Iris Collimator Research Articles

A pencil beam dose calculation model for CyberKnife system.

CyberKnife system is initially equipped with fixed circular cones for stereotactic radiosurgery. Two dose calculation algorithms, Ray-Tracing and Monte Carlo, are available in the supplied treatment planning system. A multileaf collimator system was recently introduced in the latest generation of system, capable of arbitrarily shaped treatment field. The purpose of this study is to develop a model based dose calculation algorithm to better handle the lateral scatter in an irregularly shaped small field for the CyberKnife system. A pencil beam dose calculation algorithm widely used in linac based treatment planning system was modified. The kernel parameters and intensity profile were systematically determined by fitting to the commissioning data. The model was tuned using only a subset of measured data (4 out of 12 cones) and applied to all fixed circular cones for evaluation. The root mean square (RMS) of the difference between the measured and calculated tissue-phantom-ratios (TPRs) and off-center-ratio (OCR) was compared. Three cone size correction techniques were developed to better fit the OCRs at the penumbra region, which are further evaluated by the output factors (OFs). The pencil beam model was further validated against measurement data on the variable dodecagon-shaped Iris collimators and a half-beam blocked field. Comparison with Ray-Tracing and Monte Carlo methods was also performed on a lung SBRT case. The RMS between the measured and calculated TPRs is 0.7% averaged for all cones, with the descending region at 0.5%. The RMSs of OCR at infield and outfield regions are both at 0.5%. The distance to agreement (DTA) at the OCR penumbra region is 0.2 mm. All three cone size correction models achieve the same improvement in OCR agreement, with the effective source shift model (SSM) preferred, due to their ability to predict more accurately the OF variations with the source to axis distance (SAD). In noncircular field validation, the pencil beam calculated results agreed well with the film measurement of both Iris collimators and the half-beam blocked field, fared much better than the Ray-Tracing calculation. The authors have developed a pencil beam dose calculation model for the CyberKnife system. The dose calculation accuracy is better than the standard linac based system because the model parameters were specifically tuned to the CyberKnife system and geometry correction factors. The model handles better the lateral scatter and has the potential to be used for the irregularly shaped fields. Comprehensive validations on MLC equipped system are necessary for its clinical implementation. It is reasonably fast enough to be used during plan optimization.

Dosimetric comparison between cone/Iris‐based and InCise MLC‐based CyberKnife plans for single and multiple brain metastases

We performed an evaluation of the CyberKnife InCise MLC by comparing plan qualities for single and multiple brain lesions generated using the first version of InCise MLC, fixed cone, and Iris collimators. We also investigated differences in delivery efficiency among the three collimators. Twenty‐four patients with single or multiple brain mets treated previously in our clinic on a CyberKnife M6 using cone/Iris collimators were selected for this study. Treatment plans were generated for all lesions using the InCise MLC. Number of monitor units, delivery time, target coverage, conformity index, and dose falloff were compared between MLC‐ and clinical cone/Iris‐based plans. Statistical analysis was performed using the nonparametric Wilcoxon‐Mann‐Whitney signed‐rank test. The planning accuracy of the MLC‐based plans was validated using chamber and film measurements. The InCise MLC‐based plans achieved mean dose and target coverage comparable to the cone/Iris‐based plans. Although the conformity indices of the MLC‐based plans were slightly higher than those of the cone/Iris‐based plans, beam delivery time for the MLC‐based plans was shorter by 30%∼40%. For smaller targets or cases with OARs located close to or abutting target volumes, MLC‐based plans provided inferior dose conformity compared to cone/Iris‐based plans. The QA results of MLC‐based plans were within 5% absolute dose difference with over 90% gamma passing rate using 2%/2 mm gamma criteria. The first version of InCise MLC could be a useful delivery modality, especially for clinical situations for which delivery time is a limiting factor or for multitarget cases.PACS number(s): 87.53.Ly, 87.55.D‐

