Qalitätssicherung an Multileafkollimatoren. Dr. Lutz Müller Würzburg jan 2004

Transcription

1 Qalitätssicherung an Multileafkollimatoren Dr. Lutz Müller Würzburg jan 2004

2 IMRT Verification - present Target Volume Constraints Inverse Planning Algorithm Fluence Map Leaf Sequencer Leaf & Gantry sequence Desired 3-d dose distribution 3-d Dose Delivered 3-d dose distribution 2

3 Workflow of Film Verification CT scan of phantom Irradiate patient s plan to phantom Apply patient s plan to phantom Calculate dose distribution in Phantom Process film read in 3D data OmniPro - IMRT COMPARE AND EVALUATE DATA: isodoses, + - * /,? Scan Film calibrate register reconstruct 3d 3

4 Equipment: Body Phantom Film application fields: 1. Universal body 2. Head & Neck 3. Stereotactic Material: water-equivalent RW3 (polystyrene) Dimensions: U: 36 (W) x 18 (H) x 33 (D) H&N: 18 x 18 x 18 cm 4

5 Royal Marsden (UK) Testing Case: Prostate & Pelvic Nodes in Alderson Phantom DICOM imported plan isodose comparison calibrated film gamma evaluation 5

6 Routine IMRT Verification - 3D/ 2 ½D Dosimetry checks the quality of treatment planning process and of the delivery process - In routine applications, only the delivery needs to be verified - Check it: measure fluence/sequence offline or online - Trust it: accurate, frequent QA, specially of MLC, is a MUST 6

7 IMRT Verification 2D Fluence Delivered fluence Leaf Sequence Target Volume Constraints Inverse Planning Algorithm Fluence Map Leaf Sequencer Leaf & Gantry sequence Desired 3-d dose distribution 3-d Dose Delivered 3-d dose distribution 7

8 Usage of a-si EPID (Elekta i-view) OmniPro-I mrt connects to ELEKTA s database recalibrates images (linear response) 8

9 Step-and-shoot delivery w/ EPID individual segments (integration w/o deadtime) composed image 9

10 EPID vs. Film 50% isodoses EPID... Film integrated EPID (normalized) calibrated Film 10

11 EPID vs. Plan (Helax TMS) integrated EPID (normalized) calculated plan data 11

12 I mrt QA Device Imaging device Positioning tripod Gantry adapter frame scintillators 12

13 I mrt QA: Operating Principle Scintillator Incident Radiation Reflected Light 12-bit CCD Camera Mirror 12 Bit parallel Data out 13

14 Complex IMRT Field BIS Measurement Eclipse Calculation x profile y profile I mrt QA Eclipse 14

15 MLC QA precise measurement of all leaf positions is needed! 1 cm Gap 0.5mm means 5% error 15

16 MLC QA with Film Abutting Fields 16

17 Positioning precision of?-mlc (3D-LINE) Avg. Pos = 24.6 mm Std. Dev. = 0.36 mm 17

18 Positioning precision of?-mlc (3D-LINE) pos. deviation from average # of leaves n = discard [cm] 18

19 Hysteresis in Leaf Positioning 19

20 Hysteresis in Leaf Positioning Leaf Positioning Hysteresis 16 # of leaves < 0.1 mm 0.1< dx < 0.2 mm >0.2 mm avg(close) - avg(open) [cm] 20

21 MLC QA - Gravity 90 Gantry Orientation vs. 270 Gantry Orientation 21

22 MLC QA Picket Fence MLC pairs form a narow slot moving across the field, stopping and reaccelerationg at predefined positions 22

23 MLC QA Leaf Speed Test Leaf pairs form gaps moving with different speed Delivery with beam interrupts 23

24 MLC QA Picket Fence Regular Pattern Error introduced 1.0 mm 0.9 mm 0.8 mm 0.7 mm 0.6 mm 0.5 mm 0.4 mm 0.3 mm 0.2 mm 0.1 mm 24

