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IMRT Quality Assurance With Varian Machines

IMRT Quality Assurance With Varian Machines. Niko Papanikolaou, PhD Chief of Medical Physics Department of Radiation Oncology University of Arkansas for Medical Sciences Little Rock, Arkansas, USA. Outline. Discuss the QA procedures that we follow in our clinic for patient specific IMRT QA

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IMRT Quality Assurance With Varian Machines

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  1. IMRT Quality AssuranceWith Varian Machines Niko Papanikolaou, PhD Chief of Medical Physics Department of Radiation Oncology University of Arkansas for Medical Sciences Little Rock, Arkansas, USA

  2. Outline • Discuss the QA procedures that we follow in our clinic for patient specific IMRT QA • MLC based planning using the Philips Pinnacle RTP delivered on a Varian 2100EX that is monitored by the IMPAC RV system

  3. IMRT QA References • Guidance document on delivery, treatment planning, and clinical implementation of IMRT Med. Phys 30(8) August 2003 • Implementing IMRT in clinical practice IJROBP 58 (5) 2004 Not prescriptive but suggestive

  4. Elements of an IMRT QA Program • Proper training of all the staff that is involved in the treatment plan development and delivery of the IMRT treatments • Appropriate selection of beam orientations, objectives and MLC sequencing parameters are critical to the process • Policies and Procedures that describe all processes • Hardware and software tools to make the process accurate and efficient

  5. The Varian Unit • Non-linear difference between the light field projection and the linear leaf displacement that is accounted for by the leaf position encoding • X-ray field is wider than light field by a nearly constant value for all leaves and all leaf positions (1mm) • Monitor units can be delivered in increments of 0.01MU

  6. Varian Unit- Step&Shoot Delivery • The control system defines how quickly a leaf pattern can be set followed by an MU delivery. Factors are: • Time to set and verify the MLC • Time to initiate and terminate irradiation for a step • Time needed to log the delivery of the step • Method used to activate accelerator beam on • Varian system cycles at a rate of one step and shoot instance per second. Beam switching is so fast that the speed limiting factor is the mechanical motion

  7. Varian Unit- Step&Shoot Delivery • Control loop cycle determines the accuracy of the first and last segments given within a field • Low MU and high dose rates exacerbate the problem • MLC controller receives updates of the delivered dose every 50msec

  8. Testing the Varian MLC • A suite of tests have been developed and distributed by Varian that evaluate: • Leaf positional accuracy • Kinetic properties of dMLC • Dosimetric accuracy of fractional dose delivery • Tests are designed to evaluate both the step and shoot and dynamic delivery modes

  9. MLC test • Picket Fence - leaf position • Synchronized segmented stripes – inter-leaf friction • Non-synchronized segmented stripes - inter-leaf friction • X and Y wedges – accuracy of MLC to produce X-Y wedges • Pyramids – positional accuracy in complex shapes • Complex fields A, B – complex intensity patterns • Continuous stripes – interleaf friction and speed

  10. test7 test1 test2

  11. Equipment and Tools • Phantom(s) They come in many sizes and shapes and the question is which one is the right one for the job ? A simple phantom that is easy to use, fast to setup and large enough to accommodate the typical patient dose distributions

  12. Equipment and Tools • Ion chamber(s) A reliable ion chamber that is not too small but not too large either. The choice of ion chamber is not as critical, but a good choice could make a difference on certain occasions

  13. EDR Equipment and Tools • Film The choice of film is very important. But even more important is the calibration of the film and the stability of the film processing environment and chemistry

  14. Equipment and Tools

  15. Equipment and Tools There are several software options for film analysis and IMRT QA currently available. (eg. RIT, ImRT, Protos and others) For a routine task such as the IMRT QA important factors are: • efficiency, • ease of use, • accuracy • documentation

  16. Independent MU calculations

  17. Independent MU calculations

  18. Independent MU calculations

  19. The Pinnacle IMRT QA

  20. Pinnacle IMRT QA Tools • Automatically generate orthogonal DRRs to verify isocenter placement • Use the profile tool to view the dose profile across the plan between any two points • Compute the dose distribution for any beam at a given depth in a flat water phantom • Compute the dose distribution at any plane in the dose matrix • Transfer the beams from an IMRT plan to a user-defined QA phantom • Export distributions for comparative analysis

  21. Solid phantom Film Planar dose verification (all beams) Planar dose map

  22. Phantom setup

  23. Dose Calculation

  24. Planar Dose Comparison

  25. Measurement setup SAD=100 cm SFD=102 cm SSD=94 cm 6 cm buildup 10 cm backscatter

  26. Calibration Film-Step Wedge

  27. Calibration Curve

  28. Patient Film Registration

  29. Patient Film Comparison-Profile

  30. Patient Film Comparison-Isodose

  31. Patient Film Comparison-Gama

  32. Patient Film Comparison-3D

  33. Summary • A robust MLC QA program is a pre-requisite to a successful IMRT QA practice • Patient specific IMRT QA is done by: • Independent dose calculation/verification • Ion chamber measurement for absolute point dose comparison • Film measurement for 2D relative isodose comparison

  34. Thank you

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