Radiosurgery Unit Research Articles

TH‐EF‐204‐01: Introduction

Joanna E. Cygler, Jan Seuntjens, J. Daniel Bourland, M. Saiful Huq, Josep Puxeu Vaque, Daniel Zucca Aparicio, Tatiana Krylova, Yuri Kirpichev, Eric Ford, Caridad BorrasStereotactic Radiation Therapy (SRT) utilizes small static and dynamic (IMRT) fields, to successfully treat malignant and benign diseases using techniques such as Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiation Therapy (SBRT). SRT is characterized by sharp dose gradients for individual fields and their resultant dose distributions. For appropriate targets, small field radiotherapy offers improved treatment quality by allowing better sparing of organs at risk while delivering the prescribed target dose. Specialized small field treatment delivery systems, such as robotic‐controlled linear accelerators, gamma radiosurgery units, and dynamic arc linear accelerators may utilize rigid fixation, image guidance, and tumor tracking, to insure precise dose delivery to static or moving targets. However, in addition to great advantages, small field delivery techniques present special technical challenges for dose calibration due to unique geometries and small field sizes not covered by existing reference dosimetry protocols such as AAPM TG‐51 or IAEA TRS 398. In recent years extensive research has been performed to understand small field dosimetry and measurement instrumentation. AAPM, IAEA and ICRU task groups are expected to provide soon recommendations on the dosimetry of small radiation fields. In this symposium we will: 1] discuss the physics, instrumentation, methodologies and challenges for small field radiation dose measurements; 2] review IAEA and ICRU recommendations on prescribing, recording and reporting of small field radiation therapy; 3] discuss selected clinical applications and technical aspects for specialized image‐guided, small field, linear accelerator based treatment techniques such as IMRT and SBRT. Learning Objectives: To learn the physics of small fields in contrast to dosimetry of conventional fields To learn about detectors suitable for small fields To learn about the role of Monte Carlo simulations in determination of small field output factors To provide an overview of the IAEA small field dosimetry recommendations To provide an overview of the content of the ICRU report on Prescribing, Reporting and Recording of Small Field Radiation Therapy. To learn about special technical considerations in delivering IMRT and SBRT treatments To appreciate specific challenges of IMRT implementationJ. Seuntjens, Natural Sciences and Engineering Research Council; Canadian Institutes of Health Research

TH‐EF‐204‐02: Small Field Radiation Therapy: Physics and Recent Recommendations From IAEA and ICRU

Joanna E. Cygler, Jan Seuntjens, J. Daniel Bourland, M. Saiful Huq, Josep Puxeu Vaque, Daniel Zucca Aparicio, Tatiana Krylova, Yuri Kirpichev, Eric Ford, Caridad BorrasStereotactic Radiation Therapy (SRT) utilizes small static and dynamic (IMRT) fields, to successfully treat malignant and benign diseases using techniques such as Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiation Therapy (SBRT). SRT is characterized by sharp dose gradients for individual fields and their resultant dose distributions. For appropriate targets, small field radiotherapy offers improved treatment quality by allowing better sparing of organs at risk while delivering the prescribed target dose. Specialized small field treatment delivery systems, such as robotic‐controlled linear accelerators, gamma radiosurgery units, and dynamic arc linear accelerators may utilize rigid fixation, image guidance, and tumor tracking, to insure precise dose delivery to static or moving targets. However, in addition to great advantages, small field delivery techniques present special technical challenges for dose calibration due to unique geometries and small field sizes not covered by existing reference dosimetry protocols such as AAPM TG‐51 or IAEA TRS 398. In recent years extensive research has been performed to understand small field dosimetry and measurement instrumentation. AAPM, IAEA and ICRU task groups are expected to provide soon recommendations on the dosimetry of small radiation fields. In this symposium we will: 1] discuss the physics, instrumentation, methodologies and challenges for small field radiation dose measurements; 2] review IAEA and ICRU recommendations on prescribing, recording and reporting of small field radiation therapy; 3] discuss selected clinical applications and technical aspects for specialized image‐guided, small field, linear accelerator based treatment techniques such as IMRT and SBRT. Learning Objectives: To learn the physics of small fields in contrast to dosimetry of conventional fields To learn about detectors suitable for small fields To learn about the role of Monte Carlo simulations in determination of small field output factors To provide an overview of the IAEA small field dosimetry recommendations To provide an overview of the content of the ICRU report on Prescribing, Reporting and Recording of Small Field Radiation Therapy. To learn about special technical considerations in delivering IMRT and SBRT treatments To appreciate specific challenges of IMRT implementationJ. Seuntjens, Natural Sciences and Engineering Research Council; Canadian Institutes of Health Research