25 MLC QA Picket Fence 1.0 mm 0.9 mm 0.8 mm 0.7 mm 0.6 mm 0.5 mm 0.4 mm 0.3 mm 0.2 mm 0.1 mm 1.0 mm 0.5 mm 25

26 Leaf Position Readout Precision measured displacement [cm] y = x R 2 = programmed error [cm] 26

27 Leaf Timing Diagram leaf position (cm) left leaf right leaf left leaf (error) right leaf (error) frame number 27

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29 I mrt - QA Key Features - Fast and simple set-up: less than 5 minutes. - Positioning on table or in gantry accessory holder - Acquisition of individual segments or entire IMRT field - Minimum acquisition time: 120 msec per 2D image - Intensity resolution: 12 bit - Spatial resolution 0.4 x 0.4 mm - Field size up to 400 x 400 mm - Acquisition and analysis integrated in OmniPro-IMRT software platform 29

30 Verification of intensity modulated pattern calculated plan Individual segments Delivery error! I mrt-qa After error analysis & correction 30

31 Use of 2-d Dose Distribution for Verification (Measurements at IGR Paris) 1 cm solid water on BIS, Cu plate corresponds to 2 cm H 2 O CadPlan calculates in 3 cm depth 3 cm 5 cm 31

32 Dynamic Wedge Dynamic Wedge 1.2 Dose/rel. Signal [%] Position [cm] Eclipse BIS 32

33 Verification of IMRT Quantity 3-D Dose Distribution 2-D Dose/Fluence Leaf Positions MLC QA Calculation Apply Plan to Phantom. Calculate 3-D Dose Distribution Calculate Fluence Pattern or 2-D Dose Distribution Leaf Positions from TPS Measurement Put Films in the Phantom. Process, Scan, Calibrate Films. Compose 3-D Dose Distribution BIS Integrated Fluence Pattern Leaf Positions from single BIS Images MU/Dose Check Penumbra measurement Dose in a reference Point Needed for TPS Set-up Ion Chamber in Phantom Small Ion Chamber or Diode in 3-D-Phantom 33

34 IMRT dosimetry applications - Verification of the planned versus delivered dose - Verification of the IMRT delivery prior to a treatment - MLC QA - Monitor Unit calculation - Penumbra measurements 34

35 Startup behaviour Startup Elekta 16MV Startup Behaviour Dose Rate [arb. units] Time [ms] 35

36 Dose Comparison Methods Profiles Isodoses Difference Gamma 36

37 Gamma evaluation (3mm/3%) 37

38 BIS2G attached to the gantry The Gantry is turned around 90 degrees. BIS-System already assembled into the adapter. The accelerator is turned around 180 degrees. 38

39 BIS vs. Cadplan 39

40 Fluence reconstruction more complex n m?, y? ( x, y) y' ) dx' dy' x? Open field w/o penumbra?? S( x? x', y? y' ) T ( x? x', y? Double-Gaussian Transmission Kernel (from penumbra) L.Ma, P.Geis, A.L.Boyer, Med Phys 24(8), 1997 through MLC 1 or 0 40

41 I( x 0 Fit of penumbra region Double-Gaussian Kernel:, y 0 ) 2 2 2?? ( x? x0)? ( y? y0)? ( x? x0)? ( y? ( x, y)? exp??? exp 2?? 1? 2?? I0 2 x, y y 0 ) 2??? rel. fluence square field 6cm position [cm] BIS kernel0.6-03/5 Fit to Penumbra region (square field):? 1 =0.6mm? 2 =3 mm????? 41

42 Double-Gaussian Kernel BIS measured Cadplan / double gaussian kernel 42

43 Gamma Index Cadplan raw fluence Double Gaussian Kernel?x = 3mm,?D = 3%

44 MLC QA with BIS 2G BIS 2G is an ideal tool for accurate and fast MLC QA: 1. High spatial resolution (0.4x0.4mm), 2. High acquisition speed (120ms/frame) and 3. Gantry mount MLC QA techniques: leaf positioning accuracy and reproducability acceleration and deceleration of leafs gravity effect leaf transmission (leakage) visualization on-line MLC movement 44

45 IMRT dosimetry applications - Verification of the planned versus delivered dose - Verification of the IMRT delivery prior to a treatment - MLC QA - Monitor Unit calculation - Penumbra measurements 45