Computer Planning Process Flashcards
Why is the slice thickness important?
Smaller slice thickness allows for better resolution of the digitally reconstructed images (2-3mm typically).
For example if we used 5mm for the Lens of the eye, you wouldn’t get the full contour and therefore dose calculations (DVH) would be incorrect
How can the contrast impact upon the planning process?
If the substance is iodine based. It can change the relative electron density of the structure and effect the dose distribution.
Changing the relative electron density will change the monitor units used for patients
Performing one scan for with contrast (for contouring/outline of structures) and another for dose calculations for accuracy
What is DICOM? what is it used for?
Images are transferred to the TPS via DICOM (Digital Imaging and Communications in
Medicine)
This is a file format that is recognised by all medical devices
What does DICOM allow transfer of?
- RT structures
- Dose
- Treatmentparameters
What affects isocentre placement?
PTV Location
PTV size and shape
Why is important to scroll through the entire CT data set?
- Check for differing patient anatomy
- Artefacts
- Check the data set orientation
- patient positioning
- Slice thickness
Why is the dose grid size important?
IT represents the region where the TPS calculates the dose. Anything outside this grid will not be taken into account.
You can change the pixel size. If you make the pixel size small this improves the resolution. However this takes longer to calculate.
What is laser localisation?
Point were vertical lines and transverse lines that traverses the 3 BBs intersect. Shifts are made from the laser localisation point to the isocentre.
Why is the couch removed on pinnacle?
Ensures any structures below this are excluded in the dose calibration. Couch removal is a limitation of Pinnacle due to the carbon fibre treatment couch having effects on beam attenuation and patient skin reactions (Tait, 2017)
What are the dosimetric effects of external devices (e.g the couch)?
If we don’t take the couch into consideration our dose distributions and calculations are essentially incorrect. This can have negative impacts for the patient.
There is potential overdose from the beam NOT passing through the couch, and potential under dose i areas passing through the couch.
Can external devices cause increased ____?
Surface dose
Why dose the couch cause an increased surface dose?
As the couch top or external devices are an additional object in the path of the beam the beam will be attenuated. This can lower the depth of Dmax and consequently increase the surface dose which can result in negative skin reactions
Why should couches be included in the TPS?
-Couch tops affect the dose distribution.
• Different TPSs handle structures external to the patient
contour differently.
• Some completely ignore these structures while others do not if the density is above a certain threshold.
What is an example of a TPS that supports the addition of the treatment couch?
Monaco
This TPS supports the automatic incorporation of patient support devices.
How are treatment couches implemented into the TPS?
The couch top structure includes as separate entities, the rails, the couch shell, and the interior of the couch top.
• The CT number for each of these components can be assigned.
• During commissioning, the medical physicist should verify the accuracy of these structures in the TPS by comparing measurements to dose calculations and adjusting the CT number assignments to optimize the agreement