TH‐EF‐204‐03: Determination of Small Field Output Factors, Advantages and Limitations of Monte Carlo Simulation

Joanna E. Cygler, Jan Seuntjens, J. Daniel Bourland, M. Saiful Huq, Josep Puxeu Vaque, Daniel Zucca Aparicio, Tatiana Krylova, Yuri Kirpichev, Eric Ford, Caridad BorrasStereotactic Radiation Therapy (SRT) utilizes small static and dynamic (IMRT) fields, to successfully treat malignant and benign diseases using techniques such as Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiation Therapy (SBRT). SRT is characterized by sharp dose gradients for individual fields and their resultant dose distributions. For appropriate targets, small field radiotherapy offers improved treatment quality by allowing better sparing of organs at risk while delivering the prescribed target dose. Specialized small field treatment delivery systems, such as robotic‐controlled linear accelerators, gamma radiosurgery units, and dynamic arc linear accelerators may utilize rigid fixation, image guidance, and tumor tracking, to insure precise dose delivery to static or moving targets. However, in addition to great advantages, small field delivery techniques present special technical challenges for dose calibration due to unique geometries and small field sizes not covered by existing reference dosimetry protocols such as AAPM TG‐51 or IAEA TRS 398. In recent years extensive research has been performed to understand small field dosimetry and measurement instrumentation. AAPM, IAEA and ICRU task groups are expected to provide soon recommendations on the dosimetry of small radiation fields. In this symposium we will: 1] discuss the physics, instrumentation, methodologies and challenges for small field radiation dose measurements; 2] review IAEA and ICRU recommendations on prescribing, recording and reporting of small field radiation therapy; 3] discuss selected clinical applications and technical aspects for specialized image‐guided, small field, linear accelerator based treatment techniques such as IMRT and SBRT. Learning Objectives: To learn the physics of small fields in contrast to dosimetry of conventional fields To learn about detectors suitable for small fields To learn about the role of Monte Carlo simulations in determination of small field output factors To provide an overview of the IAEA small field dosimetry recommendations To provide an overview of the content of the ICRU report on Prescribing, Reporting and Recording of Small Field Radiation Therapy. To learn about special technical considerations in delivering IMRT and SBRT treatments To appreciate specific challenges of IMRT implementationJ. Seuntjens, Natural Sciences and Engineering Research Council; Canadian Institutes of Health Research

