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
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
Can we hit the target? n we put tthe dose where we want tit?
How accurate is radiosurgery? ctic Radiosurgery, AAPM Report No. 54, 1995 r sources: MRI Distortion Image Fusion Relocatable frames Dosimetric
Frames & CT
Isocentric Accuracy: The Winston-Lutz Test
Mechanical Uncertainties e projection of the ll centered within the field?
Isocentric Accuracy: The Winston-Lutz Test he projection of the ll centered within the field? d results 0.5 mm
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
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
End-to-end localization evaluation
Phantom Specifications iplan Stereotactic Coordinates Structure AP LAT VERT AP LAT VERT Cylinder 00 0.0 00 0.0 30.00 10 1.0 04 0.4 30.8 Cube 20.0-17.0 40.0 20.8-17.1 42.4 Cone -35.0-20.0 40.0-34.6-19.7 40.8
nd-to-end Localization Accuracy
End-to-End Localization Accuracy (Surely my vendor has checked this)
Verification of MLC shapes and isocenter
System Accuracy Simulate the entire procedures: Scan, target, plan, deliver Phantom with film holder Pin denotes isocenter
Resulting film provides measure of targeting accuracy Offset from intended target
as well as falloff for a multiple arc delivery 1 0.8 0.6 0.4 02 0.2 0-0.5 0 0.5 1
Lucy Phantom Phantom Hidden Target Test scan,,p plan, localize assess
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
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
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
at do we do about MR spatial distortion? Use Image Fusion
Fusion Verification
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
Small field depth dose show familiar trends 6 12 18 24 30 36 42 60 80 100 50 100 150 200 250 300 350 400 Depth (mm)
imilar machines have similar characteristics 12A 36A 12 36 UNMC 0 50 100 150 200 250 300 350 400 Depth (cm)
.5 3 Penumbra: Cones versus MMLC Cones MMLC.5 2.5 1.5 0 0 20 40 60 80 100
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
eference diode output to ntermediate field size tput ctor Reading (FS) diode = X Reading (Ref ) diode Reading (Ref ) IC Reading (Ref) IC
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
diosurgery beams exhibit a sharp decrease in output with decreasing field size Significant uncertainty Don t use high energy
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
servations of some treatment units: 6 mm x 6 mm MLC 110 100 Percent Dept th Dose 90 80 70 60 50 40 30 20 10 0 Institution A Institution AB Institution C 0 50 100 150 200 250 300 350 400 450 Depth (mm)
0 0 0 0 0 0 0 0 0 0 0 6.0 % 10.3 % Institution A Institution B 0 0 50 100 150 200 250 300 350 400 450 Depth (mm)
servations of some treatment units: 5 mm collimator Institution A Institution B 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Depth (mm) Depth (mm)
servations of some treatment units: 15 mm collimator.2 2 1 Institution Institution A Institution B.8 8.66.4 4.2 2 0 0 50 50 100 150 200 250 300 300 Depth (mm)
ctor put Fac Outp Relative Output Factor: 6 mm x 6 mm MLC ~45%
Institution Relative Output Factor: 42 mm x 42 mm MLC Outp put Fac ctor ~6%
Commissioning your system: Does calculation agree with measurement?
d-to-end testing imetric uncertainty Calculation arc-step Calculation = 10 o arc-step = 2 o
center 4 field box Dynamic Conformal Arcs centers
End-to-end testing Dosimetric i uncertainty Absolute Dosimetry
Independent MU Calculations
to-end dosimetric evaluation
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
Conventional Localization
Absolute Dosimetry
Lucy Prototype Imaging Insert contouring and simetry based on contours Circular volumes for image fusion
MR Fusion
MR based contouring, treatment plan and dosimetry
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
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
Stereophotogrammetry in Radiotherapy Spatial Resolution: 0.05 mm Temporal Resolution: 0.0303 s Localization Accuracy: 0.2 mm Optical togrammetry
Stereophotogrammetry in Radiotherapy
X-ray Stereophotogrammetry
Calibration of Frameless SRS Systems Establish spatial dimensions Specify the Specify the isocenter
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
Results of Phantom Data (mm) Lat. Long. Vert. 3D vector Average -0.06-0.01 0.05 1.11 Standard Deviation 0.56 0.32 0.82 0.42 Sample size = 50 trials (justified to 95% confidence level, +/- 0.12mm)
Comparison in 35 SRS patients and 565 SRT fractions
ingle Fraction 1.2 Lateral 1 Sup/Inf Ant/Post 0.8 0.6 04 0.4 0.2 0-6.0-4.0-2.0 0.0 2.0 4.0 6.0 8.0 (mm) AP Lat Axial 3D vector ingle Average 0.05-0.06 0.26 1.01
Multiple Fractions 1.2 0.8 Lateral 1 Sup/Inf Ant/Post 0.6 04 0.4 0.2 0-6.0-4.0-2.0 0.0 2.0 4.0 6.0 8.0 (mm) AP Lat Axial 3D vector ltiple Average 017 0.17 017 0.17 047 0.47 236 2.36
perior / Inferior 1.2 1 Multiple Fraction Single Fraction 08 0.8 0.6 0.4 0.2 0-6.0-4.0-2.0 0.0 2.0 4.0 6.0 8.0 less localization appears equivalent to frame-based rigid fixation
ocalization using planted fiducials
Localization using implanted fiducials
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, 1995 6875-1 (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