Cranial Radiosurgery Research Articles

Temozolomide combined with radiotherapy in the treatment of recurrent cranial meningioma previously treated with multiple surgical resections and two sessions of radiosurgery: a case report and literature review.

Meningioma is a common primary brain tumor in adults. The treatment approach depends on its grade, size and symptoms. The case reported here was a repeatedly recurring transitional meningioma, previously treated with several surgical procedures and two sessions of cranial radiosurgery. At the time of last relapse, temozolomide was administered alone and combined with external beam radiation therapy. At the last follow-up after 38 months, the disease was stable. In conclusion, in this case of plurirecurrent meningioma, treatment with temozolomide alone and in combination with radiotherapy was feasible and associated with limited morbidity, providing clinical benefit and long local disease control. We discuss the case comparing our approach with other experiences reported in the literature.

Application of systems and control theory-based hazard analysis to radiation oncology.

Both humans and software are notoriously challenging to account for in traditional hazard analysis models. The purpose of this work is to investigate and demonstrate the application of a new, extended accident causality model, called systems theoretic accident model and processes (STAMP), to radiation oncology. Specifically, a hazard analysis technique based on STAMP, system-theoretic process analysis (STPA), is used to perform a hazard analysis. The STPA procedure starts with the definition of high-level accidents for radiation oncology at the medical center and the hazards leading to those accidents. From there, the hierarchical safety control structure of the radiation oncology clinic is modeled, i.e., the controls that are used to prevent accidents and provide effective treatment. Using STPA, unsafe control actions (behaviors) are identified that can lead to the hazards as well as causal scenarios that can lead to the identified unsafe control. This information can be used to eliminate or mitigate potential hazards. The STPA procedure is demonstrated on a new online adaptive cranial radiosurgery procedure that omits the CT simulation step and uses CBCT for localization, planning, and surface imaging system during treatment. The STPA procedure generated a comprehensive set of causal scenarios that are traced back to system hazards and accidents. Ten control loops were created for the new SRS procedure, which covered the areas of hospital and department management, treatment design and delivery, and vendor service. Eighty three unsafe control actions were identified as well as 472 causal scenarios that could lead to those unsafe control actions. STPA provides a method for understanding the role of management decisions and hospital operations on system safety and generating process design requirements to prevent hazards and accidents. The interaction of people, hardware, and software is highlighted. The method of STPA produces results that can be used to improve safety and prevent accidents and warrants further investigation.

Evaluation of the setup margins for cone beam computed tomography–guided cranial radiosurgery: A phantom study

The aim of this study is to evaluate the setup margins from the clinical target volume (CTV) to planning target volume (PTV) for cranial stereotactic radiosurgery (SRS) treatments guided by cone beam computed tomography (CBCT). We designed an end-to-end (E2E) test using a skull phantom with an embedded 6mm tungsten ball (target). A noncoplanar plan was computed (E2E plan) to irradiate the target. The CBCT-guided positioning of the skull phantom on the linac was performed. Megavoltage portal images were acquired after 15 independent deliveries of the E2E plan. The displacement 2-dimensional (2D) vector between the centers of the square field and the ball target on each portal image was used to quantify the isocenter accuracy. Geometrical margins on each patient׳s direction (left-right or LR, anterior-posterior or AP, superior-inferior or SI) were calculated. Dosimetric validation of the margins was performed in 5 real SRS cases: 3-dimesional (3D) isocenter deviations were mimicked, and changes in CTV dose coverage and organs-at-risk (OARs) dosage were analyzed. The CTV-PTV margins of 1.1mm in LR direction, and 0.7mm in AP and SI directions were derived from the E2E tests. The dosimetric analysis revealed that a 1-mm uniform margin was sufficient to ensure the CTV dose coverage, without compromising the OAR dose tolerances. The effect of isocenter uncertainty has been estimated to be 1mm in our CBCT-guided SRS approach.

