Customer all authors are affiliated with Artemis

Customer Acceptance Test and Commissioning of two different algorithms for M6 FI+ CyberKnife SystemMageshraja K1, Jefy Ninan1a, Prasobh.

C2, Dr. S. C. Pande3a, Dr. Aditya Gupta3b Harpreet. K3c, Ajay Khatri3c1,2,3cMedical Physicist Department of Radiation Oncology,1aChief Medical Physicist Department of Radiation Oncology, 3aHead Department of Radiation Oncology, 3bHead Department of Neurosurgery, all authors are affiliated with Artemis Hospital,Sector-51,Gurgaon,Haryana,IndiaABSTRACTINTRODUCTION CyberKnife M6 FI+ very precise robotic stereotactic radio surgery system so it possible to deliver high radiation dose to the tumour and minimum dose to surrounding normal tissues. Multi Plan treatment planning used in the CyberKnife system.

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Multi Plan treatment planning station having two different algorithms, Ray tracing and Monte Carlo. Commissioning data should precise and accurate because these data used as reference and commission the treatment planning system. Our CyberKnife M6 FI+ system commission done according to the accuracy recommendation. MATERIALS AND METHODS clinical beam data measurement carried about using Radiation Field Analyzer (RFA), Diode E, Pinpoint chamber, Semiflex ionization chamber and Unidose E Eletrometer. FC-65 ionisation chamber used in absolute dose calibration.

The mechanical accuracy of the robot and image stability was verified using GAFchromic film (EBT3), E2E and Iris QA tool kits along with softwares. StereoPHAN used in the patient specific QA point dose measurement. RESULTS Tissue phantom ratio (TPR), Off center ratio (OCR), Output factor (OF), Percentage depth dose (PDD) and open beam profile all clinical beam data measured. Absolute dose calibration output done in the cyber knife system, E2E performed for two different moods static and motion. Iris aperture size measured for all the field sizes. The patient specific QA delivered for both algorithms. DISCUSSION Clinical beam data measurement within ±1% of composite data set, OF overall standard deviation of the fixed and Iris collimators are 0.

0026 and 0.0063. The absolute dose was calibrated to 1cGy per MU.E2E,Iris QA and Laser and radiation coincidence values are within the tolerance. Patient specific QA point dose measurement for Ray tracing and Monte carlo 3.5% and 4%. INTRODUCTIONA CyberKnife M6 FI+ robotic stereotactic radio surgery system (Accuray, Sunnyvale, CA), treatment unit in which miniature type linear accelerator has mounted on an industrial robot (1,3).

CyberKnife have sub-millimetre accuracy it can treat tumours anywhere in the body like brain, spine, liver, prostate, lung with the help of frameless real-time image guidance technology and computer-controlled robotic(1,2).Due to the high degree of accuracy and precision, it capable to deliver very high radiation dose to the target with minimal dose to the nearest critical organs and surrounding normal tissues (6).The CyberKnife linear accelerator (LINAC) single energy 6 MV with flatting filter free (FFF) at constant dose rate 1000 MU/min, 9.3 GHz X Band (2).The compact light weight LINAC head attached to the end of a robotic arm that is producing the non-isocentric beam angles during treatment with help of robotic 6 degrees of freedom. There are two different type of secondary collimator Systems, Fixed and Iris with 12 different aperture sizes 5, 7.5, 10, 12.5, 15, 20, 25, 30, 35, 40, 50 and 60mm (2).

