Commissioning and Calibrating a Linear Accelerator State-of-the-Art in 2010

Transcription

1 Commissioning and Calibrating a Linear Accelerator State-of-the-Art in 2010 Indra J. Das, PhD, FACR Department of Radiation Oncology Indiana University of School of Medicine & Midwest Proton Radiation Therapy Institute (MPRI), Indiana

2 Preface Med. Phys. 35(9), , 2008

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4 2008

5 Planning for Commissioning Data

6 Planning for Commissioning Time Time= [(PDD + 5 profiles)/beam energy] x ( open + 4 wedges) x 60 points/scan x [(1 s/pts + (1s/movement and delay)] x (15 fields x 2 energies) 10 5 s 30 h

7 Rational For Commissioning Beam Data First, it is not evident that manufacturing procedures for all linear accelerators have produced a level of reproducibility acceptable for clinical use. For example, variations in beam parameters have been noted between beams with the same nominal energies. Second, on-site changes made during installation and acceptance of the user s accelerator e.g., changes in beam energy and/or profiles from beam steering will not be modeled in the golden data. Third, the beam characteristics of the soft wedges are made by moving jaws that depend on the speed parameters of the jaws and a deviation at site could affect the beam profile of the soft wedge. TG-106 Fourth, although acceptable agreement with the golden data set may be found in individual checks, it may be that some clinical setups will have multiple errors, which combine to produce unacceptable results.

8 Rational For Not Using Golden Data Hrbacek, et al Med. Phys. 34, , 2007.

9 Rational For Not Using Golden Data x i x <, x i =? (0.5, 1.0 0r 2%) Hrbacek, et al Quantitative evaluation of a beam-matching procedure using one dimensional gamma analysis, Med. Phys. 34, , 2007.

10 Question Time required for commissioning a dual energy linear accelerator with photon and electron beam is 1. 1 day 2. 3 days 3. 1 week weeks 5. 2 months Answer: 4 Reference: Das et al, TG-106, Med. Phys. 35(9), , 2008

11 Definition of Detectors Standard chamber 10 1 cm 3 The active volume for a standard Farmer-type ionization chamber is on average 0.6 cm 3. Minichamber 10 2 cm 3 The active volume for a mini-ionization chamber is on average 0.05 cm 3. Microchamber 10 3 cm 3 The active volume for a microionization chamber is on average cm 3 and ideally suited for small field dosimetry such as radiosurgery,gamma knife, CyberKnife, and IMRT

12 Setting Water Tank & Detector Air Water

13 Know Your Connectors

14 Understand Detector, Connector & Cable See Poster: Srivastava et al, SU-GG-T-270, 2010

15 Quality of Cables

16 Question When setting ion chamber in water tank, the correct position of the chamber as viewed in water tank (as seen in figure) is: 1. Position # 1 2. Position # 2 3. Position # 3 4. Position # 4 5. Position # 5 Air Water Answer: 3 Reference : Das et al, TG-106, Med. Phys. 35(9), , 2008

17 Setup and Possible Errors

18 Electrometer Null Setting Cable subtraction Proper bias >300 V for ion chamber 100 V for diamond 0 v for all diodes Proper gain Proper mode

19 Choose Consistent & Correct Polarity

20 Chambers & Gain

21 Selection of detector for beam data

22 Choice of Detector Orientation Radiation beam Chamber Orientations Y Z X Z X Y Scan Direction

23 Detector Orientation

24 Relative Dose MV, 2x2 cm2 field, Illustration of chamber volume effects Diode, dmax PTW Pinpoint, dmax RK chamber, dmax Distance Off Axis (cm)

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26 Question The possible setup error that causes the photon beam dose profile in figure is due to: Gantry tilt 2. Collimator rotation Relative Dose 1.5 cm 10 cm 20 cm 3. Tank arm tilt 4. Gantry and arm tilt Gantry tilt, collimator rotation and tank arm tilt Distance (cm) Answer: 3 Reference: Das et al, TG-106, Med. Phys. 35(9), , 2008

27 Question The accurate measurement of the buildup and surface dose could be achieved by a: Surface & Buildup Dose, 6 MV 1. Diode detector 2. Parallel plate chamber 3. Cylindrical chamber 4. Diamond detector 5. PinPoint chamber Answer: 2 Dose (%) Depth (mm) 0.6 cc 0.3 cc cc Markus A16 IC-4 PinPoint Diamond PFD SFD Reference: Das et al, TG-106, Med. Phys. 35(9), , 2008