Evaluation of initial setup accuracy and intrafraction motion for spine stereotactic body radiation therapy using stereotactic body frames

PurposeThe purposes of this study were (1) to evaluate the initial setup accuracy and intrafraction motion for spine stereotactic body radiation therapy (SBRT) using stereotactic body frames (SBFs) and (2) to validate an in-house-developed SBF using a commercial SBF as a benchmark. Methods and materialsThirty-two spine SBRT patients (34 sites, 118 fractions) were immobilized with the Elekta and in-house (BHS) SBFs. All patients were set up with the Brainlab ExacTrac system, which includes infrared and stereoscopic kilovoltage x-ray-based positioning. Patients were initially positioned in the frame with the use of skin tattoos and then shifted to the treatment isocenter based on infrared markers affixed to the frame with known geometry relative to the isocenter. ExacTrac kV imaging was acquired, and automatic 6D (6 degrees of freedom) bony fusion was performed. The resulting translations and rotations gave the initial setup accuracy. These translations and rotations were corrected for by use of a robotic couch, and verification imaging was acquired that yielded residual setup error. The imaging/fusion process was repeated multiple times during treatment to provide intrafraction motion data. ResultsThe BHS SBF had greater initial setup errors (mean±SD): −3.9±5.5mm (0.2±0.9°), −1.6±6.0mm (0.5±1.4°), and 0.0±5.3mm (0.8±1.0°), respectively, in the vertical (VRT), longitudinal (LNG), and lateral (LAT) directions. The corresponding values were 0.6±2.7mm (0.2±0.6°), 0.9±5.3mm (−0.2±0.9°), and −0.9±3.0mm (0.3±0.9°) for the Elekta SBF. The residual setup errors were essentially the same for both frames and were −0.1±0.4mm (0.1±0.5°), −0.2±0.4mm (0.0±0.4°), and 0.0±0.4mm (0.0±0.4°), respectively, in VRT, LNG, and LAT. The intrafraction shifts in VRT, LNG, and LAT were 0.0±0.4mm (0.0±0.3°), 0.0±0.5mm (0.0±0.4°), and 0.0±0.4mm (0.0±0.3°), with no significant difference observed between the 2 frames. ConclusionsThese results showed that the combination of the ExacTrac system with either SBF was highly effective in achieving both setup accuracy and intrafraction stability, which were on par with that of mask-based cranial radiosurgery.

How Low Should You Go: Choice of Minimum Dose Prescription in Cranial Radiosurgery.

Background: In radiosurgery, the convention has been to prescribe radiation dose to a “covering isodose volume”. This is presumed to be the minimum dose received by the entire tumor. Our purpose was to review our practice, assess different means of specifying a prescription isodose, and test how reliable they were in the face of various calculation methodologies.Methods: Different minimum doses were calculated using three calculation methods for 20 brain targets and compared to the original physician intent. Monte Carlo (MC) calculations were run down to 2% (1 sigma) statistics. Voxel size depended on the imaging field of view and the calculation engine. For pencil beam convolution ("finite size pencil beam" - FSPB) calculations with or without heterogeneity correction (Methods 1-2), the calculation grid matched the CT scan resolution. For MC calculations (Method 3), the highest available in-plane resolution was 256 x 256 pixels. The median voxel volume was thus 0.58 mm3 (0.47 to 0.95 mm3) for FSPB and 2.3 mm3 (1.87 to 3.80) for MC.Results: The absolute minimum target dose varied substantially between the three calculation methods — up to 25% difference between Methods 1 and 3. The differences were reduced when comparing near-minimum doses with absolute minimal volumes ΔV, DPTV-ΔV. The median difference in the isodose covering the PTV-0.03 cm3 was 0% for Methods 1 and 2 and 3.6% for Methods 1 and 3. The median difference in the isodose covering the PTV-0.01 cm3 was 0% for Methods 1 and 2 and 2.2% for Methods 1 and 3.In our data, the smaller the volume in which the minimum dose is calculated, the more sensitive this calculation was to dose calculation parameters. The standard deviation of the difference between physician intent and the isodose covering the PTV-0.01 cm3 was 2.9% (range from -3.3% to 9.3%).Conclusion: In radiosurgery, absolute minimum doses are sensitive to changes in dose calculation grids and dose calculation algorithms. Based on our experience, standardizing dose prescription to the isodose volume covering the PTV-0.01 cm3 or the PTV-0.03 cm3 would have little impact on clinical practice and would be relatively insensitive to dose calculation parameters.

Development, Implementation, and Use of a Local and Global Clinical Registry for Neurosurgery.

