skin and mucous membranes and very rarely known to develop in the brain or spinal cord. We report here a case of multiple intracranial capillary hemangiomas presenting with transient neurological deficit. The patient underwent stealth MRI brain utilizing 15 ml of Magnevist for stereotactic localization and biopsy was done to establish the diagnosis. It is pivotal to differentiate benign hemangiomas from primary cerebral vascular tumors including hemangioblastoma, hemangioendothelioma and hemangiopericytoma, as later are known for more aggressive biologic behavior.

A customizable 3-dimensional digital atlas of the canary brain in multiple modalities

Songbirds are well known for their ability to learn their vocalizations by imitating conspecific adults. This uncommon skill has led to many studies examining the behavioral and neurobiological processes involved in vocal learning. Canaries display a variable, seasonally dependent, vocal behavior throughout their lives. This trait makes this bird species particularly valuable to study the functional relationship between the continued plasticity in the singing behavior and alterations in the anatomy and physiology of the brain. In order to optimally interpret these types of studies, a detailed understanding of the brain anatomy is essential. Because traditional 2-dimensional brain atlases are limited in the information they can provide about the anatomy of the brain, here we present a 3-dimensional MRI-based atlas of the canary brain. Using multiple imaging protocols we were able to maximize the number of detectable brain regions, including most of the areas involved in song perception, learning, and production. The brain atlas can readily be used to determine the stereotactic location of delineated brain areas at any desirable head angle. Alternatively the brain data can be used to determine the ideal orientation of the brain for stereotactic injections, electrophysiological recordings, and brain sectioning. The 3-dimensional canary brain atlas presented here is freely available and is easily adaptable to support many types of neurobiological studies, including anatomical, electrophysiological, histological, explant, and tracer studies.

Neuroanatomy of a neurobehavioral disturbance in the left anterior thalamic infarction

Background and purposeCognitive and behavioural symptoms represent primary clinical manifestations of anterior thalamic infarcts (ATIs) in the tuberothalamic artery territory. The aim of the study is to understand the pathomechanism...

Effect of fiducial marker defects on stereotactic target localization in the Leksell stereotactic system

The stereotactic procedure in neurosurgery is a minimally invasive technique used to treat intracranial lesions. The fiducial markers of a frame-based stereotactic procedure are important for defining the stereotactic coordinate system and in locating the target. These markers are often defective in stereotactic images owing to the presence of air bubbles in the imaging indicator. We have assessed the effect of these defects on the registration of an image and on the localization of a target. The virtual phantom method was employed to simulate various types of defect. The virtual images were registered using the Leksell GammaPlan® (LGP) radiosurgery planning system, and the image definition and the target localization errors were assessed. As a result, the effect of the defects was most severe when the majority of the marker disappeared, but not all, especially in the posterior region. The mean and maximum image definition errors were 0.1 and 1.4 mm, which caused the mean target localization error to be 0.2 and 0.0 mm in LGP version 5.34 and 8.3.1, respectively. It is recommended to exclude images with defective fiducial markers during the image definition procedure to minimize subsequent errors, though the newest version of LGP (version 8.3.1) corrects localization errors.

Targeting of the pedunculopontine nucleus by an MRI-guided approach: a cadaver study

Laboratory evidence suggests that the pedunculopontine nucleus (PPN) plays a central role in the initiation and maintenance of gait. Translational research has led to reports on deep brain stimulation (DBS) of the rostral brainstem in parkinsonian patients. However, initial clinical results appear to be rather variable. Possible factors include patient selection and the wide variability in anatomical location of implanted electrodes. Clinical studies on PPN DBS efficacy would, therefore, benefit from an accurate and reproducible method of stereotactic localization of the nucleus. The present study evaluates the anatomical accuracy of a specific protocol for MRI-guided stereotactic targeting of the PPN in a human cadaver. Imaging at 1.5 and 9.4T confirmed electrode location in the intended region as defined anatomically by the surrounding fiber tracts. The spatial relations of each electrode track to the nucleus were explored by subsequent histological examination. This confirmed that the neuropil surrounding each electrode track contained scattered large neurons morphologically consistent with those of the subnucleus dissipatus and compactus of the PPN. The results support the accuracy of the described specific MR imaging protocol.