Fixed collimators are static aperture in size and Iris collimator aperture is adjustable under computer control. It contains 2 stacked hexagonal banks of tungsten segments that together produce a 12 sided aperture (a regular dodecagon)(2,7). Commissioning beam data are very important to get good treatment outcome. Because these measured data consider as a reference and simultaneously used in the Multi plan treatment planning system. The quantities required to measure for commissioning and quality assurance purposes of a cyber system include absolute dose calculation, using the IAEA TRS-398 protocol (5). According to accuracy commissioning recommendation, Mechanical accuracy of the robot and image stability and patient specific QA (point dose measurement),Clinical dosimetry measurements such as tissue-phantom ratios (TPRs), off center ratio (OCRs), secondary collimator output factors (OFs), percentage depth dose (PDDs), open beam profile presented here(2,10). All the measurement results compared with accuracy composite beam data set and tolerance values.MATERIALS AND METHODSAll the clinical beam data (Ray-Tracing and Monte Carlo) acquired usingRadiation Field Analyzer a computer controlled measuring system (SCANLIFT MP3-Therapy beam analyzer: PTW, Freiburg, Germany),TM60017 Diode E (PTW)TM31014 Pinpoint chamber (PTW)TM31010 Semiflex ionization chamber (PTW) andUnidose E Eletrometer (PTW)The absolute dose calibration of the accelerator output was accomplished usingTM30013 Farmer chamber (PTW) andUnidose E EletrometerThe mechanical accuracy of the robot and image stability was verified usingGAFchromic film (EBT3, Ashland Speciality Ingredients, Bridgewater, NJ- 08807)Film Ball Cube II (EBT2,H.

A.Y.E.S. Manufacturing Services, Sunnyvale, CA-95054)Film Mini Ball Cube II (EBT2)Kit XLT Phantom film (EBT2)EPSON perfection V800 photo scannerHead and neck phantom (Accuray,Sunnyvale,CA-94089)Synchrony QA tool (Accuray)CIRS-Xsight lung tracking phantom kit & 4D planning phantom (CIRS Tissue Simulation & Phantom Technology, Norfolk, USA))Imagej softwareIris QA software (Accuray) andE2E software (Accuray) The patient specific QA (point dose measurement) was verified usingStereoPHANTM (Sun Nuclear, 3275 Sun tree Blvd, Melbourne, FL 32940) andTM31014 Pinpoint chamber Unidose E EletrometerRESULTSClinical Beam DataClinical beam data measurements are done for the commissioning of two different algorithms, Ray-tracing and Monte Carlo (2). Ray-tracing algorithm need TPR, OCR and OF measurements (10). Monte Carlo algorithm PDD and Open field profiles in addition to the Ray-tracing algorithm (2).A.

1.TPR MeasurementThe Tissue phantom ratio (TPR) is the ratio of absorbed dose at a given point to the dose to the dose at a fixed reference depth using constant SAD (4). The reference depth for the cyber knife system is 15mm (Dmax) for all collimator sizes and SAD is 800 mm (2). The TPR measurement was carried out using Diode E position in the RFA water phantom.TPR measurement taken in different depth 0, 3, 5, 8, 10, 13, 20, 30, 50, 100, 150, 200, 250 and 300mm for all the field sizes for both collimators at constant SAD 800mm (Table 1 and 2). We use a cubic spline fit to generate a TPR curve depths from 0 to 300mm and normalize the values for each collimator to the depth of 15mm as shown in the Figure 1 and 2.

Depth (mm) Collimators (mm)5 7.5 10 12.5 15 20 25 30 35 40 50 600 0.566 0.51 0.

487 0.474 0.463 0.46 0.459 0.46 0.462 0.

462 0.469 0.4833 0.824 0.773 0.717 0.706 0.7 0.

678 0.678 0.683 0.

681 0.693 0.692 0.6955 0.94 0.897 0.

856 0.843 0.835 0.818 0.813 0.815 0.

811 0.816 0.817 0.828 1.005 0.

981 0.956 0.948 0.937 0.

924 0.919 0.922 0.92 0.923 0.924 0.

91910 1.013 0.999 0.985 0.977 0.

969 0.968 0.959 0.

96 0.959 0.957 0.96 0.95913 1.008 1.005 0.

999 1 0.998 0.992 0.991 0.

989 0.989 0.989 0.993 0.98915 1 1 1 1 1 1 1 1 1 1 1 120 0.

982 0.986 0.986 0.987 0.99 0.998 1 1.001 1.004 0.

999 1.003 0.99930 0.929 0.936 0.936 0.947 0.951 0.