Physicians are being challenged to obtain data for outcomes research and measures of quality practice in medicine. We developed a prospective data collection system (registry) that provides data points across all elements of a neurosurgical stereotactic radiosurgery practice. The registry architecture is scalable and suitable for any aspect of neurosurgical practice. Our purpose was to outline the challenges in creating systems for high quality data acquisition and describe experiences in initial testing and use. Over a two year period, a multicenter team working with software engineers developed a comprehensive radiosurgery registry based on a MS-Sequel® server platform. Three neurosurgeons at one center were responsible for final editing. Alpha testing began in September 2012 and server-based beta testing began in February 2013. The major elements included demographics, disease-based items (47 categories for different brain tumors, vascular malformations, and functional disorders) with relevant clinical grading systems, treatment-based items (imaging, physics, clinical), and follow-up data (clinical, imaging, subsequent therapeutics). Nine hundred patients were entered into the registry at one test center, with new entries and follow-up data entered daily at the point of contact. With experience, the mean time for one new entry was 6 minutes. Mean time for one follow-up entry was 45 seconds. The system was made secure for individual use and amenable for both data entry and research. Analytics used different filters to create customized outcomes charts as selected by the user (e.g., survival, neurologic function, complications). A local or multicenter prospective data collection registry was created for use across 47 clinical indications for stereotactic cranial radiosurgery. Further refinement of fields and logic is ongoing. The system is reliable, robust, and allows use of rapid analytical tools. Large medical registries will become widely used for collection and analysis of large data sets and should have broad applicability to many other elements of neurosurgical and medical practice.

WE-A-304-01: Strategies and Technologies for Cranial Radiosurgery Planning: MLC-Based Linac

The high fractional doses, stringent requirements for accuracy and precision, and surgical perspective characteristic of intracranial radiosurgery create considerations for treatment planning which are distinct from most other radiotherapy procedures. This session will introduce treatment planning techniques specific to two popular intracranial SRS modalities: Gamma Knife and MLC-based Linac. The basic treatment delivery characteristics of each device will be reviewed with a focus on how those characteristics determine the paradigm used for treatment planning. Basic techniques for treatment planning will be discussed, including considerations such as isodose selection, target and organ-at-risk definition, quality indices, and protection of critical structures. Future directions for SRS treatment planning will also be discussed. Learning Objectives: 1. Introduce the basic physical principles of intracranial radiosurgery and how they are realized in the treatment planning paradigms for Gamma Knife and Linac radiosurgery. 2. Demonstrate basic treatment planning techniques. 3. Discuss metrics for evaluating SRS treatment plan quality. 4. Discuss recent and future advances in SRS treatment planning. D. Schlesinger receives research support from Elekta, AB

WE‐G‐BRA‐06: Application of Systems and Control Theory‐Based Hazard Analysis to Radiotherapy

Purpose:The process of delivering radiation occurs in a complex socio‐technical system heavily reliant on human operators. Furthermore, both humans and software are notoriously challenging to account for in traditional hazard analysis models. High reliability industries such as aviation have approached this problem through using hazard analysis techniques grounded in systems and control theory. The purpose of this work is to apply the Systems Theoretic Accident Model Processes (STAMP) hazard model to radiotherapy. In particular, the System‐Theoretic Process Analysis (STPA) approach is used to perform a hazard analysis of a proposed on‐line adaptive cranial radiosurgery procedure that omits the CT Simulation step and uses only CBCT for planning, localization, and treatment.Methods:The STPA procedure first requires the definition of high‐level accidents and hazards leading to those accidents. From there, hierarchical control structures were created followed by the identification and description of control actions for each control structure. Utilizing these control structures, unsafe states of each control action were created. Scenarios contributing to unsafe control action states were then identified and translated into system requirements to constrain process behavior within safe boundaries.Results:Ten control structures were created for this new CBCT‐only process which covered the areas of hospital and department management, treatment design and delivery, and vendor service. Twenty three control actions were identified that contributed to over 80 unsafe states of those control actions resulting in over 220 failure scenarios.Conclusion:The interaction of people, hardware, and software are highlighted through the STPA approach. STPA provides a hierarchical model for understanding the role of management decisions in impacting system safety so that a process design requirement can be traced back to the hazard and accident that it is intended to mitigate.Varian Medical Systems, Inc.

Failure mode and effects analysis and fault tree analysis of surface image guided cranial radiosurgery.

