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Session V: New Therapy Technologies
Patient DVH‐based QA metrics using the Planned Dose Perturbation Algorithm
March 31, 2012
Hosang Jin, Ph.D.Assistant Professor
University of Oklahoma
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Conflict of Interest
• This talk mainly deals with a commercial product – 3DVH – from the Sun Nuclear Corporation. The speaker has not received any research funding from the company and has no disclosures.
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Outline
• Conventional IMRT QA metric• Patient DVH‐based QA
– Planned Dose Perturbation (PDP) Algorithm– DVH‐based QA using PDP (clinical cases)– Limitations of PDP
• Other per‐patient dose QA methods• Conclusions
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Conventional IMRT QA methods• Per‐patient phantom measurement (dose in phantom)
• Ion chamber+film or 2D/3D diode/IC array• test
– Proposed by Dan Low in 1998– Based on tolerance of dose difference and DTA (distance‐to‐agreement)
– Tolerance: 95% passing rate with 3% and 3 mm (most commonly used; very site and machine specific)
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Gamma test (SNC MapCHECK2)
Measurement Plan
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Gamma test with a wrong beam?
Measurement Plan
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Clinically approvedoriginal plan
Error‐induced plan by changing optimization constraints
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Insensitivity of the test
2D measured dose distribution
Binary gamma plots: red pixels are points that failed a 2%/2 mm gamma analysis
Acceptable plan Unacceptable plan
Kruse, Med Phys, 37, 2010
IBA I’mRT Matrixx
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Insensitivity of the test
• There is no clear distinction in overall gamma score between the acceptable and unacceptable IMRT plans.
• While planar dosimetry may comprise one facet of an effective IMRT QA protocol, gamma scores could not reliably identify a plan with poor dosimetric accuracy.
Kruse, Med Phys, 37, 2010
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+ IMRT QA performed in phantom geometry not in patient geometry
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Patient‐DVH‐based QAStep 1: Conventional per‐beam QA or arc QA(ArcCHECK or MapCHECK)
Dose errorper each beam (2D)
Step 3: Comparison (Reference (original) vs. Comparison (perturbed))
Treatment planning system
Step 2: Planned dose perturbation (PDP)
Independent workstation(3DVH) Perturb
ed dose (3D)
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What is PDP?• Any errors detected by the conventional per‐beam planar dose QA
method is used to perturb the original 3D patient dose.• PDP uses perturbation methodology designed specifically for
Compton effects of high energy photons.• PDP alters dose only if and where dose differences are detected in
conventional dosimetry array systems.• PDP does not require secondary dose calculation that is a new
source of error.
Zhen et al, Med Phys, 38, 2011
Note: the test and passing rates are NOTstored/used.
Accumulating the total dose perturbation over all voxels and
beams
Error mask
Dose along the beam perturbed by error
A built‐in PDP model
CMF: contribution modifying function
2D “error mask” [Dose difference + local percentage
errors]
Conventional IMRT QA
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Accuracy of PDP
Error‐free plan Error‐induced plan
2D simulated measurements
2D calculationdistribution
2D error masks
3D error‐induced plan
3D PDP‐corrected
plan
3D error‐free plan
Compa‐rison
PDP
White paper
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NOTE: No actual QA measurements were not performed; free from measurement‐induced errors
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• 15 clinical IMRT cases• Varian Eclipse (AAA) and 2100C• EDR2+a Farmer chamber
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Results
Conclusions
Chamber:Difference: ~1%(when corrected for 1% diode array offset, 3DVH vs. chamber is statistically same.)
Film
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Clinical case I: Prostate boost
• Prostate boost: 2160 cGy• Total number of fields: 7; Total MUs: 387• D test passing rate (3%/3 mm): composite 98.3%, per‐
beam QA: 100.0 0.0%
Reference: Planning Comparison: PDP
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3D gamma test1%/1 mm: 83.9%2%/2 mm: 99.5%3%/3 mm: 100.0%
Dose difference histogram
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Clinical case II: Head and Neck
• Tongue: 7200 cGy• Total number of fields: 9, Total MUs: 1077• D test passing rate (3%/3 mm): composite 99.8%, per‐
beam QA: 99.8 0.5%
Reference: Planning Comparison: PDP
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3D gamma test1%/1 mm: 62.2%2%/2 mm: 96.0%3%/3 mm: 99.8%
Dose difference histogram
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Limitation I
Low resolution High resolution
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Limitation I – cont’d
Low resolution High resolution
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Limitation II
Sun Nuclear: White paper
What if the heterogeneity correction of the TPS is inaccurate?
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Limitation III
Zhen et al, Med Phys, 38, 2011
Treatment planning(Plan approval) IMRT QA TreatmentPass
Fail(e.g.) 95% pass ratewith 3%/3 mm
Conventional QA procedure
Treatment planning(Pre‐Plan approval)
QA delivery TreatmentDVH‐based QA
(Post‐plan approval)
Fail
Pass
Acceptable tolerances?Action levels?Physician’s review?
New QA procedure
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Dose QA method I: Direct fluence map
• Patient geometry QA• Clinically relevant comparison (DVH and 3D dose difference)• More sources of error: de‐convolution of fluence map and
independent dose calculation• Need of commissioning of systems• Commercially available: OSL Dosimetry Check (EPID) and
IBA Compass (IC array) systems
Step 2: 3D dose calculation using measured fluence maps
and patient CT data
Step 3: Comparison/Analysis
Fluence map
2D array (IC)
EPID
Step 1: Measurement of direct fluence map
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Dose QA method II –transmission dosimetry
• Actual patient geometry• Potentially adaptive treatment• Dose reconstruction: source of error• Need of pre‐treatment QA
EPID
Step 1: Measurement during treatment
Step 2: 3D dose reconstruction using backprojection algorithm
Wendling et al, Med Phys, 36, 2009Step 3: Comparison/Analysis
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Conclusions• It should be noted that high passing rates in conventional QA do not alone imply accurate dose calculation and/or delivery
• The PDP algorithm was shown to accurately predict the DVH impact and clinically relevant dose using conventional planar QA results.
• However, it could introduce more complex and inefficient QA in the busy clinic.
• Most importantly, acceptable tolerances, action levels, and potential changes in QA procedures should be explored extensively.
• Limitations of the PDP should be further investigated.
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Acknowledgement
• Imad Ali, Ph.D.• Salahuddin Ahmad, Ph.D.• Vance Keeling
• Stacey Geier (Sun Nuclear; providing 3DVH materials)