Quality Assurance in Stereotactic. Radiotherapy. Swedish Cancer Institute Seattle, WA

Transcription

1 Quality Assurance in Stereotactic iosurgery and Fractionated Stereotactic Radiotherapy David Shepard, Ph.D. Swedish Cancer Institute Seattle, WA Timothy D. Solberg, Ph.D. University of Texas Southwestern Medical Center Dallas, TX

2 Quality Assurance in Linac SRS/SBRT Outline echanical aspects Linac Frames eam data acquisition mmissioning of TP system d-to-end evaluation aging and Image Fusion ameless Radiosurgery eferences and Guidelines

3 Can we hit the target? n we put tthe dose where we want tit?

4 How accurate is radiosurgery? ctic Radiosurgery, AAPM Report No. 54, 1995 r sources: MRI Distortion Image Fusion Relocatable frames Dosimetric

5 Frames & CT

6 Isocentric Accuracy: The Winston-Lutz Test

7 Mechanical Uncertainties e projection of the ll centered within the field?

8 Isocentric Accuracy: The Winston-Lutz Test he projection of the ll centered within the field? d results 0.5 mm

9 aily Lutz test is extremely important because: e mechanical isocenter n shift over time e AMC board in Varian uches can fail e cone or MLC may not be positioned perfectly after rvice re (right) and After (left) ively simple couch adjustment After Before

10 A Lutz test with the MLC is also important because: he Cone-based Lutz test does not ll you anything about the echanical isocenter of the MLC he MLC may not be repositioned erfectly after service utz test with 12 x 12 mm MLC field

11 End-to-end localization evaluation

12 Phantom Specifications iplan Stereotactic Coordinates Structure AP LAT VERT AP LAT VERT Cylinder Cube Cone

13 nd-to-end Localization Accuracy

14 End-to-End Localization Accuracy (Surely my vendor has checked this)

15

16 Verification of MLC shapes and isocenter

17 System Accuracy Simulate the entire procedures: Scan, target, plan, deliver Phantom with film holder Pin denotes isocenter

18 Resulting film provides measure of targeting accuracy Offset from intended target

19 as well as falloff for a multiple arc delivery

20 Lucy Phantom Phantom Hidden Target Test scan,,p plan, localize assess

21 Imaging Uncertainties e CT for geometric accuracy e MR for target delineation I contains distortions which ede direct correlation with CT a at the level required for SRS reotactic Radiosurgery AAPM Report No. 54 her References Sumanaweera, JR Adler, S Napel, et al., Characterization ti of tial distortion in magnet resonance imaging and its implications

22 nabe, GM Perera, RB Mooij, Image distortion in MRI-based r gel dosimetry of Gamma Knife stereotactic radiosurgery 1.8 ± 0.5 mm shift of MR images relative to CT and delivered d dose Shifts occur in the frequency encoding direction Due to susceptibility artifacts between the phantom and fiducial markers of the Leksell localization box

23 nabe, GM Perera, RB Mooij, Image distortion in MRI-based r gel dosimetry of Gamma Knife stereotactic radiosurgery quency Encoding = L/R Frequency Encoding = A/P

24 at do we do about MR spatial distortion? Use Image Fusion

25 Fusion Verification

26 Dosimetric Uncertainty dard Diode Small field measurements can be challenging; Diodes and small ion chambers are well suited to SRS dosimetry, but their characteristics / response must be well understood. Stereotactic Diode

27 Small field depth dose show familiar trends Depth (mm)

28 imilar machines have similar characteristics 12A 36A UNMC Depth (cm)

29 .5 3 Penumbra: Cones versus MMLC Cones MMLC

30 e-measure reference between each chance in e-measure reference between each chance in eld size Diode Warnings!!! iodes exhibit enough energy dependence that tios between large and small field measurements e inaccurate at the level required for radiosurgery asure output factors 2x2 cm 2 using diode 2x2 and 4x4 cm 2 using diode & chamber 4x4 cm 2 using chamber iode response will drift over time

31 eference diode output to ntermediate field size tput ctor Reading (FS) diode = X Reading (Ref ) diode Reading (Ref ) IC Reading (Ref) IC

32 eference diode output to ntermediate field size O! Reading (6 mm) diode Reading (100 mm) diode ES! Reading (6 mm) diode Reading (24 X Reading (24 mm) IC Reading (100 mm) IC mm) diode

