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Y3: Superficial Radiotherapy > Clinical Electron Beams > Flashcards

Clinical Electron Beams Flashcards

1
Q

What are electrons?

A

A directly ionising particle that interacts with intra-atomic coulomb fields.

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2
Q

Briefly describe the cause of bremsstrahlung.

A

Inelastic collisions with nuclei.

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3
Q

Breifly describe the cause of ionisation and excitation of electrons.

A

Inelastic collisions with atomic electrons.

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4
Q

Briefly describe the cause of directional change and negligible energy loss of electrons.

A

Elastic scattering & soft collisions with atomic electrons.

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5
Q

What are the 4 basic electron interactions?

A

Excitation, Ionisation, Bremsstrahlung and Characteristic radiation.

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6
Q

What makes electrons clinically attractive?

A

They deposit dose superficially.

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7
Q

What is the equation for electron fluence?

A

Φ = dN / da

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8
Q

What is the equation for electron energy fluence?

A 
Ψ = dN / da * E(bar)
where E(bar) is the average incident electron energy
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9
Q

What is the electron path length?

A

Total distance travelled before coming to rest.

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10
Q

What is the electron range?

A

The sum of the individual path lengths in the original direction of travel.
Range < path length

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11
Q

Define range straggling.

A

Electrons with same initial energy travel to different depths due to different interaction histories. Higher energies exhibit more range straggling.

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12
Q

What is electron scatter dependent upon?

A

Atomic number of medium

Energy of the electron beam

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13
Q

What is the cause of the electron PDD shape?

A

Excitation and ionisation in the build up region:
Scattering results in increasing energy deposited in shallow layers of tissue = ‘Build up effect’.

Decrease after peak: Excitation and ionisation results in deposition of energy at shallow depths, electron beam loses energy, electrons reach end of range and PDD decreases rapidly.

Bremsstrahlung tail.

Increase energy – shallower gradient & surface dose increases due to more ion pairs created.
Lower energy = deviate path more = scatter = sharper drop off

Relative change from surface to depth = not as high for higher energy as less deflection.

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14
Q

Describe the features of electron isodose shapes.

A
  • Applicator size is greater than width of 80% isodose
  • Surface dose is dependent on beam energy, distance from applicator, use of cutouts
  • High isodose levels are constricted laterally at depth
  • Increased scatter at low electron energies results in low dose isodoses ‘bulging’ outwards
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15
Q

What is the advantage of using SXT over electrons?

A

SXT gives tight penumbras, electrons do not. This is better for OARs in tight proximity.

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16
Q

What literature is relevant to clinical use of electron beams?

A

ICRU 71: Prescribing, Recording and Reporting Electron Beam Therapy

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17
Q

How many reporting levels are specified in ICRU 71?

A

3

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18
Q

What is the criteria used to choose a reference point?

A
  • Dose at point clinically relevant
  • Point easy to define, unambiguous
  • Dose accurately determined
  • Point in region where there is no steep gradient

(generally chosen to be at Dmax but away from inhomogeneity & steep gradients).

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19
Q

What is the criteria used to choose a reference point in a single field?

A
  • At centre of PTV
  • On central axis of radiation beam
  • Preferably at level of peak dose

Also report dose to any OAR too

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20
Q

What is required to give dose data?

A

A CT scan.

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21
Q

Why is the 90% isodose usually chosen for prescribing instead of the 95% in electron therapy?

A

It is difficulat to encompass the PTV with the 95%, thus the 90% is used for prescribing but higher is accepted within the target.

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22
Q

Describe the 3 reporting levels for electron beam therapy.

A

Level 1:

  • Dose at ICRU reference point
  • Peak absorbed dose on CAX of beam
  • Max dose to PTV
  • Min dose to PTV
  • Usually based on measurements on beam axis.
  • ICRU reference point often the point of peak dose on CAX

Level 2 & 3:

  • Same as level 1
  • Dose to OARs (determined from DVH or dose distribution, which required TPS and accurate diagnostic information)
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23
Q

Define the therapeutic interval.

