Patient Specific QA for Monte Carlo Lung SBRT on Cyberknife: Is It Necessary? J. Fabien, MS; Y. Zhang, MS; J. Brindle, PhD; D. Dobbins, CMD; T. Podder, PhD; B. Wessels, PhD University Hospitals Seidman Cancer Center Case Western Reserve University Medical Center
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Patient Specific QA for Monte Carlo Lung SBRT on …chapter.aapm.org/pennohio/2013FallSympPresentations/FA3...The Lung SBRT Issue • Originally, all Cyberknife planning used Ray Tracing
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Patient Specific QA for Monte Carlo Lung SBRT on Cyberknife:
Is It Necessary?J. Fabien, MS; Y. Zhang, MS; J. Brindle, PhD; D. Dobbins, CMD;
T. Podder, PhD; B. Wessels, PhD
University Hospitals Seidman Cancer CenterCase Western Reserve University Medical Center
The Lung SBRT Issue
• Originally, all Cyberknife planning used Ray Tracing algorithm
• 2012: Started using Monte Carlo algorithm
• RTOG clinical trials exclude Ray Tracing
• Realized a 10-20% deficiency in dose and resulting loss of prescription coverage
• After dosimetry effects were seen, ALL lung and T-spine patients were calculated with MC
Monte CarloRay Tracing
The Lung SBRT Issue
54 Gy
Ray Tracing
The Lung SBRT Issue
Monte CarloRay Tracing
The Lung SBRT Issue
Ray Tracing
The Lung SBRT Issue
Monte CarloRay Tracing
The Lung SBRT Issue
Monte CarloRay Tracing
PTV = 98.6%CTV = 100%
PTV = 2.2%CTV = 19.8%
The Lung SBRT Issue
Ray Tracing
The Lung SBRT Issue
Monte CarloRay Tracing
The Lung SBRT Issue
Ray Tracing
Ray tracing calculation using equivalent path length:
• TPR (FS, Deff)
• OCR (FS, R800, Deff)
• OF (FS, SAD)
OFTPRSAD
OCRMUD
2800
Ray TracingSecond check using MuCheck (Oncology Data Systems):
Ray Tracing dose = 6000 cGyMuCheck dose = 6026 cGy
Ray Tracing
Ray Tracing dose = 6000 cGyMuCheck dose = 6026 cGy
• Much more complex
• Much more accurate with heterogeneities
• No software second check exists
• Phantom measurement is necessary to verify dose
Monte Carlo
The Method
• Use heterogeneous phantom
• Overlay chamber position in low gradient area
• Calculate mean MC dose to chamber volume
• Deliver patient plan to phantom, measure dose to chamber
1. Transfer patient plan onto hetero phantom
2. Calculate low resolution dose to check & adjust chamber position
3. Calculate high resolution
4. Calculate with Monte Carlo
MC chamber mean dose = 85.30 Gy
5. Deliver patient plan to hetero phantom with ion chamber
MC chamber mean dose = 85.30 GyMeasured chamber dose = 84.14 Gy -1.4%