Linear Accelerators - Electrons Flashcards
What are the main components in a linear accelerator?
Electron gun Microwave generator Modulator Accelerator waveguide Bending magnet Head components
What are the head components in a linear accelerator in photon mode?
Primary collimators Photon target Photon monitor chamber Mirror Jaws
What are the head components in a linear accelerator in electron mode?
Primary collimators Primary scattering foil Secondary scattering foil Electron chamber Jaws Applicator
Other than head components, what changes when changing from photon to electron mode on a linear accelerator?
Electron gun current is reduced
How does the electron beam profile change as it travels through the head of the linear accelerator?
It enters the head as a pencil beam
After the first scattering foil, it had a broader shape but still thin, it appears as a Gaussian profile.
After the second scattering foil, it broadens further but still is not flat, it appears as a broader Gaussian profile.
After exiting the applicator, it is a flat and symmetric beam with sharp penumbra.
How does energy affect an electron beam PDD?
As energy increases: Surface dose increases Depth of Dmax increases Fall off has a shallower gradient The bremsstrahlung tail increases
What is the cause of the build up region in electrons?
The build-up region in an electron PDD occurs because as you go deeper the paths of the electrons become more oblique. As a result there are more interactions per unit depth. Eventually the electrons are scattered such that they are moving in all directions and the dose begins to decrease.
This is in contrast to photon build-up where the build-up region occurs at the depth corresponding to the range of the secondary electrons, after this there is no more overlap and the dose begins to decrease.
What correction is made for electrons in terms of distance away from the source?
VSD: Virtual Source Distance
( I(0) / I(g) ) ^ 1/2 = [ g / ( f + d ) ] + 1
where I0 is the reading at 100cm SSD at depth d and Ig is the reading with gap g at same depth. f is the VSD.
How can VSD be determined?
The ionisation chamber is placed in air and the 1/SQRT(current) is plotted against distance from 100cm SSD. The graph will intercept 0 at f (the VSD).
What term is used to describe electron beam quality?
R(50,d).
The depth on the CAX in water at which the absorbed dose is 50% of the maximum vaolue at 100cm in a large field. (The field size is large enough when an electron from the edge does not travel to the central axis to contribute to the dose. )
What is the equation to find z(ref) from R(50,d)?
z(ref) = 0.6 * R(50,d) - 0.1cm
The depth must take into account the effective point of measurement.
Corrections to obtain water equivalent depth of chamber window (correct for density of chamber) must be applied.
Why did the electron CoP change from using D(max) to using z(ref)?
Need consistency nationally (ideally internationally), issue with Dmax: different manufacturers have different head arrangements causing different scatter which can lead to diffs in Dmax.
Hence different manufacturers have different surface doses & bremsstrahlung tails.
R50D are more consistent between manufacturers. Zref should be similar as at this depth you get consistency between stopping power ratios between different clinical beams, and so more consistency between manufacturers.
What are the features of a small electron field?
- Lateral equilibrium is lost
- Penumbras can overlap
- R50,D defined in a large field is STILL the parameter defining the beam as a whole
- Stopping power ratios are the same
How is R(50,d) determined? What equation is used?
Measure PDD, using ion chamber.
This measures R(50,I) : the CAX depth in water at which the measured ionisation is 50% of the maximum value.
Ion chamber needs converting to dose (measures ionisation), so multiply by stopping power ratios to get dose.
R(50,D) = 1.029 * R(50,I) - 0.063 (cm) where R(50,I) is between 2-10cm and 2-24MeV.
How are depth dose curves measured for electron beams according to the electron CoP?
- Ionisation Chamber:
- 1.Raw chamber reading gives ionisation curve
- At each depth correct the reading for:
- Stopping Power ratios
- Perturbation (negligible for some chambers. Parallel plate = 1, so ignore the factor)
- Ion recombination
- At each depth correct the reading for:
- Temperature and Pressure corrections unnecessary if phantom left sufficient time to attain equilibrium
- Remember to account for Effective Point Of Measurement.
- P-type diode:
- 1 Depth dose can be measured directly, HOWEVER:
- Must validate against ionisation chamber
- Must NOT use a shielded photon diode (as high Z shield next to sensitive volume shields it from low energy scattered photons = unsuitable for electons)
- 2 The stopping power ratios of silicon to water vary little with electron energy (and therefore depth) as opposed to the stopping power ratios of air to water.
- Film / other dosimetry systems:
- Must validate against ionisation chamber
- Scanner ‘issues’ – requires good knowledge of processing