A

Distance between the selected isodose suitable for the purposes of the treatment.
Often 80% or 90% isodose
Increasing energy = increases range & interval. But lower energies are desired due to their steeper drop off.

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24
Q

What factors should be considered when doing clinical planning for electron beam therapy?

A
  • Therapeutic interval
  • Dose at ICRU Reference Point
  • Dmax and Dmin
  • Surface dose
  • Critical structures
  • Inhomogeneities
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25
Q

What happens to PDDs with increasing electron energy?

A
  • Surface dose increases
  • Depth of dose max increases
  • d50, d80 & Rp increase in depth
  • Gradient of fall-off decreases
  • X-ray contamination level increases
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26
Q

What is the effect of inhomogeneities on PDDs and dose distributions in electron beam therapy?

A

Inhomogeneities in medium impact on the scattering of the electrons in the medium. The change in scattering and hence the dose deposited depends on density and atomic composition of inhomogeneity.
Thus we need to consider dose in heterogeneity, beyond and adjacent of inhomogeneity.

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27
Q

What is the effect of bone on electron dose distributions?

A
  • increased attenuation
  • greater scattering per linear depth in medium
  • increase dose in bone
  • decreased dose beyond bone
  • increased dose adjacent bone

Density/Scatter does not change dose deposition for electrons as much as say SXT (due to dependence on Z).

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28
Q

What effect does high Z and high density have on electron dose distributions?

A

More electrons are scattered away from high density, high Z materials to low density, lower Z material.
This results in a increased electron fluence and therefore dose scatter lobes in the lower density or low Z material .

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29
Q

What is the effect of a small air cavity on an electron isodose distribution?

A
  • Decrease dose adjacent to cavity
  • Hot spot immediately beyond cavity
  • Increased dose beyond cavity
30
Q

What direction do electron isodoses run to an irregular surface?

A

Parallel to the surface

31
Q

What direction do electron isodoses run to a smooth surface but beam that is incident obliquely?

A

Parallel to surface but direction is tilted.

32
Q

What issues may an invaginated surface cause in electron beam therapy?

A

Hotspots may arise due to variation in densities.

33
Q

What can be used to modify surface dose and what effect does it have? (in both electron and photon therapy)

A

Bolus. It shifts the PDD closer to the surface (does not change the shape).

34
Q

What type of bolus is used?

A

Tissue equivalent material (wax, superflab, wet gauze)

35
Q

What may impact the choice of bollus thickness?

A
  • therapeutic interval may decrease

- depth of treatment decreases

36
Q

What are the two options available to increase the therapeutic ratio in electron therapy?

A

Increase the energy or use thin foils to increase scattered radiation and hence dose.

37
Q

What is the principle of using foils to increase the therapeutic ratio in electron therapy?

A

High Z foils (cerrobend) can produce the same angular scatter change as bolus but with less energy loss resulting in greater therapeutic interval. This is mor effective with lower energies.
Surface dose increases (like with bolus) but dose remains higher at depth because of lower energy loss - sort of ‘stretching’ of the PDD.

38
Q

What are the options for changing the shape of an electron field?

A 
Applicator
Cut out (standard or customised)
39
Q

What is the equation for the margin of electron fields?

A

margin = geometric edge - R85/2

40
Q

What is the effect of changing SSD in electron therapy?

A

Penumbra broadens, high dose region shrinks = increase field further = messy….

41
Q

What are the advantages of using lead cut outs for electon therapy?

A
  • Spares adjacent normal tissues
  • Dose homogeneity
  • Reduces penumbra width
  • Minimises effect of patient movement
42
Q

What is the equation for thickness of lead cut out in electron therapy?

A

Lead thickness (mm) should be at least = initial energy (MeV) / 2

43
Q

What is the equation for thickness of an alloy cut out in electron therapy?

