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The HESE Radiotherapy Methods for Pretreatment Patient QA
Using Intensity Modulated Techniques
C. MarcelinoMedical Physics ExpertLenicare – Serviço de RadioterapiaHospital do Espírito Santo de Évora, E.P.E.
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Why we need this pretreatment patient QA?
What can we use?
How we do it? What show our results?
What comes next?
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Why we need this pretreatment patient QA?
http://publicdomainarchive.com
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
In intensity modulated therapies, like IMRT or VMAT, the segmentation of an individual treatment field leads to complex patterns of intensity distributions as well as non-trivial MU settings.
The whole treatment plan must therefore be checked before being applied to the patient.
In addition, patient specific dosimetry should be performed for each individual treatment plan.
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Commissioning
Acceptance Tests
Data Acquisition
TPS Algorithm Validation
Quality Control Program
Daily Tests
Monthly Tests
Quartely Tests
Annual Tests
Treatment Protocols
Patient Setup
Planning Targets
Constraints
Objectives
Until now…… never less we still need
Patient QA
Measurement Based
Independent MU/Calculation
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
IMRT or VMAT delivery techniques have the potential of delivering a highly conformal dose to the target while spreading the dose to the surrounding tissue and thereby sparing organs at risk form receiving high radiation doses.2
The VMAT delivery single arc technique originally investigated by Otto, evaluated for different approaches using multiple coplanar or noncoplanarintensity-modulated arcs for complex treatment.
In VMAT the MLCs are moving dynamically while the dose rate and the gantry speed are also varied throughout the rotation of the gantry.
Potential sources of errors are imprecision of MLC controller that may result in overshooting, undershooting, or dropped segments, as well as inaccuracies of the gantry motion and monitor units MU controller.
Just to get you on the focus…
[2] Korreman S. et al. Dosimetric verification of RapidArc treatment delivery. Acta Oncologica, 2009; 48: 185 - 191
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Thomas J. LoSasso, Ph.D.IMRT Delivery System QA
Memorial Sloan-Kettering Cancer CenterNew York, New York
https://medicalphysics.org/documents/IMRT_ch22.pdf
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
What we can use?
http://pt.freeimages.com/
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Ionisation Chambers
▪ Available in different shape and sizes
▪ Dosimetric response is well understood.
▪ Absolute dose measurements
▪ Excellent spatial resolution – small size
▪ Reusable and readily available readout
▪ Versatility in placement
Measurement based for 1D…
Diodes and Mosfet
▪ Finite size
▪ Volume Averaging
▪ Suitable for homogeneous regions
▪ Angular dependency (diodes)
▪ Very dependent on calibration (diodes)
▪ Over-response for scatter (Mosfet)
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Detector Arrays (Chambers or Diodes)
▪ Measured dose in several points simultaneously
▪ Long term stability and low dose rate dependence
▪ Excellent spatial resolution (<1mm)
▪ Tissue equivalence (EBT3)
▪ Very weak energy dependence (EBT3)
Measurement based for 2D…
Films (Radiograhic Radiochromic)
▪ Low spatial resolution
▪ Detector ageing effects (Diodes)
▪ Volume averaging (similar to ion chamber)
▪ For absolute dosimetry requires proper calibration, measurement and processing
▪ Show inter- and intra-batch variation of the film sensitivity
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
EPID (Electronic Portal Image Device)
Primary fluence prediction
▪ High spatial Resolution
▪ Instantaneous read out
▪ No straighforward correlation with dose delivery
▪ No end-to-end test
Measurement based for 2D…
Back projection of transit fluence
▪ High spatial Resolution
▪ Instantaneous read out
▪ Direct comparison with dose delivery
▪ End-to-end test
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Gel Dosimeters
▪ 3-D information in one irradiation
▪ Energy and dose-rate independent
▪ High sensitivity and linear response.
Measurement based for 3D or 4D…
▪ Sensitive to time, preparation and temp.