TH‐EF‐204‐06: Closing

Joanna E. Cygler, Jan Seuntjens, J. Daniel Bourland, M. Saiful Huq, Josep Puxeu Vaque, Daniel Zucca Aparicio, Tatiana Krylova, Yuri Kirpichev, Eric Ford, Caridad BorrasStereotactic Radiation Therapy (SRT) utilizes small static and dynamic (IMRT) fields, to successfully treat malignant and benign diseases using techniques such as Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiation Therapy (SBRT). SRT is characterized by sharp dose gradients for individual fields and their resultant dose distributions. For appropriate targets, small field radiotherapy offers improved treatment quality by allowing better sparing of organs at risk while delivering the prescribed target dose. Specialized small field treatment delivery systems, such as robotic‐controlled linear accelerators, gamma radiosurgery units, and dynamic arc linear accelerators may utilize rigid fixation, image guidance, and tumor tracking, to insure precise dose delivery to static or moving targets. However, in addition to great advantages, small field delivery techniques present special technical challenges for dose calibration due to unique geometries and small field sizes not covered by existing reference dosimetry protocols such as AAPM TG‐51 or IAEA TRS 398. In recent years extensive research has been performed to understand small field dosimetry and measurement instrumentation. AAPM, IAEA and ICRU task groups are expected to provide soon recommendations on the dosimetry of small radiation fields. In this symposium we will: 1] discuss the physics, instrumentation, methodologies and challenges for small field radiation dose measurements; 2] review IAEA and ICRU recommendations on prescribing, recording and reporting of small field radiation therapy; 3] discuss selected clinical applications and technical aspects for specialized image‐guided, small field, linear accelerator based treatment techniques such as IMRT and SBRT. Learning Objectives: To learn the physics of small fields in contrast to dosimetry of conventional fields To learn about detectors suitable for small fields To learn about the role of Monte Carlo simulations in determination of small field output factors To provide an overview of the IAEA small field dosimetry recommendations To provide an overview of the content of the ICRU report on Prescribing, Reporting and Recording of Small Field Radiation Therapy. To learn about special technical considerations in delivering IMRT and SBRT treatments To appreciate specific challenges of IMRT implementationJ. Seuntjens, Natural Sciences and Engineering Research Council; Canadian Institutes of Health Research

On the development of a comprehensive MC simulation model for the Gamma Knife Perfexion radiosurgery unit

This work presents a comprehensive Monte Carlo (MC) simulation model for the Gamma Knife Perfexion (PFX) radiosurgery unit. Model-based dosimetry calculations were benchmarked in terms of relative dose profiles (RDPs) and output factors (OFs), against corresponding EBT2 measurements. To reduce the rather prolonged computational time associated with the comprehensive PFX model MC simulations, two approximations were explored and evaluated on the grounds of dosimetric accuracy. The first consists in directional biasing of the 60Co photon emission while the second refers to the implementation of simplified source geometric models. The effect of the dose scoring volume dimensions in OF calculations accuracy was also explored. RDP calculations for the comprehensive PFX model were found to be in agreement with corresponding EBT2 measurements. Output factors of 0.819 ± 0.004 and 0.8941 ± 0.0013 were calculated for the 4 mm and 8 mm collimator, respectively, which agree, within uncertainties, with corresponding EBT2 measurements and published experimental data. Volume averaging was found to affect OF results by more than 0.3% for scoring volume radii greater than 0.5 mm and 1.4 mm for the 4 mm and 8 mm collimators, respectively. Directional biasing of photon emission resulted in a time efficiency gain factor of up to 210 with respect to the isotropic photon emission. Although no considerable effect on relative dose profiles was detected, directional biasing led to OF overestimations which were more pronounced for the 4 mm collimator and increased with decreasing emission cone half-angle, reaching up to 6% for a 5° angle. Implementation of simplified source models revealed that omitting the sources’ stainless steel capsule significantly affects both OF results and relative dose profiles, while the aluminum-based bushing did not exhibit considerable dosimetric effect. In conclusion, the results of this work suggest that any PFX simulation model should be benchmarked in terms of both RDP and OF results.