961 0.966 0.97 0.974 0.97 0.98 0.

98150 0.832 0.843 0.847 0.86 0.859 0.878 0.

884 0.888 0.895 0.898 0.907 0.909100 0.

638 0.651 0.656 0.

667 0.672 0.687 0.

699 0.705 0.713 0.718 0.

732 0.74150 0.496 0.

506 0.514 0.526 0.

527 0.54 0.552 0.557 0.562 0.

568 0.586 0.593200 0.382 0.399 0.405 0.414 0.418 0.

429 0.439 0.442 0.

45 0.456 0.465 0.475250 0.304 0.314 0.321 0.33 0.

333 0.342 0.349 0.354 0.

359 0.364 0.374 0.382300 0.241 0.

252 0.257 0.264 0.269 0.276 0.281 0.

285 0.289 0.293 0.301 0.307Table 1: TPR value for all fixed collimators and normalized value to 15 mm depth.2927985-30480-47625-36195Figure 1: TPR curve for all fixed collimators and normalized value to 15 mm depth Figure 2: TPR curve for all Iris apertures and normalized value to 15 mm depth.Depth (mm) Collimators (mm)5 7.5 10 12.

5 15 20 25 30 35 40 50 600 0.557 0.517 0.483 0.

473 0.462 0.46 0.459 0.461 0.461 0.

461 0.47 0.4743 0.794 0.749 0.714 0.69 0.

677 0.67 0.673 0.674 0.

671 0.669 0.673 0.6715 0.925 0.884 0.859 0.

828 0.822 0.81 0.813 0.8 0.811 0.

809 0.808 0.818 0.993 0.975 0.952 0.943 0.

933 0.925 0.923 0.919 0.914 0.914 0.919 0.

91610 1.005 1 0.985 0.976 0.968 0.96 0.965 0.961 0.

959 0.955 0.961 0.

9613 1.006 1.005 0.995 0.998 0.

996 0.989 0.991 0.993 0.989 0.989 0.

99 0.99215 1 1 1 1 1 1.002 1 1 1 1 1 120 0.977 0.985 0.989 0.

989 0.996 1 1.005 1.002 1.006 0.998 1.009 1.01130 0.

931 0.934 0.938 0.946 0.953 0.965 0.

973 0.97 0.971 0.975 0.983 0.

9850 0.832 0.846 0.

853 0.862 0.862 0.877 0.891 0.89 0.

898 0.898 0.914 0.915100 0.64 0.649 0.658 0.67 0.

673 0.686 0.701 0.706 0.713 0.717 0.734 0.

742150 0.495 0.507 0.517 0.

525 0.533 0.544 0.551 0.

557 0.565 0.565 0.584 0.596200 0.

387 0.4 0.406 0.414 0.

417 0.432 0.439 0.445 0.

45 0.453 0.467 0.476250 0.306 0.312 0.323 0.331 0.

333 0.342 0.35 0.

354 0.36 0.362 0.372 0.382300 0.24 0.

251 0.26 0.265 0.269 0.275 0.282 0.

287 0.287 0.293 0.

302 0.307Table 2: TPR value for all Iris apertures and normalized value to 15 mm depth.A.2.

OCR MeasurementThe OCR at a particular depth is the ratio of absorbed dose at a given off-axis point relative to the dose at central axis (4). OCR measurement carried out using field chamber of Diode E, reference chamber of Semiflex ionization chamber and water phantom. Central check carried out at two different depth 15mm and 200mm for using the 60mm collimator at SSD of 800mm. Normalization done to align the radiation beam center to the detector center. Fixed collimator OCR measured by conducting orthogonal scans across at the depth of 15mm and 100mm. In Iris collimator same like Fixed collimator scans and additionally rotate the linac head in 15 degree and generate orthogonal scans in same setup. Because Iris collimator having Dodecagonal aperture (2).

