Homogeneity Corrections Flashcards
What is the deviation in treatment dose for electrons and photons from what is calculated in planning?
Electron - 5% deviation to what is calculated
Photon - up to 3% variation
What other factors can affect treatment delivery accuracy?
Natural: bone, lung, teeth, air cavities, and small intercalated spaces within bone
Man-made: mandibular plates for reconstruction, hip, leg, and arm prostheses, spinal cord fixation devices, surgical rods, stents, and various dental filling, tracheostomy tubes, fiducial markers, barium/intravenous contrast agents, breast expanders/implants
How to account for tissue heterogeneity?
- Using the CT density file
- Using manual “Density Overrides”- more preferred
Where to place the dose reference point
Soft tissue, reduced movement
Parts of hip prothesis
Acetabular and femoral stem component (could be made of different alloys), hence controured separately as different structures
Can be made with metal, plastic or ceramic
Considerations for hip prothesis
- Modify the beam orientation to avoid device even if the result is to give additional dose to critical structures adjacent to the planning target volume.
- Account for the presence of the device by using computer treatment planning programs
Physical considerations
Increased dose noticed following attenuative structure and reduced dose distal to attenuation
For energies greater than 10MV, there may be an increased dose distal to the high Z material that is attributed to pair-production interactions in the metal
How are CT-ED generated
CT Density file is generated by physicists
A tissue characterisation phantom is used with a CT Scanner to establish the relationship between electron density of various tissue and their corresponding HU
Electron vs Physical densities
Physical - mass density, g/cm3
Pinnacle uses relative electron density or physical density
Monaco uses electron density
Eclipse - limited CT-ED file