LINAC Radiosurgery in the Management of Parasagittal Meningiomas

Background: At present, there is no general agreement for the best approach to parasagittal meningiomas. Invasion of the superior sagittal sinus is frequent and responsible for relatively high recurrence rates following conventional microsurgery. Radiosurgery has the potential to treat less accessible portions of these tumors, and its application in this pathology is increasing either as a primary or a complementary therapeutic tool. Objective: To evaluate our results with LINAC radiosurgery for the treatment of parasagittal meningiomas. Methods: The patient cohort consisted of 74 patients treated for parasagittal meningioma by LINAC radiosurgery at our institution's Radiosurgery Unit during a 15-year period. Women accounted for 61% of patients. Thirteen patients (18%) underwent radiosurgery as the primary treatment for their meningioma. Results: The overall actuarial control rate was 90.6% at a mean follow-up of 49 months. In 17 patients (22.9%), there was no volumetric change. Fifty patients (67.5%) showed tumor shrinkage ranging from 15 to 80% of the original mass. In 7 patients, tumor recurrence was observed at an average time of 42.2 months after radiosurgery. All the patients with previously untreated tumors were controlled. Symptomatic transient peritumoral edema developed in 5 patients (6.7%) at a mean of 6.4 months after radiosurgery. Three patients complained of protracted headaches after treatment. Conclusions: LINAC radiosurgery was highly effective for the treatment of parasagittal meningiomas in this series. For small to medium-sized meningiomas with clear invasion of the sinusal lumen, radiosurgery is a reasonable option as a first-line treatment. Either alone or combined with conventional surgery, radiosurgery may improve the control rate for parasagittal meningiomas. © 2014 S. Karger AG, Basel

P05.05 * RADIOSURGERY IN THE MANAGEMENT OF PEDIATRIC BRAIN TUMORS

BACKGROUND: Brain tumors are the most frequent solid tumors in the pediatric age. Their treatment is usually neurosurgical, followed by chemo and/or radiation therapy. Radiosurgery allows the administration of a high radiation dose to a well defined target in 1 to 5 fractions, with minimal dose to the surrounding tissues. This fact would make it especially indicated for certain situations of children with these tumors. There are scarce publications, all retrospective studies, with specific attention to this treatment in the pediatric population. METHOD: The aim of this study has been to review our experience of the pediatric patients with brain tumors treated at our Radiosurgery Unit from 1993 to 2013. Data were obtained retrospectively from a prospectively maintained database. We have reviewed 94 patients with central nervous system tumors below the age of 16 (range 8m- 15y, median 12y) with a complete follow-up in 71.7%, during a mean of 56.2 months. RESULTS: The treated tumors have a miscellanea of different diagnosis, the most frequent being 26 low grade gliomas (LGG), 19 craniopharyngiomas (CFG), 8 ependymomas (EP), 6 medulloblastomas-PNET (M-PNET) and 6 patients with tumors related to neurofibromatosis. Among the 21 followed LGG, control was achieved in 90.5%. In CFG, and as part of the multimodal treatment of these tumors, visual, hormonal and solid tumor stability was obtained in 14 out of 18, although in 3 patients stereotactic evacuation of the cystic component was necessary. As EP and M-PNET had already received radiotherapy, radiosurgery was a good option as a boost or salvage treatment, with good local control, but with outfield relapses, that could be further treated when necessary. CONCLUSION: This advanced technique of focalized radiation therapy, with high precision and accuracy, provides dosimeric characteristics especially relevant in children that are more susceptible to irradiation. It produces promising results in selected patients.

Impact of Immobilization on Intrafraction Motion for Dedicated Cobalt Radiosurgery Unit Using Cone Beam Computed Tomography

Intracranial stereotactic radiosurgery (SRS) relies on highly accurate positioning and immobilization to ensure accurate dose delivery. This study aims to utilize cone-beam CT (CBCT) to measure geometric uncertainties associated with 2 immobilization systems used for SRS on a dedicated cobalt radiosurgery unit.