Surface image guided, Linac-based radiosurgery (SIG-RS) is a modern approach for delivering radiosurgery that utilizes optical stereoscopic imaging to monitor the surface of the patient during treatment in lieu of using a head frame for patient immobilization. Considering the novelty of the SIG-RS approach and the severity of errors associated with delivery of large doses per fraction, a risk assessment should be conducted to identify potential hazards, determine their causes, and formulate mitigation strategies. The purpose of this work is to investigate SIG-RS using the combined application of failure modes and effects analysis (FMEA) and fault tree analysis (FTA), report on the effort required to complete the analysis, and evaluate the use of FTA in conjunction with FMEA. A multidisciplinary team was assembled to conduct the FMEA on the SIG-RS process. A process map detailing the steps of the SIG-RS was created to guide the FMEA. Failure modes were determined for each step in the SIG-RS process, and risk priority numbers (RPNs) were estimated for each failure mode to facilitate risk stratification. The failure modes were ranked by RPN, and FTA was used to determine the root factors contributing to the riskiest failure modes. Using the FTA, mitigation strategies were formulated to address the root factors and reduce the risk of the process. The RPNs were re-estimated based on the mitigation strategies to determine the margin of risk reduction. The FMEA and FTAs for the top two failure modes required an effort of 36 person-hours (30 person-hours for the FMEA and 6 person-hours for two FTAs). The SIG-RS process consisted of 13 major subprocesses and 91 steps, which amounted to 167 failure modes. Of the 91 steps, 16 were directly related to surface imaging. Twenty-five failure modes resulted in a RPN of 100 or greater. Only one of these top 25 failure modes was specific to surface imaging. The riskiest surface imaging failure mode had an overall RPN-rank of eighth. Mitigation strategies for the top failure mode decreased the RPN from 288 to 72. Based on the FMEA performed in this work, the use of surface imaging for monitoring intrafraction position in Linac-based stereotactic radiosurgery (SRS) did not greatly increase the risk of the Linac-based SRS process. In some cases, SIG helped to reduce the risk of Linac-based RS. The FMEA was augmented by the use of FTA since it divided the failure modes into their fundamental components, which simplified the task of developing mitigation strategies.

Stereotactic Ablative Body Radiotherapy for Lung Cancer

Lung stereotactic ablative radiotherapy (SABR) is a method of delivering high ‘ablative’ doses of radiotherapy to tumours in the lung. It was developed at the Karolinska Institute in the early 1990s using the methods established in cranial radiosurgery with multiple beams prescribed to an isodose and using a custom designed stereotactic body frame for immobilisation. Since then, aligned with the advances in radiotherapy technology and techniques (e.g. four-dimensional computed tomography simulation and image-guided radiotherapy), there has been a rapid increase in the use of lung SABR for both early stage lung cancer and lung metastases. For peripheral primary lung cancers less than 5 cm in diameter, high rates of local control and low levels of acute and late toxicity are consistently reported in the published literature. Compared with historical control rates of stage I lung cancers treated with conventionally fractionated radiotherapy, SABR seems to offer higher rates of local control, lower levels of acute toxicity and a better quality of life after treatment. However, the full results of randomised controlled trials of SABR versus conventionally fractionated are awaited and will provide higher-level evidence. For central lung tumours, very high SABR doses can be associated with significant toxicity. Dose-adapted fractionation schedules seem to have much lower rates of toxicity and prospective trials, including the completed RTOG 0813 study and the on-going EORTC LUNGTEC study, should provide further evidence of safety and establish organ at risk tolerances. SABR can also be used for tumours metastatic to the lung with high rates of local control and is a reasonable alternative to surgery in selected patients. Going forward, prospective trials are underway to establish the safety and efficacy of SABR in oligometastatic disease. Population-based outcomes will be crucial in supporting/establishing SABR as the treatment of choice in medically inoperable patients with peripheral stage I lung cancers. Randomised phase III trials will hopefully extend the evidence base and show the safety and the utility of SABR in early central tumours and oligometastatic disease.

Stereotactic Body Radiation Therapy: A Review of Applications and Outcomes

Radiotherapy is a versatile tool used in the treatment of various types of benign and malignant neoplasms. However, conventional radiation therapy for cancerous conditions often results in collateral damage to healthy tissues due to involvement of oversized radiation fields. Over the last decade one type of precision based radiation treatment has developed. This new treatment, known as Stereotactic Radiosurgery (SRS), involves highly accurate beams of high-energy radiation that are used to destroy abnormal cells by permanently damaging their DNA. As technology progressed stereotactic cranial radiosurgery developed into a successful method for certain tumorous conditions of the head and skull. The success of stereotactic cranial radiosurgery led to further radiation application research to widen the spectrum of treatable conditions to include those located extracranially. The result of this continued research led to the development of stereotactic body radiation therapy (SBRT), a radiotherapy technique based upon principles of SRS that is used to treat small or moderate sized tumors of the body with a limited number of treatments. Stereotactic body radiation therapy combines the use of the latest tumor imaging technology as well as precision based radiation delivery mechanisms to overcome physiological barriers of normal radiation therapy such as movement of tumors in tissues. The net effect of SBRT is that a dose of radiation much larger than normal can be administered in a very precise manner, over smaller time frame, bringing about a dramatic tumor response. In this review the authors will attempt to briefly cover the subject of stereotactic body radiation therapy as well as its applications and effectiveness.