Clinical commissioning of the INCISE™ multileaf collimator for cyberknife M6™ system

Introduction The National Center for Cancer Care and Research (NCCCR) at Hamad Medical Corporation in Qatar is amongst the first institutions in Asia and Europe to have the CyberKnife® M6 FIM system (Accuray, Inc., Sunnyvale, CA, USA) including Fixed Collimators, Iris Variable Aperture Collimator and InCise™ Multileaf Collimator. Purpose To report our commissioning experiences of the CyberKnife M6 InCise™ Multileaf Collimator system. Materials and methods Accuray guidelines were used to conduct measurements. A detailed description of the equipment used, the mechanical alignment, the verification procedures, and the alignment of the detector motion with the radiation beam is described. Incise MLC testing was conducted as per Accuray procedure. Further MLC tests were developed to verify its feasibility under certain clinical settings. Results Our results were within the manufacturer specifications. Results for both the Sun Nuclear Edge detector and PTW SRS diode 60018 were found to be comparable. Measurement conditions specified by Accuray were sufficient; however the use of a reference chamber introduced additional noise for the continuous profile acquisitions. As compared to fixed and IRIS collimators, the incise MLC produced acceptable results with an average of 20% decrease in treatment delivery time. Conclusions Our results are amongst the first set of data reported on the InCise™ MLC and were within the manufacturer specifications. These results will be used to develop a benchmark reference for periodic Quality Control (QC). Disclosure Authors have nothing to disclose.

Stereotactic Arrhythmia Radioablation (STAR) of Ventricular Tachycardia: A Treatment Planning Study.

PurposeThe first stereotactic arrhythmia radioablation (STAR) of ventricular tachycardia (VT) was delivered at Stanford on a robotic radiosurgery system (CyberKnife® G4) in 2012. The results warranted further investigation of this treatment. Here we compare dosimetrically three possible treatment delivery platforms for STAR.MethodsThe anatomy and target volume of the first treated patient were used for this study. A dose of 25 Gy in one fraction was prescribed to the planning target volume (PTV). Treatment plans were created on three treatment platforms: CyberKnife® G4 system with Iris collimator (Multiplan, V. 4.6)(Plan #1), CyberKnife® M6 system with InCise 2TM multileaf collimator (Multiplan V. 5.3)(Plan #2) and Varian TrueBeamTM STx with HD 120TM MLC and 10MV flattening filter free (FFF) beam (Eclipse planning system, V.11) (Plan #3 coplanar and #4 noncoplanar VMAT plans). The four plans were compared by prescription isodose line, plan conformity index, dose gradient, as well as dose to the nearby critical structures. To assess the delivery efficiency, planned monitor units (MU) and estimated treatment time were evaluated.ResultsPlans #1-4 delivered 25 Gy to the PTV to the 75.0%, 83.0%, 84.3%, and 84.9% isodose lines and with conformity indices of 1.19, 1.16, 1.05, and 1.05, respectively. The dose gradients for plans #1-4 were 3.62, 3.42, 3.93, and 3.73 with the CyberKnife® MLC plan (Plan #2) the best, and the TrueBeamTM STx co-planar plan (Plan #3) the worst. The dose to nearby critical structures (lung, stomach, bowel, and esophagus) were all well within tolerance. The MUs for plans #1-4 were 27671, 16522, 6275, and 6004 for an estimated total-treatment-time/beam-delivery-time of 99/69, 65/35, 37/7, and 56/6 minutes, respectively, under the assumption of 30 minutes pretreatment setup time. For VMAT gated delivery, a 40% duty cycle, 2400MU/minute dose rate, and an extra 10 minutes per extra arc were assumed.ConclusionClinically acceptable plans were created with all three platforms. Plans with MLC were considerably more efficient in MU. CyberKnife® M6 with InCise 2TM collimator provided the most conformal plan (steepest dose drop-off) with significantly reduced MU and treatment time. VMAT plans were most efficient in MU and delivery time. Fluoroscopic image guidance removes the need for additional fiducial marker placement; however, benefits may be moderated by worse dose gradient and more operator-dependent motion management by gated delivery.