P06.12 * ROBOTIC STEREOTACTIC HYPOFRACTIONATED RADIOTHERAPY IN MENINGIOMAS

PURPOSE: Robotic stereotactic hypofractionated radiotherapy (SRT) become an important modality in the treatment of cranial meningiomas. In this study, we analysed the treatment parameters and outcomes of our cranial meningioma patients treated with SRT. MATERIALS AND METHODS: Between 2009 and 2013, 103 cranial meningioma lesions of 93 patients were treated with CyberKnife (Accuray®) Stereotactic Radiosurgery Unit. 42 patients (45%) received SRT as primary modality, and 51 patients ( 55 %) received postoperatively. Six patients have treated with external beam radiotherapy. Of the 103 cases, 69 of the lesions were located basally, 34 of them nonbasally. The most common symptoms in patients with basal tumours (38 patients, 58%) was detoriation in the visual field and the restriction of the eye movements and in patients with non-basal tumours (8 patients, 26%) was headache. In 33 patients (32%) tumours were located just adjacent to the optic apparatus or brain stem, 27 of them (26%) were located close (<1cm) to the optic apparatus or brain stem. SRT dose and fractionation were determined according to tumor localization. Optical apparatus and brainstem maximum point dose constraints for one fraction were 10 Gy and 14 Gy; 25 Gy and 30 Gy were for 5 fractions. RESULTS: The median age was 44 (16-81). The female to male ratio was 73/20 (3.6/1). Median follow-up was 10 months (1-37 months). The median dose was 2500cGy (1500 - 3000 cGy). The median number of fraction was 5 (1-5 fractions) fraction. GTV volume ranged between 0.66 cm3 and 80.6 cm3 (Median 12.4 cm3). Median conformity index was 1.4 (1.06 to 2.03) and the median homogeneity index was 1.25 (1.09 to 1.56). Among the patients with follow up, local control rate was 98%. Only one patient with parasagittal meningioma underwent decompressive surgery due to radiation induced edema. All other side effects were either grade I or II. The most common side effect was headache associated with brain edema in 10 patients (11%). CONCLUSION: Hypofractionated SRT is an effective treatment method in patients with cranial meningiomas with excellent rate of local control and minimal side effects.

P13.12 * LINAC RADIOSURGERY IN THE MANAGEMENT OF PARASAGITTAL MENINGIOMAS

BACKGROUND: At present, there is no general agreement for the best approach to parasagittal meningiomas. Invasion of the superior sagittal sinus is frequent and responsible for relatively high recurrence rates following conventional microsurgery. Radiosurgery has the potential to treat less accessible portions of these tumors and its application in this pathology is increasing both as a primary or a complementary therapeutic tool. OBJECTIVES: To evaluate our results with LINAC radiosurgery for treatment of parasagittal meningiomas METHODS: The patient cohort consisted of 74 patients treated for parasagittal meningioma by LINAC radiosurgery at the Sheba Medical Center Radiosurgery Unit during a 15-year period. Women accounted for 61% of patients. Thirteen patients,(18%) underwent radiosurgery as the primary treatment for their meningioma. RESULTS: The overall control rate was 90.6% at a mean follow up of 49 months. In 17 patients (22.9%) there was no volumetric change. Fifty patients (67.5%) showed tumor shrinkage ranging from 15-80% of the original mass. In 7 patients tumor recurrence was observed at an average time of 42.2 months after radiosurgery. All the patients with previously untreated tumors were controlled. Symptomatic transient peritumoral edema developed in 5 patients (6.7%) at a mean of 6.4 months after radiosurgery. Three patients complained of protracted headaches post treatment. CONCLUSIONS: LINAC radiosurgery was highly effective for the treatment of parasagittal meningiomas in this series. For small to medium sized meningiomas with clear invasion of the sinusal lumen, radiosurgery is a reasonable option as a first line treatment. Either alone or combined to conventional surgery, radiosurgery may improve control rate for parasagittal meningiomas.