Technical Aspects of a Proton Stereotactic Radiosurgery Program

Our institution’s Proton Stereotactic Radiosurgery (PSRS) program has significantly evolved since its inception in 1965. A large patient volume and requirements for stringent patient setup and accurate dose delivery have posed numerous technical and logistical challenges. With the increasing availability of proton facilities, the feasibility of using protons for cranial radiosurgery is of growing interest. We provide an overview and insights of technical aspects of our proton radiosurgery program. We have reviewed our system of PSRS that has significantly evolved over decades of use and modification and now provide a summary of the technical workflow, highlighting the PSRS-specific adaptations of the common proton therapy techniques designed for fractionated treatments, including immobilization, setup verification and dose delivery. Our PSRS system is a dedicated, in-house designed stereotactic beamline that is based upon a fixed horizontal, single scattering proton beam with a large SAD to minimize penumbra. Patients can be rotated to 90 degrees around the cranio-caudal axis with a 5-degree-of-freedom positioner. Approximately 80% of PSRS cases are treated in the dedicated beamline while the remainder is triaged to a commercial fully rotational gantry system. Patient immobilization is achieved by either a non-invasive cranial ring via custom dental and occipital molds or thermoplastic mask-based systems including a bite block. All immobilization devices are adapted to minimize impact on the proton beam. Commercial planning software is employed for treatment planning with an average use of 3 beams per lesion. Custom aperture and range compensator are fabricated for each field. The system is commissioned and routinely verified to eliminate the need for patient specific dosimetry. Setup verification for treatment utilizes fiducial markers implanted into the outer table of the skull. Alternatively to the implants, anatomical landmarks on x-rays compared to DRRs can be used for setup. Approximately 200 patients are treated annually with this PSRS system, diagnoses covering malignant (e.g. brain metastases) and benign (e.g. pituitary adenomas, schwannomas, arteriovenous malformations). Efficient procedures have been developed to ensure optimal workflow between the various involved professional groups (immobilization staff, radiation therapy technicians, physicians, neurosurgeons, physicists) to achieve safe and accurate dose delivery. Our institution has developed a unique, efficient, and highly accurate system for delivering PSRS which may serve as a model for other centers.

TH‐B‐105‐01: Accuracy and Precision in Cranial Radiosurgery

Cranial radiosurgery has developed as a highly accurate treatment procedure for relatively small lesions. There is much discussion in the literature as to what degree of accuracy is possible when the treatment system is evaluated as a whole; including cranial fixation, localization, imaging, registration, contouring, dose calculation, etc. This is now commonly referred to as the end‐to‐end (E2E) test. The now‐dated AAPM report 54 (1995) suggested that the total uncertainty can reach 2–3 mm. However, many newer publications suggest a much tighter tolerance is achievable. This discussion will focus on the strengths, and weaknesses of well‐established cranial radiosurgery systems including Gamma Knife and CyberKnife, as well as linac‐based systems. The typical, and maximum accuracy of each system will be presented including benefits of mask immobilization and invasive ring fixation. The intent of these presentations is to outline the current state of cranial radiosurgery, and review relevant technologies that have improved our treatment accuracy. Limitations that still persist at this point will also be outlined and reviewed.Learning Objectives:1. Advantages of various radiosurgery technologies.2. Limitations of various radiosurgery technologies.3. Main hurdles in achieving sub‐millimeter treatment accuracy.4. Overall treatment accuracy assessed by E2E testing.