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

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

TH‐A‐BRC‐03: AAPM TG218: Measurement Methods and Tolerance Levels for Patient‐Specific IMRT Verification QA

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

TH‐A‐BRC‐02: AAPM TG‐178 Gamma Stereotactic Radiosurgery Dosimetry and Quality Assurance

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

SU-F-T-576: Characterization of Two Dimensional Liquid Filled Detector Array(SRS 1000) in High Precision Cyberknife Robotic Radiosurgery System

Purpose: The aim of the study is to characterize a two dimensional liquid filled detector array SRS 1000 for routine QA in Cyberknife Robotic Radiosurgery system. Methods: SRS 1000 consists of 977 liquid filled ionization chambers and is designed to be used in small field SRS/SBRT techniques. The detector array has got two different spacial resolutions. Till field size of 5.5×5.5 cm the spacial resolution is 2.5mm (center to center) and after that till field size of 11 × 11 cm the spacial resolution is 5mm. The size of the detector is 2.3 × 2.3 0.5 mm with a volume of .003 cc. The CyberKnife Robotic Radiosurgery System is a frameless stereotactic radiosurgery system in which a LINAC is mounted on a robotic manipulator to deliver beams with a high sub millimeter accuracy. The SRS 1000's MU linearity, stability, reproducibility in Cyberknife Robotic Radiosurgery system was measured and investigated. The output factors for fixed and IRIS collimators for all available collimators (5mm till 60 mm) was measured and compared with the measurement done with PTW pin-point ionization chamber. Results: The MU linearity was measured from 2 MU till 1000 MU for doserates in the range of 700cGy/min – 780 cGy/min and compared with the measurement done with pin point chamber The MU linearity was with in 3%. The detector arrays stability and reproducibility was excellent and was withinin 0.5% The measured output factors showed an agreement of better than 2% when compared with the measurements with pinpoint chamber for both fixed and IRIS collimators with all available field sizes. Conclusion: We have characterised PTW 1000 SRS as a precise and accurate measurement tool for routine QA of Cyberknife Robotic radiosurgery system.

TH‐A‐BRC‐01: AAPM TG‐135U1 QA for Robotic Radiosurgery

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

Dosimetric and radiobiological comparison of CyberKnife M6™ InCise multileaf collimator over IRIS™ variable collimator in prostate stereotactic body radiation therapy.

The impetus behind our study was to establish a quantitative comparison between the IRIS collimator and the InCise multileaf collimator (MLC) (Accuray Inc. Synnyvale, CA) for prostate stereotactic body radiation therapy (SBRT). Treatment plans for ten prostate cancer patients were performed on MultiPlan™ 5.1.2 treatment planning system utilizing MLC and IRIS for 36.25 Gy in five fractions. To reduce the magnitude of variations between cases, the planning tumor volume (PTV) was defined and outlined for treating prostate gland only, assuming no seminal vesicle or ex-capsule involvement. Evaluation indices of each plan include PTV coverage, conformity index (CI), Paddick's new CI, homogeneity index, and gradient index. Organ at risk (OAR) dose sparing was analyzed by the bladder wall Dmax and V37Gy, rectum Dmax and V36Gy. The radiobiological response was evaluated by tumor control probability and normal tissue complication probability based on equivalent uniform dose. The dose delivery efficiency was evaluated on the basis of planned monitor units (MUs) and the reported treatment time per fraction. Statistical significance was tested using the Wilcoxon signed rank test. The studies indicated that CyberKnife M6™ IRIS and InCise™ MLC produce equivalent SBRT prostate treatment plans in terms of dosimetry, radiobiology, and OAR sparing, except that the MLC plans offer improvement of the dose fall-off gradient by 29% over IRIS. The main advantage of replacing the IRIS collimator with MLC is the improved efficiency, determined from the reduction of MUs by 42%, and a 36% faster delivery time.