OCR values were calculated by average in each side of the cross plan and in plan scans. Therefore each entry in the OCR data table is the average of four measurement values for Fixed collimator and eight 28486101031240190501033145measurement values for Iris collimator as shown in the Figure 3 and 4.284797523622029210235585Figure 3: OCR curve for fixed collimators at 15mm and 100mm depth.Figure 4: OCR curve for all Iris apertures at 15mm and 100mmdepth.

A.3. Output FactorsThe output factor (OF) is the ratio of absorbed dose at a particular field size relative to the dose at a reference field size (2,4). The reference field size for the cyber knife system is based on the 60mm fixed collimator at SAD 800mm (2).The measurement carried out by using the Diode E, water phantom and Unidose E electrometer. All the measurement carried out at the depth of 15mm (Dmax). Meter reading are taken five times continually for each field size and calculated the average value. Both secondary collimators Fixed and Iris average value of each field size is normalize to reference field size of 60mm Fixed collimator value (2).

All the measured values are compared with the composite data set (Accuray) as shown in the Table 3.Output FactorCollimator Size Fixed Cone IrisOutput Factor Composite Ratio To Composite Standard Deviation Output Factor Composite Ratio To Composite Standard Deviation5.0 mm 0.6802 0.675 1.008 0.031 0.5511 0.

541 1.018 0.0527.5 mm 0.

8342 0.829 1.006 0.024 0.8054 0.796 1.012 0.

02610.0 mm 0.8808 0.878 1.004 0.018 0.884 0.

877 1.008 0.01812.5 mm 0.9158 0.

914 1.002 0.013 0.917 0.915 1.002 0.

01215.0 mm 0.9388 0.938 1.

001 0.008 0.9388 0.938 1 0.

00920.0 mm 0.963 0.962 1.001 0.005 0.

9615 0.962 0.999 0.00625.0 mm 0.9743 0.

974 1 0.004 0.9729 0.974 0.999 0.

00730.0 mm 0.9812 0.98 1.001 0.

004 0.98 0.98 1 0.

00535.0 mm 0.986 0.985 1.001 0.

003 0.9845 0.985 0.

999 0.00640.0 mm 0.

9894 0.989 1.001 0.003 0.9885 0.989 0.999 0.00550.

0 mm 0.9952 0.995 1 0.003 0.9947 0.995 0.999 0.00560.

0 mm 1 1 1 0 0.9993 1 0.999 0.005 Table 3: Comparison of output factors with the Accuray provided composite dataA.

4. PDD MeasurementPDD is defined as the ratio of the absorbed dose at any depth to the absorbed at a reference depth (DMAX) (4). PDD measurements are performed using 60mm fixed collimator.

Check the center of the linac beam at two depths in the water phantom 100 mm and 200 mm to verify that the linac beam is pointing straight down. The PDD measurement acquired at the depth of 1 mm to 300 mm is shown in the Figure 5 and 6. 3118485977901905097790Figure 5: Percentage depth dose (PDD) for fixed cone using 60 mm collimator.Figure 6: Percentage depth dose (PDD) for Iris using 60 mm collimator.A.5. Open Field ProfileOpen field profile measurements are done with no collimators attached to the fixed collimator housing.