Stereotactic anatomy of the human nucleus accumbens: from applied mathematics to microsurgical accuracy

Stereotactic surgery of the human NA is a neurosurgical achievement of the twenty-first century. Our purpose was to provide a clinically oriented study focused on the detailed stereotactic anatomy of the NA, with great respect to its targeting. We tried to offer a guide of NA stereotactic targeting for neurosurgeons. For our imaging study, we used cerebral magnetic resonance images (MRIs) from 26 neurosurgical patients (52 NAs). The material of our anatomic study consisted of 32 cerebral hemispheres (32 NAs) from 18 normal human brains, which we have in our Department (Department of Anatomy) from cadaver donors. We measured and analyzed the X, X', Y, Y', Z, Z' stereotactic coordinates of the NA at specific clinically important transverse, coronal and sagittal levels. Our principal findings contain a probability-based guide for in vivo (side depended) stereotactic localization of the human NA, a standard for the NA, specific stereotactic zone of the human brain (Z=-4), two specific standard NA areas (X=7, X=8) and the most reliable stereotactically standard area of the human NA (Y=2). We provide a stereotactic anatomic guide for some common targeting necessities of the NA stereotactic surgery, resulted from detailed analysis and careful combination of the measured data of our clinically oriented study. We hope that our work will be a really useful guide for neurosurgeons applying deep brain stimulation of the NA.

Hot spots in dynamic18FET-PET delineate malignant tumor parts within suspected WHO grade II gliomas

Molecular imaging studies have recently found inter- and intratumoral heterogeneity in World Health Organization (WHO) grade II gliomas. A correlative analysis with tumor histology, however, is still lacking. For elucidation we conducted the current prospective study. Fifty-five adult patients with an MRI-based suspicion of a WHO grade II glioma were included. [F-18]Fluoroethyltyrosine ((18)FET) uptake kinetic studies were combined with frame-based stereotactic localization techniques and used as a guide for stepwise (1-mm steps) histopathological evaluation throughout the tumor space. In tumors with heterogeneous PET findings, the O(6)-methylguanine-DNA methyltransferase (MGMT) promoter methylation status and expression of mutated protein isocitrate dehydrogenase variant R132H (IDH1) were determined inside and outside of hot spot volumes. Metabolic imaging revealed 3 subgroups: the homogeneous WHO grade II glioma group (30 patients), the homogeneous malignant glioma group (10 patients), and the heterogeneous group exhibiting both low- and high-grade characteristics at different sites (15 patients). Stepwise evaluation of 373 biopsy samples indicated a strong correlation with analyses of uptake kinetics (p < 0.0001). A homogeneous pattern of uptake kinetics was linked to homogeneous histopathological findings, whereas a heterogeneous pattern was associated with histopathological heterogeneity; hot spots exhibiting malignant glioma characteristics covered 4-44% of the entire tumor volumes. Both MGMT and IDH1 status were identical at different tumor sites and not influenced by heterogeneity. Maps of (18)FET uptake kinetics strongly correlated with histopathology in suspected grade II gliomas. Anaplastic foci can be accurately identified, and this finding has implications for prognostic evaluation and treatment planning.

A new system for neuronavigation and stereotactic biopsy pantograph stereotactic localization and guidance system

Everyday, neurosurgeons face the problem of orientation within the brain but the advent of stereotactic surgery and neuronavigation have solved this problem. Frame-based stereotactic systems (FBSS) and neuronavigation systems have their own strengths and priority and pitfalls, which were the main driving force for us to design a new system. This hybrid system comprises three main parts: main frame, monitoring system, and pantograph, which are connected to each other and to the operating table by particular attachments. For using this system, after performing CT SCAN or Magnetic Resonance Imaging (MRI) the axial view will be transferred to Liquid Cristal Display (LCD). In the operating room, the head of the patient fixes to the operating table and registration is completed by two arms of pantograph. We made a simulation operation with our system on an occipital cavernous angioma and a frontal oligodendroglioma. The software, which have been used for simulation were as follows; Poser (version-7), Catia (version 5- R18), and 3 Dimension Max (version 2008). The accuracy of this system is approximately two millimeter. The advantages of this system are: easy to use, much less expensive, and compatible with different devices, which may be needed during neurosurgical operation. For countries that do not have the opportunity to have sophisticated technology and neuronavigation system, we believe that our system is a one-stop solution.

Fractionated Stereotactic Radiation Therapy for Orbital Optic Nerve Sheath Meningioma – a Single Institution Experience and a Short Review of the Literature

Optic nerve sheath meningioma (ONSM) is a rare orbital tumor that generally induces a slow progressive visual loss in affected patients. Radiotherapy (RT) has currently become the first choice to treat ONSM. In this study our experience in ONSM treatment with fractionated stereotactic radiotherapy (FSRT) is reported. Five patients with diagnosis of orbital ONSM were treated between April 2007 and December 2009 at the Radiation Oncology department of our institution. All patients underwent history and physical, and ophthalmic examinations. Orbital MRI was performed before and 6 weeks after treatment; thereafter every 6 months for the first 2 years. By previous stereotactic localization of the target, RT was delivered with 28 daily fraction of 1.8 Gy by multiple non coplanar arcs dynamically conformed by a micro multileaf-collimator. At diagnosis, in all 5 patients, visual acuity limitations of different degrees were found, while exophthalmos was present in 2, diplopy in 2, orbital pain in 1, and proptosis in 1. In all patients pre-treatment MRI showed an orbital mass involving the optic nerve. After radiotherapy, previous symptoms improved in all patients. However, after RT the MRI consistently showed a stationary status compared to the MRI before RT. At a median follow up of 26 months (range 9-37) all patients had a subjective and/or objective better visual performance than before RT without any evidence of disease progression. No late side effects were recorded. Accordingly to the current literature, our experience confirms the efficacy and the safety of FSRT in patients with orbital ONSM.

