Clinical Electron Beams Flashcards
What is changed between a linac in its photon mode or electron mode?
The electron gun current is reduced to significantly decrease the dose rate.
A primary and secondary scattering foil is positioned in the head to spread out the beam and flatten the beam, respectively.
How are electron beams collimated and why?
Electrons are collimated with applicators added to the head of the linac.
These are necessary as MLCs and jaws are not useful due to the significant scattering of the electrons in air. Therefore, the applications should be positioned as close as possible to the surface of the patient.
How can electron energy be characterised?
Possible metrics are:
1) Maximum energy, E_{m}
2) Probable energy, E_{p}
3) Mean energy
4) Energy spread
Note that mean energy and E_{p} will decrease with depth, while energy spread will increase.
How does an electron PDD change with increasing energy?
1) D_{max} will increase with energy.
2) The relative surface dose increases with energy (note that surface doses from electron beams is significantly greater than observed with photon beams).
3) The contribution from the bremsstrahlung tail increases.
What causes the build-up region for electron beams? And how is this different than with photon beams?
The deeper into the medium, the more oblique the path of the electrons. This means that there are more interactions per unit length. This results in a build-up of dose at depth.
Photon beams have a build-up region resulting from the generation of secondary charge carriers, which propogate the energy from the interaction site.
How is SDD defined for electron beams?
The SDD is defined as the position at which the electron beam appears to diverge from.
This is the result of scatter within the head of the linac. Therefore, the electrons appear to come from a different position than observed with photon beams.
How is the Virtual source distance (VSD) determined?
Solve the following for f:
( I_{0] / I_{g} )^{-1/2} = (g / [f+d] )+1,
where I_{0} and I_{g} are two beam intensities for beams with different SSDs, d is a constant representing the measurement depth, g is the different between the two SSDs used, and f is the VSD.
Note that the VSD will vary between linac models and is head specific.
What is an alternative method for determining VSD, other than simply using two SDD measurements?
Measuring the differences in beam intensities for a number of different SSDs, and plotting the square root of their ratios versus the difference in the SSDs will produce a linear scatter plot. The gradient of the resultant relationship is equal to 1/(VSD + d), where d is the measurement depth.
This can be solved for VSD.
[Alternatively, plot the square root of the ionisation current against the distance from a 100 cm SSD and the x-axis intercept will be the VSD.]
How is electron beam quality defined?
R_{50,D} - that is the depth in water where the adsorbed dose has fallen to half its maximum for a 100 cm SSD beam.
How is the reference depth, z_{ref}, for electron beams defined?
z_{ref} = 0.6 R_{D,50} - 0.1, [units of cm]
where R_{D,50} is the depth at which the beam quality is defined.
Why is R_{100,D} not used as the beam quality for electron beams?
The difference between different linac models at higher energies becomes apparent at depth of maximum dose causing in inconsistencies. Furthermore, the PDD is relatively flat about R_{100, D}; increasing uncertainty.
Why is the definition of z_{ref} used? (electron beams)
This quantity has been found return a correspondence between the stopping powers and R_{50,D} over a range of manufacturers.
How is R_{50,D} determined? (electron beams)
R_{50, I} is measured with an ion chamber.
R_{50,D} = 1.029 R_{50,I} - 0.063 [units of cm]
How is a PDD measured for an electron beam?
1) Measure the raw PDD with an ion chamber.
2) account for the change in stopping powers with changing depth, and apply the appropriate factor to convert to absorbed dose.
3) apply perturbation and ion recombination corrections.
[Alternatively, a suitably commissioned diode may be used.]
What is the empirical formula for determining whether a phantom is large enough to measure the bremsstrahlung tail for an electron beam?
= 5 + 1.25 R_{50,I} [units in cm]