EP-1470: The output stability of CyberKnife robotic radiosurgery unit: 5 years clinical experience

EP-1470: The output stability of CyberKnife robotic radiosurgery unit: 5 years clinical experience

Frameless linac-based stereotactic radiosurgery treatment for SUNCT syndrome targeting the trigeminal nerve and sphenopalatine ganglion

The short-lasting unilateral neuralgiform headache associated with conjunctival injection and tearing or SUNCT syndrome was first described in the 1970s. This paper is the first in the literature that describes the successful use of stereotactic radiosurgery (SRS) using a non-invasive frameless technique, targeting both the trigeminal nerve and the sphenopalatine ganglion in the management of intractable SUNCT. We also discuss the role of selecting peripheral targets in the management of this rare headache syndrome. Among patients treated for functional pain disorders in our radiosurgery unit using the frameless technique since August 2011, one patient with symptoms matching the International Classification of Headache Disorders-2 (ICHD-II) criteria of SUNCT syndrome was identified. The multi-disciplinary case records of this patient were retrospectively reviewed and reported. Our patient had symptoms resembling the ICHD-II diagnostic criteria of SUNCT, which was refractory to medical treatment. Ninety Gy was delivered to the trigeminal root entry zone and 80 Gy was delivered to the sphenopalatine ganglion. At 16 months' follow-up, she was pain free with minimal side effects. Frameless linear accelerator (linac)-based SRS targeting the trigeminal nerve and sphenopalatine ganglion remained successful in our patient at 16 months. Longer follow-up and further experience will determine the efficacy and safety of this approach. We suggest that frameless SRS is a convenient and attractive non-invasive option for patients with medically refractory SUNCT.

Technology resource planning in radiation oncology: application of a needs-based analytic framework to radiosurgery planning in Ontario.

With the emergence of radiosurgery as a new radiotherapeutic technique, health care decision makers are required to allocate capital radiotherapy resources to meet both current and future radiosurgery requirements. The goal of this article is to demonstrate the feasibility of applying an explicit, needs-based model to resource planning in radiation oncology. Using an analytic model that relates radiosurgery need to population size, epidemiology, level of service planned, and productivity, the current radiosurgical need for single brain metastases in Ontario was estimated. The model was populated using Ontario-specific data where possible and supplemented with information from the published literature. Multiway sensitivity analyses were performed to calculate the minimum and maximum technology requirements. The calculated number of full-time radiosurgical units required to treat patients with single brain metastases in Ontario was 5.9. Sensitivity analyses performed varying both level of service planned and productivity yielded a range of requirements from 2.5 to 12.2 full-time radiosurgery units. We have shown through the example of single brain metastases in Ontario that it is feasible to perform explicit, needs-based resource planning in radiation oncology. As the availability of new specialized technology increases, health care decision makers may use this approach to ensure the needs of their population are met while maximizing productivity and minimizing opportunity cost.

Cone Beam Computed Tomography Image Guidance System for a Dedicated Intracranial Radiosurgery Treatment Unit

Image guidance has improved the precision of fractionated radiation treatment delivery on linear accelerators. Precise radiation delivery is particularly critical when high doses are delivered to complex shapes with steep dose gradients near critical structures, as is the case for intracranial radiosurgery. To reduce potential geometric uncertainties, a cone beam computed tomography (CT) image guidance system was developed in-house to generate high-resolution images of the head at the time of treatment, using a dedicated radiosurgery unit. The performance and initial clinical use of this imaging system are described. A kilovoltage cone beam CT system was integrated with a Leksell Gamma Knife Perfexion radiosurgery unit. The X-ray tube and flat-panel detector are mounted on a translational arm, which is parked above the treatment unit when not in use. Upon descent, a rotational axis provides 210° of rotation for cone beam CT scans. Mechanical integrity of the system was evaluated over a 6-month period. Subsequent clinical commissioning included end-to-end testing of targeting performance and subjective image quality performance in phantoms. The system has been used to image 2 patients, 1 of whom received single-fraction radiosurgery and 1 who received 3 fractions, using a relocatable head frame. Images of phantoms demonstrated soft tissue contrast visibility and submillimeter spatial resolution. A contrast difference of 35 HU was easily detected at a calibration dose of 1.2 cGy (center of head phantom). The shape of the mechanical flex vs scan angle was highly reproducible and exhibited <0.2 mm peak-to-peak variation. With a 0.5-mm voxel pitch, the maximum targeting error was 0.4 mm. Images of 2 patients were analyzed offline and submillimeter agreement was confirmed with conventional frame. A cone beam CT image guidance system was successfully adapted to a radiosurgery unit. The system is capable of producing high-resolution images of bone and soft tissue. The system is in clinical use and provides excellent image guidance without invasive frames.

