Measurement based MU verification tool for lung SBRT

Lihong Qin, PhDDepartment of Radiation Oncology

Minneapolis VA Health Care System (MVAHCS)

Disclosure

• I’m a contract employee working at MVAHCS• Have no conflict interest

Outline

• Introduction and Background• Lung Insert (phantom)• Measurement and Result• Discussion

Introduction• SBRT program at Minneapolis VA:

– Only linac based lung SBRT .– Follow guidelines:

o VA National Radiation Oncology Program (NROP)o ACRo AAPM

– SBRT training from the University of Texas Southwestern Medical Center (Robert Timmerman, MD, Timothy Solberg, PhD, Paul Medin, PhD)

• We are ACR accredited facility • VA NROP contracted IROC Houston (RPC) to do a

“On-site Audits” every two years

Lung SBRT at MVAHCS

Immobilization /localizationBody Fix Philips big bore CT/ 4D

Planning Treat/imagingMonitor unit verification

TPS independent assessment• RPC lung phantom study for IMRT plan

TPS independent assessment

• RPC head/neck phantom: excellent agreements

• IROC on-site audit measurements (in water)all data IROC measured /our TPS calculation are in agreement

• Trust our TPS more than other calculations available in our clinic

Lung SBRT at MVAHCS

• 3D conformal noncoplanar• Field size: collimator size no

smaller than ~3.5 cm• Planning: average CT, 6x

photon, cc convolution, 2mm dose grid, density correction: heterogeneous

• Monitor Unit verification: program

MU verification for lung SBRT TG 114 guideline

• Simple effective depth puts our SBRT MU constantly above action level

• We try everything to bring them within 5% /

– TG 114, TG 71, TG101,TG 65– Calling friends – Staring at the 20 something % differences

Lung Insert Phantom

• We have an ArcCHECK with PMMA (Acrylic) insert and ion chamber holder

PMMA insert Diameter is 15 cmion chamber holder diameter 3cm

CC 13 chamber

Find a better way for MU verification of SBRT

1.18g/cc0.3~0.5 g/cc

Make Lung Insert Phantom

• Tissue characterization phantom specification from Gammex:– LN-300 lung 0.31 g/cm3

– LN-450 lung 0.47 g/cm3

• Western red cedar: nominal density 0.38 g/cm3

• Get the cedar

Make Lung Insert Phantom

Make Lung Insert Phantom

• Machine Shop

This is not my garage

Making Lung Insert Phantom

Steve Morin making the phantom in “his”shop at the VA He can make lots of things

LungInsert phantom

LungInsert

ArcCHECK with LungInsert

ArcCHECK with LungInsert

• Lung insert: 15 cm• 3 cm diameter PMMA

chamber holder

PMMA: 1.18 g/ccLung Insert: 0.33 g/cc

Good geometry/density with diodes and ion chamber

Measurement• Commissioned for clinic use : (save CT image as

QA phantom, copy patient plan to QA phantom)• 5 cases measured using ArcCHECK/LungInsert for

commissioning:– 4 with planned gantry/collimator angles but with couch

angle 0.– 1 case with all planned angles, including couch

• Obtained both point dose at Iso-center (Rx point) and dose distribution with Gamma analysis

• Point dose is calculated per beam.• 2 cases measured for clinic MU verification

Results: exampleMUCheck with effective depth

Results: exampleArcCHECK with LungInsert Measurement

Results

• 7 SBRT plans tested • ~ over 70 dose points measured• All below 5%, most within 2%

between TPS/measurement • 2 points above 4% in 1 case

(happens to be the case measured with all planned angles, including couch angles)

Dose distribution: the case measured with all planned angles

ResultsGamma Analysis 3%/3mm

% passed

91

92

93

94

95

96

97

98

99

100

1 2 3 4 5 6 7

Series1

7 case results % passed from 94.4 to 99.2

Discussion

• The LungInsert for ArcCHECK can be used as measurement based MU verification tool for lung SBRT

• It’s good to have the Insert made as two halves. Easier In/Out from the ArcCHECK cavity.

Discussion

• Pros: – The results obtained from our 7 cases all below

AAPM guideline action levels. – Also get a dose distribution gamma analysis – Make it in your garage

• Cons:– Machine time– Measurement? I do the measurement now

Acknowledgement

• Special thanks go to Steve Morin who made the LungInsert

• Thanks rad onc and biomed at VA• Questions?

Thank you