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

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

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

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

3D QA

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

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

Question How can I interpret and trust 3D QA?

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

Detector spacing kvct MVCT

Linearity Conventional Dose Rates Dose @ a central diode 0 500 1000 1500 2000 2500 3000 200 300400 500 600700 800 1000 R² = 1 2000 0 500 1000 1500 2000 2500 MU Reading/MU 1.00 1.05 1.10 1.15 1.20 1.25 6MV 500MU/min 10x10 100SAD Ratio 0 1000 2000 3000 MU

Linearity High Dose Rates Dose @ a central diode 0 1000 2000 3000 4000 5000 1000 2000 R² = 1 4000 500 50 0 1000 2000 3000 4000 5000 Reading/MU 1.00 1.05 1.10 1.15 1.20 1.25 6MV FFF 1400MU/min 10x10 100SAD Ratio 0 2000 4000 6000 MU MU

Linearity High Dose Rates Dose @ a central diode 0 1000 2000 3000 4000 5000 6000 1000 2000 R² = 1 4000 500 50 0 1000 2000 3000 4000 5000 MU Reading/MU 1.00 1.10 1.20 1.30 10MV FFF 2400MU/min 10x10 100SAD Ratio 0 2000 4000 6000 MU

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

Spatial Sensitivity Lateral Table Shifts 0.5mm 1mm 2mm 98% 96% 94% 92% 90% 88% 86% Pass Rate vs Shift 0 1 2 3 4 Shift (mm) 3mm

Spatial Sensitivity Longitudinal Table Shifts 0.5mm 1mm 2mm 98% 96% 94% 92% 90% 88% 86% Pass Rate vs Shift 0 1 2 3 Shift (mm) 3mm

Angular Sensitivity Gantry Angle Changes 0.5 1 2 98% 96% 94% 92% 90% 88% 86% Pass Rate vs Angle Change 0 1 2 3 Change in Angle (degrees) 3

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

Detector Response Variations

Calibration Validity Check AP PA

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

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

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

Dose Accuracy Measured 10x10 Planned 10x10

Clinical Example Chest

Phantom Plan

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

Clinical Example Head & Neck

Phantom Plan

QA Result Measured Calculated Ion Chamber = 0% Comparison Profiles

Clinical Example Prostate

Phantom Plan

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

Trend Analysis When did 3D QA indicate clinical issues?

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

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

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

Conclusions 1. Commissioning 2. Ongoing Quality Improvement

Thank you!