33 diosurgery beams exhibit a sharp decrease in output with decreasing field size Significant uncertainty Don t use high energy

34 pplied statistical methods to compare data Need proof that beam data acquisition for small fields is difficult? urveyed Beam Data from 40 identical treatment units: Percent Depth Dose Relative Scatter Factors Absolute Dose-to-Monitor Unit CF Reference Condition

35 servations of some treatment units: 6 mm x 6 mm MLC Percent Dept th Dose Institution A Institution AB Institution C Depth (mm)

36 % 10.3 % Institution A Institution B Depth (mm)

37 servations of some treatment units: 5 mm collimator Institution A Institution B Depth (mm) Depth (mm)

38 servations of some treatment units: 15 mm collimator Institution Institution A Institution B Depth (mm)

39 ctor put Fac Outp Relative Output Factor: 6 mm x 6 mm MLC ~45%

40 Institution Relative Output Factor: 42 mm x 42 mm MLC Outp put Fac ctor ~6%

41 Commissioning your system: Does calculation agree with measurement?

42 d-to-end testing imetric uncertainty Calculation arc-step Calculation = 10 o arc-step = 2 o

43 center 4 field box Dynamic Conformal Arcs centers

44 End-to-end testing Dosimetric i uncertainty Absolute Dosimetry

45 Independent MU Calculations

46 to-end dosimetric evaluation

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48 PC SRS Phantom s not include MRI imaging s not include image fusion vides only a spherical target cludes image-guided guided d positioning i Hidden Target nt dosimetry limited by TLD accuracy ited information on dose distribution t results require return of phantom

49 Conventional Localization

50 Absolute Dosimetry

51 Lucy Prototype Imaging Insert contouring and simetry based on contours Circular volumes for image fusion

52 MR Fusion

53 MR based contouring, treatment plan and dosimetry

54 What about Frameless Systems? frameless stereotactic system provides alization accuracy consistent with the safe ivery of a therapeutic dose of radiation en in one or few fractions, without the aid an external reference frame, and in a nner that is non-invasive. meless stereotaxis is inherently image guided required: obilization need not be linked to localization

55 reo)photogrammetry - the principle behind ameless technologies hotogrammetry is a asurement technology in which the three- ensional coordinates f points on an object are determined by asurements made in two or more hotographic images aken from different positions

56 Stereophotogrammetry in Radiotherapy Spatial Resolution: 0.05 mm Temporal Resolution: s Localization Accuracy: 0.2 mm Optical togrammetry

57 Stereophotogrammetry in Radiotherapy

58 X-ray Stereophotogrammetry

59 Calibration of Frameless SRS Systems Establish spatial dimensions Specify the Specify the isocenter

60 How do we know the system is targeting properly? d-to-end evaluation that mimics a patient procedure entify target & plan X-ray Irradiate DRR Set up in treatment room

61 Results of Phantom Data (mm) Lat. Long. Vert. 3D vector Average Standard Deviation Sample size = 50 trials (justified to 95% confidence level, +/- 0.12mm)

62 Comparison in 35 SRS patients and 565 SRT fractions

63 ingle Fraction 1.2 Lateral 1 Sup/Inf Ant/Post (mm) AP Lat Axial 3D vector ingle Average

64 Multiple Fractions Lateral 1 Sup/Inf Ant/Post (mm) AP Lat Axial 3D vector ltiple Average

65 perior / Inferior Multiple Fraction Single Fraction less localization appears equivalent to frame-based rigid fixation

66 ocalization using planted fiducials

67 Localization using implanted fiducials

68 Radiosurgery Guidelines RO/AANS Consensus Statement on stereotactic radiosurgery uality improvement, 1993 G Radiosurgery QA Guidelines, 1993 M Task Group Report 54, 1995 pean Quality Assurance Program on Stereotactic adiosurgery, (Germany) Quality Assurance in Stereotactic adiosurgery/radiotherapy, 2004 M Task Group 68 on Intracranial stereotactic positioning ystems, 2005 Practice Guidelines for the Performance of Stereotactic adiosurgery, 2006 Practice Guidelines for the Performance of Stereotactic Body adiation Therapy, 2006 M Task Group 101, Stereotactic t ti Body Radiotherapy, 2008

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