28 6 MV 60 Deg Wedge, 10 cm depth: water vs Profiler 250 Diode Array Profile Water Profile 200 Relative Dose Distance Off Axis(cm)

29 18 MV 60 Degree Wedge, 10 cm depth, 100 cm SAD: Water vs Profiler 200 Diode Array profile Water Profile Relative Dose Distance Off Axis(cm)

30 Scanning Speed

31 Scanning Speed

32 Percent Depth Dose Arithmetic Mean (AM) smoothing: 60 Degree Wedge PDD Unsmoothed AM x 1 AM x 2 AM x 3 AM x Depth(cm)

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34 Electron beam depth doses shown in figure represents the problem of 1. Noise in the cable 2. Electrometer gain 3. Bias on the electrometer 4. Speed of scanning 5. Tuning of accelerator Question Depth Dose (%) MeV 9 MeV MeV MeV MeV MeV 0.2 Answer: Reference: Das et al, TG-106, Med. Phys. 35(9), , 2008 Depth, z (cm)

35 Future of Beam Data Commissioning Standardization of linear accelerators Monte Carlo based commissioning Newer Radiation Detectors & Cables Newer Scanning Systems Smart algorithms

36 Aubin et al., Med Phys, 37(5), , 2010

37 Monte Carlo Codes Aubin et al., Med Phys, 37(5), , 2010

38 Simulation of intensity at target Aubin et al., Med Phys, 37(5), , 2010

39 Simulated Profiles Aubin et al., Med Phys, 37(5), , 2010

40 Profiles for different fields

41 Depth Dose Simulation Aubin et al., Med Phys, 37(5), , 2010

42 Detectors Detector Manufacturer Type volume SFD Scanditronix Photon diode 1.7x10-5 cm 3 PFD Scanditronix Photon diode 1.9x10-4 cm 3 Exradin A-16 Standard Imaging Ion chamber 0.007cm 3 Wellhofer-IC4 Scanditronix Ion chamber 0.40 cm 3 Pinpoint PTW Ion chamber 0.015cm cc PTW Ion chamber 0.125cm 3 0.3cc PTW Ion chamber 0.3 cm 3 0.6cc PTW Ion chamber 0.6 cm 3 Diamond PTW Diamond 0.003cm 3 Markus PTW Parallel plate 0.055cm 3 Edge Detector Sun Nuclear Diode 10-5 cm 3 Other

43 Relative sensitivity Sensitivity vs Volume of Detectors PFD cc SFD 0.3cc 0.125cc Markus A-16 PinPoint IC4 1.0E E E E E+00 Volume (cm 3 )

44 Question Photon beam dose profiles taken with various detectors as shown in figure is possibly due to: 110 Beam Profile 1. Speed of scanning 2. Beam asymmetry 3. Pb piece in the beam 4. Hysteresis of scanning system 5. Orientation of scanning detector Dose (%) cc 0.3 cc cc Markus A16 IC-4 PinPoint Diamond PFD SFD Distance (mm) Answer: 5 Reference: Das et al, TG-106, Med. Phys. 35(9), , 2008

45 Comparison of Large Tank and Small SRS Cylinder Tank for SRS, TMR & Profiles Moving Tank System for TMR ARM Inc., Port Saint Lucie, FL 34983

46 Direct TMR Data Acquisition No SSD to SAD calculations required, No cubic spline fit of a limited number of fixed data points needed Calculated TPR 0.6 Measured TPR Calculated TPR ~2% less at depth 0 Cubic spline fit of 12 data points Nikesch et al, CyberKnife Center, Palm Beach, FL

47 Scanner Orientation Advantage

48 Sun Nuclear 3D Scanner Ring drive maintains consistent scanning direction 2. Diameter drive has maximum scanning range of 640mm 3. Vertical drive has maximum travel of 400mm

49 Conclusions Golden Data should be taken as a reference only Understand time and amount of data to be taken View each parameters properly, double check by another individual Use proper detector for each type of data collection Set optimum speed for scanning, do not rush

50 -Conclusions Understand the limits and measuring condition Question every unusual data set Do not smooth data too much Write report for future reference Future technology & resources could help commissioning simpler

51 Thanks