3D Diode Array Commissioning: Building Confidence in 3D QA … · 2020. 10. 1. · ArcCHECK Introduction • Designed for Helical & Arc Delivery • 1386diodes in a helical geometry

CONE HEALTHCancer Center

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

Caroline Yount, MSCONE HEALTH 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



3D QA



3D QA



3D QA



ArcCHECK Introduction

• Designed for Helical & Arc Delivery

• 1386 diodes in a helical geometry21cm diameter, 21cm length1cm spacing, 3.28cm equivalent depth0.64mm2 active detector area

• 4th Dimension = Time50ms 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 spacing2. Linearity3. Sensitivity4. Individual detector response variations5. Absolute dose accuracy

Then… Trend analysis



Detector spacing

kVCT MVCT



Linearity – Conventional Dose Rates

6MV500MU/min10x10100SAD

200200200300

400500

600700

800

1000

2000

R² = 1

050

010

0015

0020

0025

0030

00

0 500 1000 1500 2000 2500

Dos

e @

a c

entr

al d

iode

MU1.

001.

051.

101.

151.

201.

25

0 1000 2000 3000R

eadi

ng/M

UMU

Ratio



50200200200

500

1000

2000

4000

R² = 1

010

0020

0030

0040

0050

00

0 1000 2000 3000 4000 5000

Dos

e @

a c

entr

al d

iode

MU

Linearity – High Dose Rates

6MV FFF1400MU/min10x10100SAD

1.00

1.05

1.10

1.15

1.20

1.25

0 2000 4000 6000R

eadi

ng/M

UMU

Ratio



Linearity – High Dose Rates

10MV FFF2400MU/min10x10100SAD

50200200200500

1000

2000

4000

R² = 1

010

0020

0030

0040

0050

0060

00

0 1000 2000 3000 4000 5000

Dos

e @

a c

entr

al d

iode

MU1.

001.

101.

201.

30

0 2000 4000 6000R

eadi

ng/M

UMU

Ratio



Spatial Sensitivity

Table = 0mm Table shifted laterally = 0.5mm



Spatial Sensitivity

0.5mm

1mm

2mm

3mm

86%88%90%92%94%96%98%

0 1 2 3 4Shift (mm)

Pass Rate vs Shift

Lateral Table Shifts



Spatial Sensitivity

0.5mm

1mm

2mm

3mm

86%88%90%92%94%96%98%

0 1 2 3

Shift (mm)

Pass Rate vs Shift

Longitudinal Table Shifts



Angular Sensitivity

0.5°

1°

2°

3°

Gantry Angle Changes

86%88%90%92%94%96%98%

0 1 2 3Change in Angle (degrees)

Pass Rate vs Angle Change



Detector Response Variations

• Sensitivity differences between the ArcCHECK detectors

• These differences can be measured• Individual correction factors applied to subsequent raw measurements









Calibration Validity Check

PAAP




In‐house array calibration:

Factory array calibration:




1.5%

New In‐house array calibration:

1%



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

Profiles

Ion Chamber = -2%



Clinical Example – Head & Neck



Phantom Plan



QA Result

Measured Calculated

Comparison

Profiles

Ion Chamber = 0%



Clinical Example ‐ Prostate



Phantom Plan



QA Result

Measured Calculated

Comparison

Profiles

Ion Chamber = -1.2%



Trend Analysis

“When did 3D QA indicate clinical issues?”



Trend Analysis

Patient‐specific Chamber Readings

Setup error, high gradients, modulation too high

‐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



Trend Analysis

Patient‐specific Gamma Pass Rates

Gamma 3%/3mm <1

0%

10%

20%

30%

40%

50%

60%

70% Last 100 QAs



Future work

1. Better characterization with FFF2. Higher resolution characterization for SRS3. Applying results to patient’s CT for DVH

analysis



Conclusions

1. Commissioning2. Ongoing Quality Improvement



Thank you!

Documents

3D Diode Array Commissioning: Building Confidence in 3D QA … · 2020. 10. 1. · ArcCHECK Introduction • Designed for Helical & Arc Delivery • 1386diodes in a helical geometry