Continuous versus step-by-step scanning mode of a novel 3D scanner for CyberKnife measurements

The purpose of the study is to investigate the continuous versus step-by-step scanning mode of a commercial circular 3D scanner for commissioning measurements of a robotic stereotactic radiosurgery system. The 3D scanner was used for profile measurements in step-by-step and continuous modes with the intent of comparing the two scanning modes for consistency. The profile measurements of in-plane, cross-plane, 15 degree, and 105 degree were performed for both fixed cones and Iris collimators at depth of maximum dose and at 10cm depth. For CyberKnife field size, penumbra, flatness and symmetry analysis, it was observed that the measurements with continuous mode, which can be up to 6 times faster than step-by-step mode, are comparable and produce scans nearly identical to step-by-step mode. When compared with centered step-by-step mode data, a fully processed continuous mode data gives rise to maximum of 0.50% and 0.60% symmetry and flatness difference respectfully for all the fixed cones and Iris collimators studied.

SU‐E‐T‐706: Small Field Dosimetry: Output Factors for Fixed Cones, Iris Collimator and MLC Fields for the CyberKnife M6™ System with Newly Released InCise™ Multileaf Collimator

Purpose:Accuray has recently released a new collimator, the InCise™ Multileaf Collimator (MLC), for clinical use with the CyberKnife M6™ System. This work reports the results of measurements of output factors (OF) for fields shaped by the InCise™ Multileaf collimator, Iris collimator and fixed cones.Methods:The MLC consists of 41 pairs of 2.5 mm wide leaves projecting a minimum and maximum field size of 7.6 mm × 7.5 mm and 110 mm × 97.5 mm at 800 mm SAD. OF measurements were made using 6 different detectors: PTW stereotactic and electron diodes, PTW microDiamond detector, Sun Nuclear Edge diode, IBA SFD diode and PFD 3G photon diode. Measurements were made for 14 MLC field sizes and 12 cone/Iris field sizes (5mm to 60 mm diameter field sizes). All measurements were made in a Wellhofer water tank at the depth of maximum dose. Correction factors from Francescon et al (PMB 59, 2014, N11‐N17; 17, 2012 3741–3748) were used to correct the diode responses for the fixed cone and Iris measurements.Results:For the MLC fields no correction factors are currently available. For the cones and Iris collimator, the average of corrected OF for all diodes ranged from 0.651 ± 0.014 (5mm cone) to 0.994 ± 0.001 (50 mm cone); for the Iris collimator the corresponding values are 0.510 ±0.004 and 0.997 ±0.000 respectively. The OF for the MLC field sizes ranged from 0.806 ±0.009 for the 7.6 mm × 7.5 mm field to 1.023± 0.003 for the 110 mm × 97.5 mm field size.Conclusion:The present results are the first set of output factor data reported for the InCise™ MLC, Iris collimator and the fixed cones for the CyberKnife M6™ system. Good agreement is obtained among all the corrected data for the cones and Iris data.