MO‐F‐500‐01: Clinical Applications of Surface Imaging

Non‐invasive 3D surface matching systems have become clinically available for patient positioning and tracking throughout treatment. This session will focus on the current clinical applications of surface imaging, specifically for breast radiotherapy and cranial radiosurgery. First, acceptance and commissioning of surface imaging systems will be reviewed. Next, workflows and resulting data will be presented for the use of surface imaging to position patients receiving whole‐breast irradiation. Additionally, workflows and data describing the use of surface imaging to track patients during breath‐hold breast cancer treatments and frameless radiosurgery will also be described. The session outline is: a. An overview of commissioning and acceptance testing of surface imaging systems, including incorporation of routine QA into current clinical practice for a comprehensive IGRT program b. The use of surface imaging for daily positioning of patients receiving whole‐breast radiotherapy c. Surface imaging applications for breath‐hold treatment of left‐sided breast cancer patients d. Surface imaging applications for frameless cranial radiosurgery.Learning Objectives:1. Learn to incorporate QA for surface imaging into current QA procedures for IGRT.2. Understand the advantages and limitations of surface imaging for clinical use in breast radiotherapy and cranial radiosurgery.3. Learn about the potential use of surface imaging for real‐time motion tracking.Jan‐Jakob Sonke's research is sponsored by Elekta Corporation.Elekta Corporation sponsors research by Jan‐Jakob Sonke.

SU‐E‐T‐416: Targeting Accuracy of a TrueBeam System for Multi‐Lesion Cranial Radiosurgery with Single Isocenter

Purpose: To investigate the targeting accuracy of a Varian TrueBeam System for delivering intracranial radiosurgery to multiple lesions using a single isocenter (ISO). Methods: The study was performed using hidden target (BB) tests along with a MV portal imager (EPID). Six BBs were inserted into a head phantom simulating the locations of multiple metastatic tumors. One BB acted as the common ISO. The distances from the ISO to the other BBs were 2.9 to 7.1 cm. Simulating the actual patient treatment in our clinic, the phantom was immobilized using a BrainLAB U‐shape frame and a head mask. CT image was acquired with 1 mm slice thickness. For a selected couch, gantry and collimator angle combination, a 3×3 cm MLC‐shaped field was created for each BB. The centers of BB and the field were aligned, and the distance from the plan was read out on the DRR. Before shooting the tests, CBCT was used to setup the phantom, and the translational/rotational mismatch was corrected iteratively until less than 1 mm and 0.1 degree respectively. The actual positions of the BBs relative to their corresponding field centers were measured on the portal images. The deviation between the measured and planned BB positions quantifies the geometric targeting error. The test was performed for various couch, gantry and collimator angle combinations (total 88 tests). Results: The mean±SD of the BB position deviations were 0.0±0.4 and 0.2±0.4 mm in the image transverse‐and longitudinal‐direction respectively; and 0.5±0.3 mm for the vector length. Six of 88 BB tests (6.8%) exceeded the vector length of 1 mm with a maximum of 1.2 mm Conclusion: The mechanical targeting accuracy of the TrueBeam System at our institution is acceptable for delivery of intracranial radiosurgery with a single isocenter for treatment of multiple targets. Application of this technique is being investigated.

PD-0603: Evaluation of 3 rotational techniques for cranial radiosurgery according to the target shape and their number.

PD-0603: Evaluation of 3 rotational techniques for cranial radiosurgery according to the target shape and their number.

Image fusion pitfalls for cranial radiosurgery.

Stereotactic radiosurgery requires imaging to define both the stereotactic space in which the treatment is delivered and the target itself. Image fusion is the process of using rotation and translation to bring a second image set into alignment with the first image set. This allows the potential concurrent use of multiple image sets to define the target and stereotactic space. While a single magnetic resonance imaging (MRI) sequence alone can be used for delineation of the target and fiducials, there may be significant advantages to using additional imaging sets including other MRI sequences, computed tomography (CT) scans, and advanced imaging sets such as catheter-based angiography, diffusor tension imaging-based fiber tracking and positon emission tomography in order to more accurately define the target and surrounding critical structures. Stereotactic space is usually defined by detection of fiducials on the stereotactic head frame or mask system. Unfortunately MRI sequences are susceptible to geometric distortion, whereas CT scans do not face this problem (although they have poorer resolution of the target in most cases). Thus image fusion can allow the definition of stereotactic space to proceed from the geometrically accurate CT images at the same time as using MRI to define the target. The use of image fusion is associated with risk of error introduced by inaccuracies of the fusion process, as well as workflow changes that if not properly accounted for can mislead the treating clinician. The purpose of this review is to describe the uses of image fusion in stereotactic radiosurgery as well as its potential pitfalls.

Dynamic conformal arc cranial stereotactic radiosurgery: implications of multileaf collimator margin on dose-volume metrics.