Abstract P6-02-04: Breast Lesion Excision System — Intact®(B.L.E.S): A Novel Stereotactic Method of Biopsy Utilizing Radiofrequency for Suspicious Non-Palpable Mammographic Lesions

Abstract AIM: The Breast Lesion Excision System by Intact®(B.L.E.S) is a novel stereotactic, vacuum-assisted breast biopsy device that utilizes radiofrequency in order to completely excise any suspicious (BIRAD ≥4) non palpable mammographic lesion. The purpose of this clinical study is to evaluate the efficiency and success rate of the B.L.E.S system in the accurate diagnosis of any suspicious non-palpable mammographic lesions. MATERIAL AND METHODS: This study included 105 patients with suspicious non palpable mammographic lesions that visited the Breast Unit of the First Propaedeutic Surgical Clinic at Ippocration hospital from January 2008 to April 2010. The mammographic findings included suspicious microcalcifications, solid lesion and radial scars, all classified as BIRAD ≥4. The method was performed by the same surgical-radiologist team utilizing the Fisher's stereotactic table which includes a breast compression plate, incorporated digital mammography device, a robotic arm that carries the biopsy needle and a vacuum apparatus. The system is controlled by a central computer that performs the stereotactic localization. We utilize the 15mm or the 20mm tissue basket depending on the size of the lesion, trying to completely excise it with healthy margins, if possible. The procedure is performed under local anaesthesia (20ml of 2% Lydokaine) and is completed with a single needle pass. A 5-6 mm skin incision is performed and a retrieval basket is deployed to circumscribe the lesion, while radiofrequency is used to ablate the surrounding breast tissue. The basket containing the captured specimen is removed through the incision and the wound is dressed with steril strip. We routinely perform mammography on the specimen to insure the success of the biopsy and we place a titanium clip in the biopsy cavity to mark the area. RESULTS: One hundred and five patients with non palpable mammographic lesions underwent biopsy with the BLES — Intact® system. All lesions were excised completely, but not all with healthy margins especially in cases of DCIS with intraductal component. The histological findings included: Fibroadenoma: 38 (36.1 %), Invasive Ductal Breast Cancer with associated DCIS 14 (13.3 %), papilloma: 11 (10.4 %), Fibrocystic changes 11 (10.4 %), Papillary Apocrine Metaplasis 7 (6.6 %), Stromal Fibrosis: 7 (6.6%), mammary duct ectasia: 6 (5.71%), DCIS :5 (4.7%), Liponecrosis: 2 (1.9 %), Intramamary lymph node: 2 (1.9%), Invasive Ductal Carcinoma: 1 (0.95%), Invasive Ductal Carcinoma with associated lobular in situ (LCIS): 1 (0.95%). CONCLUSION: Suspicious non palpable mammographic lesions can be safely removed for histologic diagnosis utilizing the BLES — Intact® system. The method has the unique advantage of completely removing the lesion without cutting through it. This way the specimen retains its original architectural structure intact, which means even more accurate histologic diagnosis and thus lower underestimation rates. Furthermore, the BLES system alliviates all concern for possible tumor dissemination through the needle tract since it comes in no contact with the tumor and can remove the lesion with healthy margins. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P6-02-04.

