Beam parameters Flashcards
What is the quality index of a photon beam?
TPR20,10
What is the beam quality specifier of an electron beam?
R_50,D
How can profiles be measured?
Using an array - e.g. MatriXX, IC profiler or using a water tank and scanning across with a suitable chamber
What parameters can be obtained from profiles?
Flatness
Symmetry
Field width
How is the beam quality physically measured?
Water tank; for photons at 20cm and 10cm deep (10cmx10cm field, 100cm SSD) and for electrons a profile is taken and the R_50,D found. The PDI needs to be converted to PDD or just convert I_50.
How is field width defined?
FWHM of the profile - lateral distance between profile lines at the 50% dose level
How is the penumbra region defined? Where does this come from?
The distance between the 80% and 20% dose on the profile.
IEC definition
What is beam flatness?
A measure of how beam strength varies across the central 80% of the FWHM.
If “horns” are seen in the profile, what are they due to?
The flattening filter (not seen in FFF linacs). The filter is designed such that the field is flat for reference conditions. because of the FF shape more photons are absorbed in the centre.
The FF hardens the beam, and gives a gradually increasing radial intensity.
What is beam symmetry?
Symmetry looks at the dose at equivalent points on either side of the beam and compares them within the 80% of FWHM.
What is the tolerance on flatness and symmetry according to IPEM 81? How does this compare to the IEC specification?
+/- 2% of reference value from IPEM 81.
IEC 2007 suggests 3% asymmetry.
What is an equation for flatness (there are multiple)?
F = 100*(Dmax - Dmin)/(Dmax + Dmin)
F = 100*Dmax/Dmin
What is the equation for symmetry?
S = Max(|Point L - Point R|)
What does beam flatness depend on?
Szie and shape of phantom, depth of measurement.
What part of the linac may be responsible for out-of-tolerance beam symmetry?
Most likely the ionisation chamber which monitors and feedbacks the symmetry.
The waveguide, electron gun, bending magnets and target could also all play a part in this
Why is a flattened beam NOT necessary, meaning that FFF machines are useful and becoming more common?
For treatment techniques like IMRT, the beam is deliberately modulated so that it is not flat; therefore it is not necessary to spend a long time ensuring that the beam is flat in the first place.
Are flatness and symmetry still used in FFF machines?
Yes, they are still useful to monitor beam energy and steering but require different interpretations.
