Medical Accelerator

F. Foppiano, M.G. Pia, M. Piergentili

M. Piergentili Genoa 8 March 2004

Problem Statement

• build a simulation tool which determines the dose distributions given in a phantom by the head of a linear accelerator used for IMRT.

• Many algorithms were developed to estimate dose distributions, but the most sophisticated ones resort to some approximations too. These approximations might affect the outcome of dose calculation, especially in a complex treatment planning as IMRT.

step and shoot

User Descriptions

• Oncology is the main  utilization field of radiotherapy.

• The goal of radiotherapy is delivering the required therapeutic dose to the tumor area with high precision, while preserving the surrounding healthy tissue

• Accurate dosimetry is at the basis of radiotherapy treatment planning

• Tipical user of this product is a medical physicist who have to make a treatment planning and needs to verify the distribution dose released by the beam. 

Phases of Treatment planning • to acquire patient's data • to position and to immobilize the patient • to acquire the anatomy of the target • to set the beam • to calculate the distribution dose and

the length (of time) of the treatment • This simulation is used in the last part of

treatment planning (dose verification).

Varian Clinac 2100

•Flattening filter serves to homogenize the photon beam

•Each pair of jaws can be rotated through an axis that is perpendicular to the beam axis

•Details regarding the exact composition and shape of all these objects are still incomplete (they will arrive soon)

Intensity Modulated Radiation Therapy

IMRT generates tightly conforming dose

distributions.

This microscopic control allows IMRT to produce dose

distribution patterns that are much closer to the desired patterns than possible previously

User Requirements

• Rigorous software process• OO DesignOO Design• Geometry ModelingGeometry Modeling• Select physics processesSelect physics processes• Dosimetric analysisDosimetric analysis• User InterfaceUser Interface

Specific User Requirements

1.Geometry

UR 1.1 The phantom will correspond to an available's one on the market

UR 1.2 The user shall be able to change the position of the collimators jaws x and y

UR 1.3 The user shall be able to change the configuration of the MLC (selecting the distances of the leaves from the central axis source-isocentre) 

3. Primary particles

UR 3.1 The user shall be able to define the mean energy and standard deviation of the electrons delivered by the head;

4. Physical processes

UR 4.1 The user shall be able to define the physical processes involved for e-, e+, gamma

5. Detector

UR 5.1 The Phantom is the detector;

UR 5.2 The information is the energy deposit due to primary and secondary particles.

Specific User Requirements6. Events

UR 6.1 The user shall be able to retrieve information about  the energy deposit due to the primary particle delivered by the gantry and all the secondary particles generated.

7. Visualization

UR 7.1 The user shall be able to visualize:

• the experimental set-up

• the tracks of the particles.

• the isodose plots.

• the PDD (Percent Depth Dose)

• the flatness

8. GUI

UR 8.1 There will be a section in which the user can be able to select the phantom's characteristics.   

UR 8.3 There will be a section in which the user can be able to select the beam's characteristics.   

UR 8.4 There will be a section in which the user can be able to select the configurations of the collimators 

Specific User Requirements9. Analysis

UR 9.1 The user shall be able to store the information about the primary particles energy.

UR 9.2 The user shall be able to store the information about the energy deposit in the phantom.  

UR 9.3 The user shall be able to calculate the isodoses.

UR 9.4 The user shall be able to calculate the PDD. 

UR 9.5 The user shall be able to calculate the flatness.

Specific requirements: constraint requirements

UR A.1 The system should work on the following platforms:

oLinux;

oWindows.

What has been done?

The user can choose the energy and standard deviation of the primary particles energy distribution (Gaussian)

The primary particles (e-) leave from a point source with random direction (0˚< θ < 3˚)

The head components modeled include: the target, primary and secondary collimators, the flattening filter, the mirror and the air

The flattening filter is modeled as a cone

What has been done?

Physical processes:

• Multiple scattering • Bremsstrahlung• Ionisation• Annihilation• Photoelectric effect • Compton scattering • Rayleigh effect• gamma conversion

Depth and transverse dose distributions are measured in a water phantom

What’s Next

•Real shape and dimensions of the components

•Monitor chamber

•Multi Leaf Collimator

•tests

•Comparison with experimental results (exp measurements will be taken at IST)

Possible improvements

To simulate the ionisation chamber inside the water phantom

Reduce Calculation time

Graphical user interface

Treatment planning:

•CT interface (to insert the geometry of the patient inside the simulation)•Inverse planning (we state our clinical objectives mathematically and let the IMRT optimisation process determine the beam parameters that will lead to the desired solution, these objectives should not be unrealistic)