Model‐based calculations of off‐axis ratio of conic beams for a dedicated 6 MV radiosurgery unit

Because the small-radius photon beams shaped by cones in stereotactic radiosurgery (SRS) lack lateral electronic equilibrium and a detector's finite cross section, direct experimental measurement of dosimetric data for these beams can be subject to large uncertainties. As the dose calculation accuracy of a treatment planning system largely depends on how well the dosimetric data are measured during the machine's commissioning, there is a critical need for an independent method to validate measured results. Therefore, the authors studied the model-based calculation as an approach to validate measured off-axis ratios (OARs). The authors previously used a two-component analytical model to calculate central axis dose and associated dosimetric data (e.g., scatter factors and tissue-maximum ratio) in a water phantom and found excellent agreement between the calculated and the measured central axis doses for small 6 MV SRS conic beams. The model was based on that of Nizin and Mooij ["An approximation of central-axis absorbed dose in narrow photon beams," Med. Phys. 24, 1775-1780 (1997)] but was extended to account for apparent attenuation, spectral differences between broad and narrow beams, and the need for stricter scatter dose calculations for clinical beams. In this study, the authors applied Clarkson integration to this model to calculate OARs for conic beams. OARs were calculated for selected cones with radii from 0.2 to 1.0 cm. To allow comparisons, the authors also directly measured OARs using stereotactic diode (SFD), microchamber, and film dosimetry techniques. The calculated results were machine-specific and independent of direct measurement data for these beams. For these conic beams, the calculated OARs were in excellent agreement with the data measured using an SFD. The discrepancies in radii and in 80%-20% penumbra were within 0.01 cm, respectively. Using SFD-measured OARs as the reference data, the authors found that the calculated OARs were more accurate than those measured with a microchamber or film dosimetry. The model produces sufficiently accurate conic beam dosimetric data that can be used to validate direct measurement results for such beams.

Traitement des métastases cérébrales par radiochirurgie stéréotaxique : étude de 33 cas liés à un accident de « surexposition »

The consequences of a dosimetric radiosurgery accident are not the same as a conventional radiotherapy accident. The objective of this study was to estimate the clinical and radiological outcome of patients treated by radiosurgery for metastasis during the period of the overexposure accident that occurred in the Toulouse Radiosurgery Unit. Between April 2006 and March 2007, 33 patients with 57 metastases were treated in the Toulouse Radiosurgery Unit (Novalis ®, BrainLab). An initial error in the estimation of the scatter factors led to an overexposure to radiation. The median age was 55 years [range, 35–85]. Twenty-one patients (64%) harbored a single metastasis. The primary tumor location was lung (16 cases), kidney (nine cases), breast (four cases), and others (four cases). The mean tumoral volume was 3.2 cm 3 [0.04–14.07]. The mean prescribed dose at the isocenter was 20 Gy [range, 10–23], the mean delivered dose was 31.5 Gy [range, 13–52], and the mean overdose was 61.2% [range, 5.6–226.8]. In order to evaluate the consequences of the overdose, three parameters were analyzed: a risk index using dose and volume, the volume of parenchyma that received more than 12 Gy, and the mean dose in a sphere of 20 cm 3 surrounding the target volume. Median actuarial survival was 14.1 months, the survival rate was 79.4 % at six months, 59.1% at 12 months, and 27.2% at 24 months. The rate of tumor control was 80.7%. No morbidity was observed. There was no correlation between death and the parameters studied. The survival rates and times observed in our study of the patients treated for brain metastases by radiosurgery and overexposed were among the good results of the international literature. Deaths were not related to the overdose and no side effect was noted. This dosimetric accident has not had worse consequences in this population.

