Linear Accelerators - Electrons Flashcards

1
Q

What are the main components in a linear accelerator?

A
Electron gun
Microwave generator
Modulator
Accelerator waveguide
Bending magnet
Head components
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2
Q

What are the head components in a linear accelerator in photon mode?

A
Primary collimators
Photon target
Photon monitor chamber
Mirror
Jaws
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3
Q

What are the head components in a linear accelerator in electron mode?

A
Primary collimators
Primary scattering foil
Secondary scattering foil
Electron chamber
Jaws
Applicator
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4
Q

Other than head components, what changes when changing from photon to electron mode on a linear accelerator?

A

Electron gun current is reduced

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5
Q

How does the electron beam profile change as it travels through the head of the linear accelerator?

A

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.

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6
Q

How does energy affect an electron beam PDD?

A
As energy increases:
Surface dose increases
Depth of Dmax increases
Fall off has a shallower gradient
The bremsstrahlung tail increases
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7
Q

What is the cause of the build up region in electrons?

A

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.

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8
Q

What correction is made for electrons in terms of distance away from the source?

A

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.

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9
Q

How can VSD be determined?

A

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).

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10
Q

What term is used to describe electron beam quality?

A

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. )

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11
Q

What is the equation to find z(ref) from R(50,d)?

A

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.

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12
Q

Why did the electron CoP change from using D(max) to using z(ref)?

A

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.

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13
Q

What are the features of a small electron field?

A
  • 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
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14
Q

How is R(50,d) determined? What equation is used?

A

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.
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15
Q

How are depth dose curves measured for electron beams according to the electron CoP?

A
  1. Ionisation Chamber:
  2. 1.Raw chamber reading gives ionisation curve
    1. At each depth correct the reading for:
      • Stopping Power ratios
      • Perturbation (negligible for some chambers. Parallel plate = 1, so ignore the factor)
      • Ion recombination
    1. Temperature and Pressure corrections unnecessary if phantom left sufficient time to attain equilibrium
    1. Remember to account for Effective Point Of Measurement.
  3. P-type diode:
  4. 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)
  5. 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.
  6. Film / other dosimetry systems:
    - Must validate against ionisation chamber
    - Scanner ‘issues’ – requires good knowledge of processing
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16
Q

What recommendations are made regarding phantoms in the elctron CoP?

A
  • Ideally water
  • Thick enough to allow measurement of Bremsstrahlung tail (thickness > 5 + 1.25 x R50,D (cm) )
  • Scan upwards (prevent rippling)
  • Minimum 5cm extra outside field edges at surface
  • Plastic phantoms allowed as know depth better (Epoxy Resin: WTE, or others recommended)
    - Polystyrene & PMMA result in larger uncertainty due to charge storage effects (trapped electrons).
    - Need validating against water phantom
17
Q

What correction can be made if the atomic number of an electron phantom is too low?

A

Add a small amount of higher atomic number. This will increase the scattering power.

18
Q

What equipment is used for the electron primary standard?

A

Calorimeter

19
Q

How does the electron primary standard work?

A

The calorimeter measures dose by measuring the temperature increase caused by incident radiation. (Actually they keep the temperature constant and monitor the settings required to keep it constant).

20
Q

What are the preferred chambers for electron secondary standards?

A
  • NE2571 cylindrical graphite Farmer (above 9MeV)
  • PTW 30004 cylindrical graphite Farmer (above 9MeV)
  • NACP parallel plate (large guard ring to stop scatter + large collecting volume)
  • Roos parallel plate (large guard ring to stop scatter + large collecting volume)

(used to have Markus but guard ring was too small, so scatter was large and lead to high readings)

21
Q

What is the calibration process for electrons at the NPL?

A
  1. define reference depth in water for beams at NPL
    R(50,D) = 1.029 * R(50,I) - 0.063 (cm)
  2. Use range scaling to get depth in graphite
    z(ref,g) = z(ref,w) * R(50,g) / R(50,w)
  3. Calibrate the chamber against the graphite calorimeter at NPL at z(ref,g)
    N(D,ref,g) = D(g) / M(ref,g)
  4. Theoretical conversion from graphite to water
    N(D,ref,w) = N(D,ref,g) * [ p(ref,w) / p(ref,g) ] * [ S(w,air) / S(g,air) ]
    where p is pertubation factor
  5. Compare user and reference chambers at NPL, at z(ref) in water
    N(D,user,w) = N(D,ref,w) * M(ref,w) / M(user,w)
22
Q

What should be done to obtain values that are not given by NPL, ie: if clinical beyond R(50d)?