Diode detector is positioned in the water phantom at the depth of 25 mm from water surface and SAD 800 mm (2). Center check is done with PTW MedPhysto software to make sure the origin of the water phantom is in coincident with central axis of the linac radiation beam. A set of orthogonal scan profiles are acquired extending from -80 mm to +80 mm in each direction as shown in the Figure 7.Figure 7: Open beam profile for inplane and crossplaneThe absolute dose calibration of the accelerator output Absolute dose calibration of the cyber knife was accomplished in accord with the IAEA TRS 398 protocol. The cyber knife output was calibrated 1 cGy per MU under reference conditions 60mm collimator, 800 mm SAD. Absolute dose calibration carried using FG 65 ionization chamber, water phantom and unidose E electrometer. To know the KQ,Q0 of the chamber need to find TPR20,10. TPR20,10 measured at two different depth 200 mm and 100 mm under the reference condition. The output measured at the reference depth of 100mm (5)The Output formalism isDQ ,100 mm = MR X ND,W X KT,P X KPol X Kion X KQ,Q0DQ = DQ ,100 mm / TPR (1OO mm)DQ ,100 mm – Absolute dose at 100 mm depthDQ -absolute dose at DMAXMR –Electrometer Reading (nC)ND,W –Chamber Calibration Factor (Gy/C) KT,P –Chamber temperature and pressure correction factorKPol –Chamber polarity correction factorKion –ion recombination factor KQ,Q0 –Beam Quality Index TPR (100 mm) – Tissue phantom ratio at 100 mm depthC. The mechanical accuracy of the robot and image stability C.1. End-to-End testThe E2E test is used to determine the total positional error for each stationary tracking mode and motion tracking mode installed on a cyber knife system (2). Stationary tracking modes include the 6D skull tracking system, the fiducial tracking system, and the Xsight spine tracking system. Head and Neck phantom and synchrony QA tool are used for stationary tracking mode. Motion tracking mode includes the Xsight lung tracking system and Synchrony Respiratory Tracking System. CIRS- Xsight lung tracking phantom kit is used in motion tracking system. (9) There are two orthogonal Radiochromic films loaded in phantoms. The plans are generated according to Accuray recommendation (2) (Table 4). Both tracking modes plans are delivered with couch positional accuracy of less than 1mm and 10. EPSON perfection V800 photo scanner was calibrated to scan the two exposed films (Axial and Sagittal) and one unexposed film. Unexposed film used for to subtract the background during analysing processes. E2E software is used to analyse the scanned film data (Table 5). The specification for total positional error for the E2E test is ? 0.95 mm for all stationary and motion tracking modes (2). Table 4:E2E planning protocol for both collimatorsTRACKING MODE FIXED(< 0. 95 mm) Iris(< 0. 95 mm)6D SKULL 0.45 mm 0.39 mmFIDUCIAL 0.56 mm 0.54 mmSYNCHRONY WITH FIDUCIAL 0.30 mm 0.35 mmX SIGHT SPINE 0.52 mm 0.48 mmX SIGHT LUNG 0.50 mm 0.45 mmTable 5:E2E analysed data valuesC.2. Laser and radiation coincidence testThe CyberKnife system uses a pinhole laser that is coincident with the radiation field central axis. The LINAC laser is reflected off an adjustable mirror and aligned to the mechanical center of the collimators. Laser and radiation coincidence test carried out using 35mm fixed collimator and two radiochromic films at two different distance. First film exposed 800MU at SDD 800 mm the laser point marked in film, same procedure repeated for second film also only change in the SDD 1600 mm. EPSON perfection V800 photo scanner was used to scan the exposed film and Imagej software used analyse the scanned data (Table 6). The tolerance value is < 1mm at SDD 800 mm and < 2mm at SDD 1600 mm (2). Collimators SDD=800 mm SDD=1600 mmFixed 0.45 1.2Iris 0.51 1.15Table 6: Laser and radiation coincidence analysed data valuesC.2. Iris QAIris is the one of the secondary collimator in CyberKnife system and its aperture size changes are computer controlled. To verify the aperture size in Iris collimator radiochromic film, Iris QA, Iris QA hardware accessories are used (2,8) .Birdcage assembly attached to collimator system and Iris QA film mount placed on birdcage (2,7). Radiochromic film position on film mount, Build up 15 mm kept top of the film. Irradiate the film with 600 MU. The each aperture sizes are repeated three