SU‐E‐T‐40: A Method for Improving Dose Gradient for Robotic Radiosurgery

Purpose:For targets with substantial volume, collimators of relatively large sizes are usually selected to minimize the treatment time in robotic radiosurgery. Their large penumbrae may adversely affect the dose gradient around the target. In this study, we implement and evaluate an inner‐shell planning method to increase the dose gradient.Methods:Ten patients previously treated with CyberKnife M6 system were randomly selected with the only criteria that PTV be larger than 2cm3. A new plan was generated for each patient, in which the PTV was split into two regions: an inner shell and a core, where the core was created by shrinking the PTV by 5mm using 3D erosion, and the shell was obtained by subtracting the core from the PTV; then a 7.5mm Iris collimator was exclusively applied to the shell, with other appropriate collimators applied to the core depending on its size. The optimization objective functions and constraints were kept the same as the corresponding clinical plans. The results were analyzed for V12Gy, V9Gy, V5Gy, and gradient index (GI).Results:Volume reduction is found for the inner‐shell method at all studied dose levels as compared to the clinical plans. The absolute dose volume reduction ranged from 0.05cm3 to 18.5cm3 with a mean of 5.6cm3 for 12Gy, from 0.2cm3 to 38.1cm3 with a mean of 9.8cm3 for 9Gy, and from 1.5cm3 to 115.7cm3 with a mean of 24.8cm3 for 5Gy, respectively. The relative GI reduction ranged from 3.2% to 23.6%, with a mean of 12.6%. Paired t‐test for GI has a p‐value of 0.0014. The range for treatment time increase is from −3 min to 20 min, with a mean of 7.0 min.Conclusion:The inner‐shell planning method can significantly increase the dose gradient outside the PTV, while maintaining good coverage, conformity, and reasonable treatment time.

SU‐E‐T‐404: Evaluation of the Effect of Spine Hardware for CyberKnife Spinal Stereotactic Radiosurgery

Purpose:Spine hardware made of high‐Z materials such as titanium has the potential to affect the dose distribution around the metal rods in CyberKnife spinal stereotactic radiosurgery (SRS) treatments. The purpose of this work was to evaluate the magnitude of such effect retrospectively for clinical CyberKnife plans.Methods:The dose calculation was performed within the MultiPlan treatment planning system using the ray tracing (RT) and Monte Carlo (MC) method. A custom density model was created by extending the CT‐to‐Density table to titanium density of 4.5 g/cm3 with the CT number of 4095. To understand the dose perturbation caused by the titanium rod, a simple beam setup (7.5 mm IRIS collimator) was used to irradiate a mimic rod (5 mm) with overridden high density. Five patient spinal SRS cases were found chronologically from 2010 to 2015 in our institution. For each case, the hardware was contoured manually. The original plan was re‐calculated using both RT and MC methods with and without rod density override without changing clinical beam parameters.Results:The simple beam irradiation shows that there is 10% dose increase at the interface because of electron backscattering and 7% decrease behind the rod because of photon attenuation. For actual clinical plans, the iso‐dose lines and DVHs are almost identical (<2%) for calculations with and without density override for both RT and MC methods. However, there is a difference of more than 10% for D90 between RT and MC method.Conclusion:Although the dose perturbation around the metal rods can be as large as 10% for a single beam irradiation, for clinical treatments with complex beam composition the effect of spinal hardware to the PTV and spinal dose is minimal. As such, the MC dose algorithm without rod density override for CyberKnife spinal SRS is acceptable.

SU‐E‐T‐163: Characterization of a Novel High Resolution 1D Silicon Monolithic Array for Small Field Commissioning and Quality Assurance

Purpose:To study the suitability of a novel 1D silicon monolithic array for dosimetry of small radiation fields and for QA of high dose gradient treatment modalities (IMRT and SBRT).Methods:A 1D array composed of 4 monolithic silicon modules of 64 mm length and 1 mm pixel pitch was developed by IBA Dosimetry. Measurements were carried out for 6MV and 15MV photons on two commercial different linacs (TrueBeam and Clinac iX, Varian Medical Systems, Palo Alto, CA) and for a CyberKnife G4 (Accuray Inc., Sunnyvale, CA). The 1D array was used to measure output factors (OF), profiles and off axis correction factors (OACF) for the Iris CyberKnife variable collimator (5–60 mm). In addition, dose profiles (at the isocenter plane) were measured for multiple IMRT and SBRT treatment plans and compared with those obtained using EDR2radiographic film (Carestream Health, Rochester NY), a commercial 2D diode array and with the dose distribution calculated using a commercial TPS (Eclipse, Varian Medical Systems, Palo Alto, CA).Results:Due to the small pixel pitch of the detector, IMRT and SBRT plan profiles deviate from film measurements by less than 2%. Similarly, the 1D array exhibits better performance than the 2D diode array due to the larger (7 mm) pitch of that device. Iris collimator OFs measured using the 1D silicon array are in good agreement with the commissioning values obtained using a commercial stereotactic diode as well as with published data. Maximum deviations are < 3% for the smallest field (5 and 7.5mm) and below 1% for all other dimensions.Conclusion:We have demonstrated good performances of the array for commissioning of small photon fields and in patient QA, compared with diodes and film typically used in these clinical applications. The technology compares favorably with existing commercial solutionsThe presenting author is founded by a Marie Curie Early Initial Training Network Fellowship of the European Communitys Seventh Framework Programme under contract number (PITN‐GA‐2011‐289198‐ARDENT).The research activity is hosted by IBA Dosimetry, Gmbh.