WE-C-BRA-05: Shortening Treatment Time in Robotic Radiosurgery by Novel Node Reduction Techniques

Purpose: The CyberKnife robotic radiosurgery unit uses small circular beams that can be targeted from 117 node positions to obtain highly conformal dose distributions. Typically a large number of node positions, beams and monitor units (MUs) are used, resulting in long treatment times per fraction. The aim of this study was to develop and evaluate node reduction techniques to reduce the long treatment times for robotic radiosurgery. Method and Materials: Two node reduction techniques were implemented in our inverse planning algorithm developed in-house. This algorithm uses ‘resampling’: repeated inverse optimization, each time replacing candidate beams with zero weight by new candidate beams. In the first reduction technique (passive reduction), node positions that were not ascribed any dose after inverse optimization were excluded from successive resampling iterations. This process was repeated until the number of node positions remained constant. In the second method (active reduction), node positions were reduced by forced exclusion of node positions with the lowest contribution until a valid solution could no longer be found. Both techniques were evaluated for two lung cases of different complexity, one prostate and one oropharynx case. Obtained plans were compared with plans without node reduction by the number of node positions, beams and MUs. All plans fulfilled the clinical dose constraints. Results: Compared to no node reduction, both passive and active node reduction resulted in a lower number of node positions (passive:−23%, active:−68%) and MUs (passive:−3%, active:−3%). The number of beams increased during node reduction (passive:+3%, active:+13%). Increasing complexity lowered the reduction in node positions. Conclusion: Results showed a dramatic reduction in the number of node positions while still fulfilling clinical dose constraints. The most efficient treatment plans were obtained by active node reduction. This technique could be used to shorten the treatment time per fraction.

A Variable Circular Collimator: A Time-efficient Alternative for a Multi-leaf Collimator in Robotic Radiosurgery?

The CyberKnife robotic radiosurgery unit uses small, non-isocentric cone beams for dose delivery. Clinical treatment plans are designed with one or two fixed collimators, requiring the tumor to be painted with many beams to obtain a highly conformal dose distribution. The large number of beams and also Monitor Units (MUs) leads to prolonged fraction duration. A Beam's Eye-View (BEV) shaping device, such as a mini Multi-Leaf Collimator (mMLC), would potentially lead to more time-efficient plans requiring less beam directions and MUs, but is presently not available for this system.

Gamma Knife output factor measurements using VIP polymer gel dosimetry

Water equivalent polymer gel dosimeters and magnetic resonance imaging were employed to measure the output factors of the two smallest treatment fields available in a Gamma Knife model C radiosurgery unit, those formed employing the 4 and 8 mm final collimator helmets. Three samples of the VIP normoxic gel formulation were prepared and irradiated so that a single shot of the field whose output factor is to be measured and a single shot of the reference 18 mm field were delivered in each one. Emphasis is given to the development and benchmarking of a refined data processing methodology of reduced uncertainty that fully exploits the 3D dose distributions registered in the dosimeters. Polymer gel results for the output factor of the 8 mm collimator helmet are found to be in close agreement with the corresponding value recommended by the vendor (0.955 +/- 0.007 versus 0.956, respectively). For the 4 mm collimator helmet, however, polymer gel results suggest an output factor 3% lower than the value recommended by the vendor (0.841 +/- 0.009 versus 0.870, respectively). A comparison with corresponding measurements published in the literature indicates that output factor results of this work are in agreement with those obtained using dosimetric systems which, besides fine spatial resolution and lack of angular and dose rate dependence of the dosimeter's response, share with polymer gels the favorable characteristic of minimal radiation field perturbation.

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