Dosimetric Accuracy of TomoTherapy Based Stereotactic Radiotherapy

Helical Tomotherapy (HT) has gained wide acceptance as a delivery technique for intra- and extra-cranial stereotactic radiotherapy using the on-board MVCT for stereotactic localization. Verifying the dosimetric accuracy of the delivery becomes more difficult for small targets. Of 70 patients treated on our HT unit since January 1, 2010, routine delivery quality assurance (DQA) has only exceeded the expected dose by more than 2.5%, for 5 patients, all with stereotactic targets in the lung and brain. The largest dose difference (+5%) was seen for the smallest target (8.4 cm3). Measurements were performed with the standard DQA procedure using small volume A1SL ion chamber and radiochromic film. Film measurements showed excellent localization accuracy, but confirmed the dose discrepancy for the 5 patients. The goal of this work is to investigate these discrepancies and to quantify the dosimetric accuracy of the HT stereotactic delivery. HT plans were run using a cylindrical water-filled phantom with contoured spherical targets of variable diameter (5 mm-40 mm, volume 0.05 cm3 to 32.4cm3). The phantom was scanned with a 48 cm field of view and 1 mm slice width. Images were imported into the TomoTherapy TPS resulting in a “Fine” dose grid voxel size of 1.9 mm x 1.9 mm x 1.0 mm, which is consistent with the smallest we use clinically for HT stereotactic procedures. Plans utilized the 1.0cm field width and a pitch of 0.287. Dose was measured using a very small volume (0.002cm3) liquid ion chamber. The phantom and chamber were positioned using MVCT imaging for lateral and vertical positioning and to approximate a longitudinal position. The final longitudinal position was determined by searching for the maximum signal for the 5 mm diameter target in 1 mm increments. Measured dose was found to be higher than that predicted by the TPS in all cases with the discrepancy between the planned and measured dose increasing with decreasing target volume. For the 5 mm, 10 mm, 15 mm, 20 mm, 30 mm and 40 mm diameter targets, measured doses were 7.9% ± 0.2%, 4.6 ± 0.3%, 2.1% ± 0.4%, 1.4% ± 0.1%, 0.7% ± 0.3%, and 0.6 ± 0.1% higher than expected from the TPS. The results confirm that the planning system underestimates the dose delivered for these very small targets, resulting in a delivered dose that is higher than predicted by the TPS. The HT system delivers acceptable doses (within 5%) down to targets 10 mm in diameter, but smaller fields incurred discrepancies of up to 8% as measured with a fine resolution dosimeter. To ensure accurate dosimetry, care must be exercised when choosing planning and verification techniques for systems not designed for stereotactic dose calculations.

Poster 72: Resting State Motor Networks are Reorganized After Stroke

Objective: This is a methodological study layered on the Veterans Affairs (VA) Cooperative Studies Program clinical trial with study participation ending in July 2009. A late-breaking clinical trial abstract is planned for submission to this meeting. Prior to this time, there were not sufficient numbers of participants nor knowledge of clinical context, to contemplate submission of a regular abstract. Robotic upper-extremity rehabilitation is quickly becoming an important experimental treatment in hemiparetic stroke. In contrast to other methods of rehabilitation, little is known about the neurobiological mechanisms underlying recovery of function achieved with this method. We hypothesized that baseline correlations would exist between biology and function and particularly that a relatively new technique of resting state connectivity analysis would provide a better measure than functional magnetic resonance imaging (fMRI). Main Outcome Measures: Participants in VA trials of robotic rehabilitation in chronic (>6mo) stroke were recruited to have transcranial magnetic stimulation (TMS) and magnetic resonance imaging (MRI) studies at specific time points during their clinical studies. There were 2 baseline sessions of TMS and 1 of MRI. TMS and MRI were then performed at the end of the period of therapy (6 or 12wks, depending on which clinical study they were enrolled) and TMS alone was performed 12 weeks later. fMRI was used to measure brain activation, related to performance of rhythmic hand grip, in predetermined regions of interest. Other MRI methods were used to determine resting state connectivity, fractional anisotropy, and lesion localization. TMS consisted of stereotactic localization of the brain representation of the biceps brachii (BB) and extensor digitorum communis (EDC) muscles on the affected side, the threshold, recruitment curve, and ipsilateral silent period for those muscles. Results: 11 participants have so far completed the study. X% had measurable responses in both muscles and no one without EDC responses had BB responses. Resting state connectivity related to motor cortex was impaired compared to normal subjects.

Single fraction radiosurgery using Rapid Arc for treatment of intracranial targets

BackgroundStereotactic-Radio-Surgery (SRS) using Conformal-Arc-Therapy (CAT) is a well established irradiation technique for treatment of intracranial targets. Although small safety margins are required because of very high accuracy of patient positioning and exact online localisation, there are still disadvantages like long treatment time, high number of monitor units (MU) and covering of noncircular targets. This planning study analysed whether Rapid Arc (RA) with stereotactic localisation for single-fraction SRS can solve these problems.MethodsTen consecutive patients were treated with Linac-based SRS. Eight patients had one or more brain metastases. The other patients presented a symptomatic vestibularis schwannoma and an atypic meningeoma. For all patients, two plans (CAT/RA) were calculated and analysed.ResultsConformity was higher for RA with additional larger low-dose areas. Furthermore, RA reduced the number of MU and the treatment time for all patients. Dose to organs at risk were equal or slightly higher using RA in comparison to CAT.ConclusionsRA provides a new alternative for single-fraction SRS irradiation combining advantages of short treatment time with lower number of MU and better conformity in addition to accuracy of stereotactic localisation in selected cases with uncomplicated clinical realization.