Linear Accelerators - Electrons

Question 1

What are the main components in a linear accelerator?

Answer 1

Electron gun
Microwave generator
Modulator
Accelerator waveguide
Bending magnet
Head components

Question 2

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

Answer 2

Primary collimators
Photon target
Photon monitor chamber
Mirror
Jaws

Question 3

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

Answer 3

Primary collimators
Primary scattering foil
Secondary scattering foil
Electron chamber
Jaws
Applicator

Question 4

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

Answer 4

Electron gun current is reduced

Question 5

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

Answer 5

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.

Question 6

How does energy affect an electron beam PDD?

Answer 6

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

Question 7

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

Answer 7

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.

Question 8

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

Answer 8

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.

Question 9

How can VSD be determined?

Answer 9

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).

Question 10

What term is used to describe electron beam quality?

Answer 10

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. )

Question 11

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

Answer 11

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.

Question 12

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

Answer 12

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.

Question 13

What are the features of a small electron field?

Answer 13

• 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

Question 14

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

Answer 14

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.

Question 15

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

Answer 15

1. Ionisation Chamber:
2. 1.Raw chamber reading gives ionisation curve
3. 2. At each depth correct the reading for:
      
      • Stopping Power ratios
      • Perturbation (negligible for some chambers. Parallel plate = 1, so ignore the factor)
      • Ion recombination
4. 3. Temperature and Pressure corrections unnecessary if phantom left sufficient time to attain equilibrium
5. 4. Remember to account for Effective Point Of Measurement.
6. P-type diode:
7. 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)
8. 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.
9. Film / other dosimetry systems:
   - Must validate against ionisation chamber
   - Scanner ‘issues’ – requires good knowledge of processing

Question 16

What recommendations are made regarding phantoms in the elctron CoP?

Answer 16

• 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

Question 17

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

Answer 17

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

Question 18

What equipment is used for the electron primary standard?

Answer 18

Calorimeter

Question 19

How does the electron primary standard work?

Answer 19

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).

Question 20

What are the preferred chambers for electron secondary standards?

Answer 20

• 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)

Question 21

What is the calibration process for electrons at the NPL?

Answer 21

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)

Question 22

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

Answer 22

Extrapolate the values (but larger uncertainties)

Question 23

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

Answer 23

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

Question 24

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

Answer 24

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.

Question 25

How is ion recombination obtained for electron equipment?

Answer 25

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

Question 26

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

Answer 26

• 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

Question 27

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

Answer 27

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

Question 28

What is the correction made for temperature and pressure?

Answer 28

[ 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.

Question 29

What is the correction made for polarity?

Answer 29

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.