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