Simulation of the radiotherapy process

 

Conventional simulation

 
During simulation the defined treatment set-up, which is prepared on the TPS, will be simulated on the simulator using the light field projection and the radiographic or fluoroscopic modes provided by the conventional simulator. This process will help to mimic the radiation therapy beams in terms of divergence. The final configuration will be recorded permanently on radiographic film or digitally store in DICOM file.

 
The simulator is a machine that emulates the geometry and the movements of the treatment unit but diagnostic quality x-rays instead of high-energy treatment rays.
The photons produced by the X ray tube are in the kilo voltage range and are preferentially attenuated by higher Z materials such as bone through photoelectric interactions. The result is a high quality diagnostic radiograph with limited soft tissue contrast but with excellent visualization of bony landmarks and high Z contrast agents.
The groups of human tissues absorb the x-rays in a different way:
- bone – higher Z materials with excellent absorption, very good representation
- soft-tissue – small atomic number, small absorption, limited contrast
- fat tissue – radiation-absorbing capability is less than bone, limited contrast like soft-tissue
- air-gas: small density, small absorption, good contrast

The parts of conventional simulator:

- high-voltage x-ray generator
- x-ray tube
- collimator system
- image processing system
- treatment couch
- control panel
- image intensifier

 

Exposure factors

The x- ray beam quality and intensity can be modified with modification of exposure factors:
- kV: tube voltage, the X-ray hardness and intensity is determined by tube voltage.
- mAs: is proportional with the number of charged particles interacting with matter of anode. The value of mAs has influence on the film density, which has effect on the contrast of image.

Operations in the simulator:

- Patient set-up
- Patient fixation using the different type of devices
- Determination of beam geometry
- Determination of field geometry and isocentre
- Acquisition of contour
- Acquisition of BEV and set-up DRR
- Marking of patient
- The simulator images are stored in digital form or in x-ray film, if we use the film, than we need the film developers.

CT-simulation

 
CT-simulator – dedicated CT scanner for use in radiotherapy treatment simulation and planning. The CT scanner has large bore (opening up to 85 cm), room lasers, including a movable sagittal laser for patient positioning and marking and flat table top and special software for virtual simulation.

 

Operations in the CT simulator:

- Patient set-up
- Patient fixation using the different type of devices
- Determination and marking of isocentre
- Acquisition of CT data and transfer to simulation workstation
- Contouring of targets and critical structures
- Determination of beam geometry
- Determination of field geometry and shielding
- Transfer of CT and beam data to the treatment planning system
- Acquisition of BEV and set-up DRR

 

Literature

 
1. Dr. Bogner Péter: Képalkotás eszközei, avagy az orvosi képalkotás fizikája
2. Duliskovich Tibor: Digitális röntgen detektor technológiák
3. E.B.Podgorsak: Radiation Oncology Physics: A Handbook for Teachers and Students, IAEA, Vienna, 2005.

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