3D Diode Array Commissioning: Building Confidence in 3D QA Technology

Transcription

1 3D Diode Array Commissioning: Building Confidence in 3D QA Technology Caroline Yount, MS CANCER CENTER

2 3D QA The complex three-dimensional (3D) shapes of intensity modulated radiation therapy (IMRT) dose distributions and the methods for IMRT dose delivery demand that the dosimetry measurement techniques be reviewed and adapted for the unique challenges posed by IMRT. TG 120

3 QA Challenges Modern technologies are utilizing rotation delivery to speed up treatment time and decrease tissue toxicity (RapidArc, VMAT, TomoTherapy, IMAT, SmartArc) Rotational beam delivery creates a challenge for patient specific QA 360 degrees of delivery

4 3D QA

5 3D QA

6 3D QA

7 3D QA

8 ArcCHECK Introduction Designed for Helical & Arc Delivery 1386 diodes in a helical geometry 21cm diameter, 21cm length 1cm spacing, 3.28cm equivalent depth 0.64mm 2 active detector area 4 th Dimension = Time 50ms update frequency Optional cavity plug insert with ion chamber holder PMMA construction

9 Detector Geometry Entrance and exit dose are measured Effectively doubling the detector density in the measurement field. Central 10x10 contains approximately 230 detectors Detectors are arranged on a HeliGrid Increases sampling rate and reduces detector overlap from Beams Eye View (BEV) Entrance and exit dose can be correlated to determine gantry angle

10 Question How can I interpret and trust 3D QA?

11 Commissioning Measurements 1. Detector spacing 2. Linearity 3. Sensitivity 4. Individual detector response variations 5. Absolute dose accuracy Then Trend analysis

12 Detector spacing kvct MVCT

13 Linearity Conventional Dose Rates a central diode R² = MU Reading/MU MV 500MU/min 10x10 100SAD Ratio MU

14 Linearity High Dose Rates a central diode R² = Reading/MU MV FFF 1400MU/min 10x10 100SAD Ratio MU MU

15 Linearity High Dose Rates a central diode R² = MU Reading/MU MV FFF 2400MU/min 10x10 100SAD Ratio MU

16 Spatial Sensitivity Table = 0mm Table shifted laterally = 0.5mm

17 Spatial Sensitivity Lateral Table Shifts 0.5mm 1mm 2mm 98% 96% 94% 92% 90% 88% 86% Pass Rate vs Shift Shift (mm) 3mm

18 Spatial Sensitivity Longitudinal Table Shifts 0.5mm 1mm 2mm 98% 96% 94% 92% 90% 88% 86% Pass Rate vs Shift Shift (mm) 3mm

19 Angular Sensitivity Gantry Angle Changes % 96% 94% 92% 90% 88% 86% Pass Rate vs Angle Change Change in Angle (degrees) 3

20 Detector Response Variations Sensitivity differences between the ArcCHECK detectors These differences can be measured Individual correction factors applied to subsequent raw measurements

21 Detector Response Variations

22 Detector Response Variations

23 Calibration Validity Check AP PA

24 Calibration Validity Check Factory array calibration: In house array calibration:

25 Calibration Validity Check New In house array calibration: 1% 1.5%

26 ArcCHECK Calibration Array calibration on standard linac Dose calibration on standard linac SAD hand calc at 3.28 cm depth (water equivalent depth)

27 Dose Accuracy Measured 10x10 Planned 10x10

28 Clinical Example Chest

29 Phantom Plan

30 QA Result Measured Calculated Comparison Ion Chamber = -2% Profiles

31 Clinical Example Head & Neck

32 Phantom Plan

33 QA Result Measured Calculated Ion Chamber = 0% Comparison Profiles

34 Clinical Example Prostate

35 Phantom Plan

36 QA Result Measured Calculated Ion Chamber = -1.2% Comparison Profiles

37 Trend Analysis When did 3D QA indicate clinical issues?

38 Trend Analysis Patient specific Chamber Readings 13.5% 12.0% 10.5% 9.0% 7.5% 6.0% 4.5% 3.0% 1.5% 0.0% 1.5% 3.0% 4.5% 6.0% 7.5% 9.0% 10.5% 12.0% 13.5% Machine dose rate low ~900 QAs Setup error, high gradients, modulation too high

39 Trend Analysis Patient specific Gamma Pass Rates 70% Last 100 QAs 60% 50% 40% 30% 20% 10% 0% Gamma 3%/3mm <1

40 Future work 1. Better characterization with FFF 2. Higher resolution characterization for SRS 3. Applying results to patient s CT for DVH analysis

41 Conclusions 1. Commissioning 2. Ongoing Quality Improvement

42 Thank you!