IRIS형 방사선검출기 콜리메이터 제작 및 MCNP 코드를 이용한 성능평가

When a radiation detector is applied to the measurement of the radioactivity of high-level of radioactive materials or the rapid response to the nuclear accident, several collimators with the different inner radii should be prepared according to the level of dose rate. This makes the in-situ measurement impractical, because of the heavy weight of the collimator. In this study, an IRIS collimator was developed so as to have a function of controlling the inner radius, with the same method used in optical camera, to vary the attenuation ratio of radiation. The shutter was made to have the double tungsten layers with different phase angles to prevent the radiation from penetrating owing to the mechanical tolerance. The performance evaluation through the MCNP code was conducted by calculating the attenuation ratio according to the inner radius of the collimator. The attenuation ratio was marked on the outer scale ring of the collimator. It is expected that when a radiation detector with the IRIS collimator is used for the in-situ measurement, it can change the attenuation ratio of the incident photon to the detector without replacing the collimator.

SU‐E‐T‐453: A Novel Daily QA System for Robotic Image Guided Radiosurgery with Variable Aperture Collimator

Purpose:A novel end‐to‐end system using a CCD camera and a scintillator based phantom that is capable of measuring the beam‐by‐beam delivery accuracy of Robotic Radiosurgery has been developed and reported in our previous work. This work investigates its application to end‐to‐end type daily QA for Robotic Radiosurgery (Cyberknife) with Variable Aperture Collimator (Iris).Methods:The phantom was first scanned with a CT scanner at 0.625 slice thickness and exported to the Cyberknife Muliplan (v4.6) treatment planning system. An isocentric treatment plan was created consisting of ten beams of 25 Monitor Units each using Iris apertures of 7.5, 10, 15, 20, and 25 mm. The plan was delivered six times in two days on the Cyberknife G4 system with fiducial tracking on the four metal fiducials embedded in phantom with re‐positioning between the measurements. The beam vectors (X, Y, Z) are measured and compared with the plan from the machine delivery file (XML file). The Iris apertures (FWHM) were measured from the beam flux map and compared with the commissioning data.Results:The average beam positioning accuracies of the six deliveries are 0.71 ± 0.40 mm, 0.72 ± 0.44 mm, 0.74 ± 0.42 mm, 0.70 ± 0.40 mm, 0.79 ± 0.44 mm and 0.69 ± 0.41 mm respectively. Radiation beam width (FWHM) variations are within ±0.05 mm, and they agree with the commissioning data within 0.22 mm. The delivery time for the plan is about 7 minutes and the results are given instantly.Conclusion:The experimental results agree with stated sub‐millimeter delivery accuracy of Cyberknife system. Beam FWHM variations comply with the 0.2 mm accuracy of the Iris collimator at SAD. The XRV‐100 system has proven to be a powerful tool in performing end‐to‐end type tests for Robotic Image Guided Radiosurgery Daily QA.