▪ Relative dosimeter - require cross-calibration
▪ Cost
3D Array or Rotational Cylinder
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
▪ Peforms the treatment plan recalculation
▪ Act like different TPS, verifying mainly the UM calculation
▪ Capable of volumetric calculation
▪ Allow several analysis between prior treatment, independent prior treatment and really measured
Independent MU Calculation…
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
How do we do it?
https://pixabay.com/
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
▪ 2 Linear Accelerators Varian Clinac DHX, equiped with Multileaf Collimator 120 leafs
▪ 1 with the VMAT (*) capability (RapidArc®)
▪ 2 Treatment Planning Systems Varian Eclipse® EX
▪ including Analytical Anisotropic Algorithm (AAA) and Electron Monte Carlo (EMC) algorithms
▪ 1 Computer Tomography GE BrightSpeed 16
▪ Oncology Information system network ARIA® v13.5
Our department…
VMAT - Volumetric Modulated Arc Therapy
varian.com
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
The treatments plans for VMAT delivery…
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Going on the validation process…
▪ Analyze the 3D distribution and select the most suitable 2D plans for validation:
▪ Targets doses, organs at risk evolved or dose gradients near organs at risk
▪ Validation plans are generated and recalculated on the CT-image of PTW Octavius II comparison between planned and measured doses.
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Going on the validation process…
1D Ion Chamber
Absolute dosePoint Dose
Arc-by-Arc and Plan dose
2D Ion Chamber Array
Absolute Dose
729 Point Doses (Máx.)
Gamma Analysis
EBT3 Films
Absolute Dose
Relative Dose
“Thousands” of pixels
Gamma Analysis
EPID
Relative Dose
“Thousands” of pixels
Gamma Analysis
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Índice Gammma ɣ▪ Compare the measured dose with the TPS calculated dose
Dose Difference
Distance to Agreement
Índice ɣ
• Dose difference suitable for low dose gradient regions• Distance-to-agreement more suitable for high gradient regions• Both gradients exist on VMAT plans• Gamma index combine both variables
Low et al, Med Phys 25 656, 1998 Depuydt et al, R&O 62(3) 309, 2002 Ju et al, Med Phys 35 879, 2008
Gamma Index γ…
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Identify that regions where dose difference (D) and distance-to-agreement (d) fail simultaneously
▪ Describes a orthogonal space defined by DTA dimension and dose difference
▪ Combines both difference concepts together by a abstract metric similar to one distance
▪ Gamma index is determined by variation of r, to find the minimal value of T and is calculated for all distribution points
Gamma Index γ…
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Analysis examples…
Film DosimetryRIT Sofware
2D Ion Chamber ArrayPTW Verisoft
EPID DosimetryPTW seven29
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Interpolation
▪ Slice width in the planning CT
▪ Grid selection on your optimizer
▪ Grid selection on your dose algorithm
Resolution
Independent of your equipment choice be aware about interpolation and resolution…
▪ Resolution of the measuring equipment
▪ Resolution on the data analysis software
▪ Evaluation criteria
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
EPID
EBT2
PTW seven29
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Eclipse PTW Verisoft PTW seven29
One case of our…
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
What show our results?
http://pt.freeimages.com/
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
▪ ICRU 83 without consensus about aceitability criteria for gamma index
Which criteria should we apply…
ESTRO Booklet Nº 9 (2008) A. Mans et al. Phys. Med. Biol. 61 (2016) 7221–7235
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
The Global statistics…
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
With 1D Ion Chamber…
▪ 445 Patients
▪ Absolute Dosimetry
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
With 2D Ion Array …▪ 1357 Patients, Absolute Dosimetry, γ criteria: 3% / 3mm (tr. 20%)
Mean = 99,46 %
SD = 1,2%
Counts = 2693
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
With 2D Film …▪ 1052 Patients, Absolute Dosimetry, γ criteria: 3% / 3mm (tr. 20%)
Mean = 97,7 %
SD = 2,60 %
Counts = 2110
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
With 2D Film …▪ 1092 Patients, Relative Dosimetry, γ criteria: 3% / 3mm (tr. 20%)
Mean = 98,7 %
SD = 2,00 %
Counts = 2116
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
The analysis of the VMAT/IMRT QA process performed with different methods was very satisfactory
All methods showed distinct advantages and drawbacks.
The most important drawbacks are the calibration process for film absolute dosimetry and understand the deviations causes between predicted and measured doses.
Conclusions…
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
What Comes next?
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Improving themachine QA,
particular the MLC
ImplementingClass Solutions
(Prostate, H&N, Lung)
Implementingindependent
MU calculation
Participation atexternal audits
The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017
Thanks for your attention!