VERIFICATION OF THE RADIOTHERAPY TREATMENT PROCESS
PART 1
TREATMENT PLANNINGSYSTEM VERIFICATION
KAREN P. DOPPKE MS.
AAPM 2003
Harvard Medical SchoolMassachusetts General HospitalDepartment of Radiation OncologyBoston, Massachusetts
CLINICAL TREATMENTPLANNING PROCESS
1. Patient Positioning and Immobilization2. Patient Contour & Image Acquisition3. Anatomy Definitions4. Beam Position and Characteristics5. Dose Calculations6. Plan Evaluation7. Plan Implementation8. Plan Review
Verification of Beam and Plan Data
Initial Testing – Commissioning / Basic Data EntryBeam Fitting and Display
Machine Settings – Match Planning System to MachineNon-Dosimetric Import / Export Data
Geometry – Beam Modifiers / Block / MLC / Bolus / ScalesDRR Projections/ Beam Display on Patient
Treatment Planning – Testing and comparing plans fromprevious system or standard plans
Measurements – Prediction of Dose by Planning System
Evaluation – Testing of System with 3-4 Study Patients Test the Planning Process from CT to Treatment
Computer controlled water phantom
Solid phantom for both chamber and film measurements
Solid phantom above with various densities - CT scanned
Small chamber or diode for measurements of small fields
Anthropomorphic Phantom if available
Possible film densitometry needed
Measurement Equipment for Verification
Physicist Review of PlanningSystem Requirements
Basic Data Requirements - New beam Data Required
Data Transfer Requirements – CT, MRI,PET, Devices
Dose Algorithm Review – Vendor Information a Must
Plan Verification Measurements
Depends on the interpolation of measured % Depth Dose.
Beam profile data for large number of depths.
Uses attenuation for devices and inverse-square corrections.
Scattering is based on field area or a “Clarkson Type” calculation.
Dose calculations are usually only in plane of calculation.
Effective path length corrections for tissue density.
Missing tissue and secondary electrons are not considered.
Correction Based Algorithms
Open Field Normal Incidence
The data from the photon working group indicated that the SD for all data points was ~ 3%.
Ref: Masterson 1991
Oblique Incidence Verification Test
Photon working group indicated that for oblique incidence and missing tissue measurements that ~ 27% points did not meet the FOM of 3% or 3 mm.
Ref: Masterson 1991
Histograms for Field/Block Edge
Histograms from the photon working group indicate thatthe 3 mm goal for penumbra was satisfied for most points.
Ref: Masterson 1991
Goal is the simulation of radiation field interactions.
Treatment beam characteristics are determined from specific energy spectra for energy of the beam usually calculated by Monte Carlo simulations.
Prediction of primary interactions and the transport of the secondary photons and electrons from the interaction site
Uses Monte Carlo calculations to generated energy deposition kernels.
Data needed for validation beam profiles, depth dose data and output factors.
Physics-Model Based Algorithms
New developments in methods to decreased the time required dose calculations on CT patient data sets makes Monte Carlo calculations for treatment planning possible.
Monte Carlo dose calculation code available as an addition to commercial treatment planning systems.
Monte Carlo Photon Dose Calculations
Measurement of % Depth Dose data required.
Angular divergence of the beam determined by the final collimator.
The angular scattering is assumed to be Gaussian.
Calculations in most commercial planning systems are not 3D.
Dose calculations that include bone and air cavities need to be reviewed carefully.
Monitor Unit calculations for irregular fields from planning systems?
Electron Dose Calculation AlgorithmsPencil Beam Modals
The development of electron dose calculations using Monte Carlo techniques has been reported and have also demonstrated good agreement with measured data.
Expected to provide more accurate dose calculations.
Will be implemented in commercial planning systems.
Electron Dose CalculationsMonte Carlo calculations
Regions for Dose Verification
1. Inner Beam - high Dose
2. Penumbral Region - 5 mm in & out beam/block edge3. Outside Beam Edge - beyond Penumbra
4. Buildup Region - in & outside of beam
5. Central Axis - depth dose6. Absolute dose - normalization point
Dose Criteria - Inner Region-High Dose
Homogeneous / Inhomogeneous
Van Dyk (1993) 3% / 3%
TG 53 (1998) 1-2% Open Fields3% Modified Fields (MLC/Blocks/Asym)1.5 Extended SSD5% Inhomogeneous7% Anthropomorphic
SGSMP (1999) 2% - 2 mm
Penumbral Region
Homogeneous / Inhomogeneous
Van Dyk (1993) 4 mm
TG 53 (1998) 2 mm / 5mm3 mm Wedge / MLC7 mm Anthropomorphic
SGSMP (1999) 2 mm
Outside Beam / Low Dose Gradient
Homogenous / Inhomogeneous
Van Dyke ( 1993) 3%
TG 53 (1998) 2%/5% Open3% Block5% Wedge/MLC/Bolus7% Anthropomorphic
SCSMP (1999) 2% / 3%3% Anthropomorphic
Buildup Region
Homogeneous / Inhomogeneous
Van Dyke (1993) 4 mm
TG 53 (1998) 20%-50%
SGSMP (1999) 3 mm
Central Axis / Depth Dose
Homogeneous / Inhomogeneous
Van Dyke (1993) 2% / 3%4% Anthropomorphic
TG 53 (1998) 1-2% / 3%5% Anthropomorphic
SGSMP (1999) 2%4% Anthropomorphic
Absolute Dose-Normalization Point
Van Dyke (1993) 1%-2%
TD 53 (1998) 0.5% Open1% Block / MLC / Asym2% Wedge3% Inhomogeneous5% Anthropomorphic
SGSMP (1999) 2%3% Inhomogeneous4% Anthropomorphic
SummaryGenerally Achievable Tolerances
1. Patient Position and Immobilization 0.5 cm
2. Patient Contour and Image Acquisition 0.1 cm
3. Anatomy Definitions including Fusion 0.2 to 0.5 cm
4. Beam Position and Characteristics 0.1 cm &<10
Beam LocationCollimator Settings and DisplayAperture Definition and DisplayGantry, Collimator and Table
Generally Achievable Tolerances
5. Dose calculations
Dose central 80% of beam central axis slice 1%
Dose central 80% of beam non-axial slice 1%
Dose in penumbra (80% to 20%) open 1-5 mm
Dose to central point in blocked field ~2%
Dose under block / MLC 2%
Generally Achievable Tolerances
6. Plan Evaluation 2-5%
Depends on dose calculation gridAccuracy of segmentationBin size of dose volume histogram
7. Plan Implementation / Plan Verification
8. Plan ReviewLarge Errors Possible !Failure to observe planner error !
References
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