times. The same step up need to done for 15 mm fixed collimator for Iris QA analysis purpose. The irradiated films and blank films are scanned using EPSON perfection V800 photo scanner. The Iris QA software used to analyse the scanned data and is as shown in the Table 7. The tolerance should be less than ± 2mm of baseline values (2,7).APERTURE SIZE MEASURED VALUE5 mm 4.80 mm7.5 mm 7.33 mm10 mm 9.84mm12.5 mm 12.38mm15 mm 14.90 mm20 mm 19.91 mm25 mm 24.95 mm30 mm 29.98 mm35 mm 34.90mm40 mm 39.88 mm50 mm 49.90 mm60 mm 59.92mmTable 7: Iris apertures exposed film data measured valueD. The patient specific QA (point dose measurement)The patient specification QA has done for both algorithms Ray-tracing and Monte Carlo using StereoPHANTM, Pinpoint chamber and Unidose E Eletrometer. The QA plan generated in Multiplan MD suite version 5.3.0 and noted the planned point dose value for both algorithms. StereoPHANTM and pinpoint chamber set in the treatment position and deliver the 2000 MU for warm up. After the zeroing QA plans are delivered. Electrometer reading, temperature and pressure values are noted. Measured Point dose values are found using the absolute dose formalism. The percentage of variation between planned value and measured value should be less than is ± 5 % (2). The measurement setup is as shown in the Figure 8.Figure 8: Patient specific QA setupDISCUSSIONThe clinical measured data for TPRs, OCRs, PDDs and Open field profile were found to be in excellent agreement with average multi-site data, which are maintained and provided by Accuray. Each of our measurements was within ±1% of the average data. The output factors (OF) for both collimators was compared with composite data from Accuray (Table 3). Fixed and Iris 5mm collimator have the maximum standard deviation and output factor ratio to composite (0.031, 0.0052) & (1.008, 1.018). The overall standard deviation of the fixed and Iris collimators are 0.0026 and 0.0063. The CyberKnife absolute dose was calibrated to 1cGy per MU and the beam quality index is 0.670 which was within the tolerance of 6MV photon beam value (0.0676±0.009). E2E is the major QA in CyberKnife because this one is combination of robot and imaging system. Our results was less than the tolerance value (Table 5). Laser and radiation coincidence was in excellent agreement within tolerance values (Table 6). The Iris aperture we found maximum deviation in 5 mm collimator 4.80 mm but this also with tolerance limit. Our clinical data used to commission the Ray tracing and Monte Carlo algorithm. The plan was delivered. Difference between planned and measured value for Ray tracing and Monte Carlo are 3.5% & 4% within acceptable limit.REFERENCESSubhash.C. Sharma, Joseph T. Ott, Jamone B. Williams, and Danny Dickow. Commissioning and acceptance testing of a CyberKnife linear accelerator. journal of applied clinical medical physics, volume 8, number 3, summer 2007CyberKnife robotic radiosurgery system Physics Essentials guide. versions 10.6.x/5.3.x/3.xSudahar Harikrishnaperumal,Gopalakrishna Kurup,Murali Venkatraman, Velmurugan Jagadeesan. A phantom study to determine the optimum size of a single collimator for shortening the treatment time in CyberKnife stereotactic radiosurgery of spherical targets. JOURNAL OF APPLIEDCLINICAL MEDICAL PHYSICS, VOLUME 13, NUMBER5, 2012 Faiz m. Khan,John P. Gibbons. The physics of radiation therapy fifth edition.Lippincott Williams and Wilkins;2014.IAEA TRS 398.Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water.Gopalakrishna kurup CyberKnife: A new paradigm in radiotherapy. Editorial: Journal of medical physics September 6, 2018Sarah-Charlotta Heidorn , Nikolaus Kremer , Christoph Fürweger. A Novel Method for Quality Assurance of the CyberKnife Iris Variable Aperture Collimator.cureus.618Ellen E. Wilcox and George M. Daskalov Evaluation of GAFCHROMIC® EBT film for CyberKnife® dosimetry. Med. Phys. 34 „6…, June 2007 Yuxi Pan,Ruijie Yang,Jun Li,Xile Zhang,Lu Liu, Junjie Wang. Film-based dose validation of Monte Carlo algorithm for CyberKnife system with a CIRS thorax phantom. J Appl Clin Med Phys 2018E. Pantelisa and C. Antypas, L. Petrokokkinos, P. Karaiskos and P. Papagiannis, M. Kozicki, E. Georgiou, L. Sakelliou, I. Seimenis. Dosimetric characterization of CyberKnife radiosurgical photon beams using polymer gels. Med. Phys. 35 „6…, June 2008