Treatment and Technical Intervention Time Analysis of a Robotic Stereotactic Radiotherapy System

The purpose of this study is to obtain a better operational knowledge of Stereotactic Body Radiotherapy (SBRT) treatments with CyberKnife(r). An analysis of both In-room Times (IRT) and technical interventions of 5 years of treatments was performed, during which more than 1600 patients were treated for various indications, including liver (21%), lung (29%), intracranial (13%), head and neck (11%) and prostate (7%). Technical interventions were recorded along with the time of the failure, time to the intervention, and the complexity and duration of the repair. Analyses of Time Between Failures (TBF) and Service Disrupting TBF(disr) were performed. Treatment time data and variability per indication and following different system upgrades were evaluated. Large variations of IRTs were found between indications, but also large variations for each indication. The combination of the time reduction Tool (using Iris(r)) and Improved Stop Handling was of major impact to shortening of treatment times. The first implementation of the Iris collimator alone did not lead to significantly shorter IRTs for us except during prostate treatments. This was mostly due to the addition at the same time of larger rotational compensation for prostate treatments (58 instead of 1.58). Significant differences of duration between the first fraction and following fractions of a treatment, representing the necessity of defining imaging parameters and explanation to patients, were found for liver (12 min) and lung treatments using Xsight(r) Spine (5 min). Liver and lung treatments represent the longest IRT's and involve the largest variability's in IRT. The malfunction rate of the system followed a Weibull distribution with the shape and scale parameters of 0.8 and 39.7. Mean TBF(disr) was 68 work hours. 60 to 80% of the service disrupting interventions were resolved within 30-60 min, 5% required external intervention and 30% occurred in the morning. The presented results can be applied in the evaluation of the required machine time in order to implement robotic radiosurgery for different indications. The analytical distributions of IRTs and technical interruptions can be used for simulations.

Improved Normal Tissue Sparing and Reduced Treatment Time Using Multileaf Collimator Robotic Radiosurgery Stereotactic Body Radiation Therapy (SBRT) Compared to Iris Collimator SBRT in Treatment of Localized Prostate Cancer

Improved Normal Tissue Sparing and Reduced Treatment Time Using Multileaf Collimator Robotic Radiosurgery Stereotactic Body Radiation Therapy (SBRT) Compared to Iris Collimator SBRT in Treatment of Localized Prostate Cancer

MO‐D‐105‐06: Dose‐Area Product as a Method for Small Field Geometric QA

Purpose: To investigate the use of dose‐area product (DAP) as a highresolution tool for QA of small field sizes as an alternative to more complex alternatives such as film or water tank scanning. Method: An 81 mm diameter PTW Bragg Peak parallel plate chamber was used in combination with an Iris variable circular aperture collimator fitted to a CyberKnife System and a vendor‐supplied alignment jig. The Iris collimator is designed to deliver field diameters between 5 mm and 60 mm, with a tolerance of ±0.2 mm. First the sensitivity of the chamber reading to influence parameters (SSD, angle misalignment, lateral displacement, output variation) was investigated. Next the dependency of DAP on field diameter was investigated. Finally the sensitivity of the method to detect small field size changes consistent with the device specification was evaluated. Comparisons were made with film and diode water scans using four CyberKnife systems and three chambers. Results: Expected variations in chamber position/orientation within the jig resulted in measurement changes <0.2%. Measurement variations due to LINAC output changes can be removed by normalising measurements to a fixed collimator reading obtained at the same time. The variation in chamber reading with field size followed a quadratic dependency over the 5mm ‐ 60mm range. The change in chamber reading associated with a 0.2 mm field diameter change ranged from 10% at 5 mm nominal to 0.5% at 60 mm, and was reasonably consistent across tests performed on four units. Measurements were more consistent than routine film tests, and were stable over two years. Conclusion: DAP provides a practical, efficient, robust, and sensitive method for routine QA of field aperture changes as small as ±0.2mm across the field size range 5 mm – 60 mm. The method may also have applications for QA of small fields formed by MLC.