The effect of multileaf collimator (MLC) margin on target and normal tissue dose-volume metrics for intracranial stereotactic radiosurgery (SRS) was assessed. 118 intracranial lesions of 83 SRS patients formed the basis of this study. For each planning target volume (PTV), five separate treatment plans were generated with MLC margins of -1, 0, 1, 2 and 3 mm, respectively. Identical treatment planning parameters were employed with a median of five dynamic conformal arcs using the Varian/BrainLab high-definition MLC for beam shaping. Prescription dose (PD) was such that 22 Gy covered at least 95% of the PTV. Dose-volume and dose-response comparative metrics included conformity index, heterogeneity index, dose gradient, tumour control probability (TCP) and normal tissue complication probability (NTCP). Target dose heterogeneity decreased with increasing MLC margin (p<0.001); mean heterogeneity index decreased from 70.4 ± 12.7 to 10.4 ± 2.2%. TCP decreased with increasing MLC margin (p<0.001); mean TCP decreased from 81.0 ± 2.3 to 62.2 ± 1.8%. Normal tissue dose fall-off increased with MLC margin (p<0.001); mean gradient increased from 3.1 ± 0.9 mm to 5.3 ± 0.7 mm. NTCP was optimal at 1 mm MLC margin. No unambiguous correlation was observed between NTCP and PTV volume. Plan delivery efficiency generally improved with larger margins (p<0.001); mean monitor unit per centigray of the PD decreased from 3.60 ± 1.30 to 1.56 ± 0.13. Conclusion Use of 1 mm MLC margins for dynamic conformal arc-based cranial radiosurgery resulted in optimal tumour control and normal tissue sparing. Clinical significance of these comparative findings warrants further investigation.

SU-E-T-401: Evaluation of Initial Setup Accuracy and Intra-Fraction Motion for Spine SBRT Using Stereotactic Body Frames.

Quantify initial setup accuracy and intra-fraction motion using stereotactic body frames (SBF) for spine SBRT. 10 patients (11 sites, 31 fractions) treated with spine SBRT using SBF immobilization were evaluated for initial setup accuracy and intra-fraction motion. Either the commercial Elekta SBF or an in-house developed SBF (BHS-SBF) were used. The BHS-SBF uses the same setup/immobilization principle as the Elekta but with increased interior space and couch indexing. Both frames include sidewalls to conform the vac-loc rigidly to the patient's sides. All patients were setup using the Brainlab ExacTrac system which includes IR and stereoscopic kV x-ray based positioning. Patients were initially positioned in the frame using skin tattoos then shifted to treatment isocenter based on IR markers affixed to the frame with known geometry relative to isocenter. kV imaging was acquired and automatic 6-D bony fusion performed. Resulting translations and rotations give the initial setup accuracy. Calculated shifts and rotations were performed using a robotic couch and verification imaging acquired. The imaging/fusion process was repeated multiple times during treatment providing intra-fraction motion data. Mean initial setup error in the VRT, LNG and LAT directions was 0.1+/-3.0 mm (0.1+/-0.6 deg), 0.5+/-5.2 mm (0.1+/-1.1 deg) and -0.3+/- 3.7 mm (0.4+/-0.8 deg) respectively. Mean 3-D error magnitude was 6.6 mm with a 95% certainty of 11.2 mm. Mean intra-fraction shifts observed in the VRT, LNG and LAT directions were -0.1+/-0.4 mm, -0.1+/-0.4 mm and 0.1+/-0.3 mm respectively. Mean 3-D intra-fraction shift magnitude was 0.6 mm with a 95% certainty of 1.4 mm. No significant difference was observed between the SBFs. Patient positioning is not sufficiently reproducible with the evaluated SBF to allow non-image guided treatment. However, provided image guidance is used for patient positioning, these frames provide excellent immobilization which is on par with mask based cranial radiosurgery.

Équipements requis pour la mise en œuvre de la radiochirurgie et de la radiothérapie en conditions stéréotaxiques intracrânienne

Since the previous special issue of Cancer Radiothérapie dedicated to radiosurgery in 1998, many important technological and computer developments have improved external beam radiotherapy treatment device performances. Whereas the Gamma Knife® was the gold standard for intracranial radiosurgery, new linear accelerator developments have led to new possibilities for the clinicians. This article describes quickly the different devices available for cranial radiosurgery or fractionated stereotactic radiotherapy.