A

Extrapolate the values (but larger uncertainties)

23
Q

What is the process for obtaining electron dose at D(max)?

A
  1. Use one of the designated chambers
  2. Use a suitable phantom
  3. Set up the phantom
  4. Set field size (as per standard output)
  5. Position chamber so EPOM is at reference depth, zref
    • Correct for non-standard density (if necessary)
    • Correct for non-water phantoms (if necessary)
  6. Take measurement and correct for:
    • ion recombination & polarity
    • temperature & pressure
    • electrometer (charge & linearity)
      M(ch,w) = M(raw) * f(T,P) * f(pol) * f(ion) * f(elec)
  7. Convert corrected reading to Dose (Gy):
    D(w) (zref,w) = M(ch,w) * N(D,w) * (R50,D)
  8. Repeat
  9. Transfer to dose at d(max) using depth dose curve
24
Q

What corrections should be made when calibrating an electron field instrument?

A

CoP states that you correct for polarity and ion recombination and warns about perturbation factors.
Also calibration coefficient:
Cf(field) = Dose / M(f)
where M(f) is corrected field reading for temp & pressure
But if field instrument is always used in the same setup as for the standard output check, then this is unnecessary.

25
Q

How is ion recombination obtained for electron equipment?

A

New chamber : Jaffe plot: Extrapolate back from a normalised 1/M vs 1/V plot
BUT NACP/Markus = not linear. High V = curves off, so can’t extrapolate back. Only use up to manufacturer recommended, departments tend to use highest V as High V = less recombination = smaller factor, but less accurate. If use small, then large factor but can extrapolate.

Dual voltage technique:
f(ion) – 1 = [ ( M(1) / M(2) ) - 1 ] / [ ( V(1) / V(2) ) - 1 ]
BUT this makes linear assumption (so do Jaffe to prove first)
Technique is only valid if:
- V1 is >= 3 x V2 (IAEA) and
- 1/M vs 1/V is linear over the range being considered
(Typically true of cylindrical chambers, but not for parallel plate)

Boagg formula:
Measuring dose per pulse (DPP):
 - Measure pulse interval and length using an oscilloscope 
 - Measure the dose for a set number of MU
From this you know:
- Pulse Repetition Frequency (PRF)
- dose
- number of MU
- time to deliver
26
Q

Why is it recommended not to use high V to minimise ion recombination?

A
  • Charge multiplication
  • Insulator charging
  • Changes in sensitive volume (large V may cause bend/shift in electrode/front window)
  • Introduces error - Better to accurately correct for ion recombination
27
Q

What is the equation provided in the electron certificate of calibration regarding ion recombination?

A

f(ion) = c +md
where d is dose per pulse
(this is done at fixed V)

28
Q

What is the correction made for temperature and pressure?

A

[ 1013.25 / P ] * [ (273.15 + T) / 293.15 ]
(or 1013.25 mBar = 760 mmHg)
Temperature should be temperature of the PHANTOM
A non-water phantom will reach temperature of the room; a water phantom will be a degree or so cooler
Pressure should be ambient air pressure – chamber should be open to air
The wall of any waterproofing should be thin enough to allow the chamber to achieve thermal equilibrium in about 5 minutes.
Cross cals = have equipment in room for hours prior.

29
Q

What is the correction made for polarity?

A

Take a reading with bias voltage applied (say -100V) M-
Reverse the polarity and wait at least ten minutes
Take a reading with reverse polarity, M+
fpol = [ | M(+) | + | M(-) | ] / [ 2 * M(-) ]

If polarity effect is insignificant it need not be applied, but it must be measured to determine if it is insignificant or not.
Polarity should be less than 0.5% for parallel plate chambers and up to 1% in Farmer chambers. If the polarity effect is more than a few percent then the chamber is not suitable for use in electron beam dosimetry.
Polarity effects may change sign with depth.

Photons = 1, electrons = close to 1.

Recomb & Pol varies with depth & field size.