VERIFICATION OF THE RADIOTHERAPY
TREATMENT PROCESS
PART 1
TREATMENT PLANNING
SYSTEM VERIFICATION
KAREN P. DOPPKE MS.
AAPM 2003
Harvard Medical School
Massachusetts General Hospital
Department of Radiation Oncology
Boston, Massachusetts
CLINICAL TREATMENT
PLANNING PROCESS
1. Patient Positioning and Immobilization
2. Patient Contour & Image Acquisition
3. Anatomy Definitions
4. Beam Position and Characteristics
5. Dose Calculations
6. Plan Evaluation
7. Plan Implementation
8. Plan Review
Verification of Beam and Plan Data
Initial Testing – Commissioning / Basic Data Entry
Beam Fitting and Display
Machine Settings – Match Planning System to Machine
Non-Dosimetric Import / Export Data
Geometry – Beam Modifiers / Block / MLC / Bolus / Scales
DRR Projections/ Beam Display on Patient
Treatment Planning – Testing and comparing plans from
previous 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
Measurement Equipment for Verification
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
Physicist Review of Planning
System 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
Correction Based Algorithms
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.
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 that
the 3 mm goal for penumbra was satisfied for most points.
Ref: Masterson 1991
Physics-Model Based Algorithms
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.
Monte Carlo Photon Dose Calculations
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.
Electron Dose Calculation Algorithms
Pencil Beam Modals
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 Calculations
Monte Carlo calculations
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.
Regions for Dose Verification
1. Inner Beam - high Dose
2. Penumbral Region - 5 mm in & out beam/block edge
3. Outside Beam Edge - beyond Penumbra
4. Buildup Region - in & outside of beam
5. Central Axis - depth dose
6. Absolute dose - normalization point
Dose Criteria - Inner Region-High Dose
Homogeneous / Inhomogeneous
Van Dyk (1993)
3% / 3%
TG 53 (1998)
1-2%
3%
1.5
5%
7%
SGSMP (1999)
2% - 2 mm
Open Fields
Modified Fields (MLC/Blocks/Asym)
Extended SSD
Inhomogeneous
Anthropomorphic
Penumbral Region
Homogeneous / Inhomogeneous
Van Dyk (1993)
4 mm
TG 53 (1998)
2 mm / 5mm
Wedge / MLC
3 mm
Anthropomorphic
7 mm
SGSMP (1999)
2 mm
Outside Beam / Low Dose Gradient
Homogenous / Inhomogeneous
Van Dyke ( 1993)
3%
TG 53 (1998)
2%/5%
3%
5%
7%
SCSMP (1999)
2% / 3%
Anthropomorphic
3%
Open
Block
Wedge/MLC/Bolus
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)
SGSMP (1999)
1-2% / 3%
5%
Anthropomorphic
2%
4%
Anthropomorphic
Absolute Dose-Normalization Point
Van Dyke (1993)
1%-2%
TD 53 (1998)
0.5%
1%
2%
3%
5%
Open
Block / MLC / Asym
Wedge
Inhomogeneous
Anthropomorphic
SGSMP (1999)
2%
3%
4%
Inhomogeneous
Anthropomorphic
Summary
Generally 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 Location
Collimator Settings and Display
Aperture Definition and Display
Gantry, 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 grid
Accuracy of segmentation
Bin size of dose volume histogram
7. Plan Implementation / Plan Verification
8. Plan Review
Large Errors Possible !
Failure to observe planner error !
References