A

Thickness of Cerrobend (mm) = Lead thickness x 1.2
where:
Lead thickness (mm) should be at least = initial energy (MeV) / 2

Often use a standard thickness of 1 cm.

44
Q

What is the impact of too thin a cut-out in electron therapy?

A

Electrons towards end of range
Depositing more energy per unit path length
Deposit high dose immediately under the cutout

45
Q

What is the effect of obliquities when using a cut-out in electron therapy?

A

Going through edges can result in higher skin doses below cut-out

46
Q

What is the definition of a small electron field?

A

The of the electron field is less than the range of the electron beam.

47
Q

What is the effect of a small electron field?

A
  • Dmax shifts towards the surface with reducing field size
  • Spectrum changes due to increased contribution of scatter to the measurement point on the CAX
  • Practical range remains unchanged
  • More electron scatter away form the central portion of the electron beam than inwards beyond Dmax.
48
Q

What is the effect on PDDs of a small electron field?

A
  • Little change in PDD between 4x4 and 30x30 cm field
  • Dmax shifts towards the surface as less dose contributing to central axis
  • Relative surface dose increases
  • Practical range remains unchanged
49
Q

Why do PDDs remain constant for field sizes larger than the practical range of the electron beam?

A

The electrons from the periphery of the field are not scattered sufficiently to contribute to the central axis depth dose.

50
Q

What is the purpose of internal shielding?

A

Protect underlying tissue (like pinna, lips, cheeks)

51
Q

What are the effects of using an internal shield?

A

Electrons are backscattered back to give a high dose adjacent to the surface of the high density material
Internal shield needs to sufficient thickness of material otherwise get some electron transmission with electrons at end of range and hence depositing high doses in tissue beyond the internal shield.

52
Q

What effect on backscatter does the energy at the internal shield have in electron therapy?

A

Lower energy = less backscatter

53
Q

What is the equation to calculate electron backscatter factor?

A

EBF (at shield) = 1 + 0.735 exp ( - 0.052 * E(s) )
EBF (at t) = exp ( - k * t )

where
k = 0.61 * E(s) ^ - 0.62
E(s) is electron energy at shield
t is distance from shield in mm

54
Q

What are the reasons of putting wax on either side of an internal shield?

A

Absorbed backscatter & health and safety (putting lead in someones mouth)

55
Q

What is the result of two abutting electron fields?

A

A hotspot at depth due to overlapping isodoses.

56
Q

What is the result of leaving a gap between two electron fields?

A

A coldspot at the surface.

57
Q

What technique is used to treat with abutting electron fields?

A

A spoiler is used to broaden the penumba, reduce energy and increase surface dose.
As the penubra is broadened, it is less essential to align the fields and gives a bit of uncertainty.

58
Q

What is the treatment area defined by in electron therapy?

A

The applicator or cut out

59
Q

What are the reference conditions for an electron calculation?

A

Fixed SSD: depends on applicator design and distance between applicator, and skin surface. Usually 100cm. with 5 cm between applicator-surface.
d(ref): depends on beam energy.
Reference field size: depends on applicator or cut out. Usually 10x10 applicator.
Full scatter conditions

Defined at Dmax, 10x10, 1 cGy/MU.

60
Q

What factor converts dose at reference conditions to dose at non-standard reference conditions?

A

(multiplied by) the output factor.

61
Q

What tis the output factor of electrons dependent upon?

A
  • Position of secondary collimators
  • Applicator (field size)
  • Cutout
  • Primary electron beam
  • Scattered electrons in linac head & applicator
  • Scattered dose in phantom
62
Q

How is the OPF for irregular fields determined?

A

Needs measuring directly.

63
Q

How does the OPF vary with applicator size and energy?

A

Increases with energy.

Increases with applicator size (approximately).

64
Q

What is the rule of thumb to calculate the depth at dose 90% for electrons?

A

d(90) = E(0) / 4

65
Q

What is the rule of thumb to calculate the depth at dose 80% for electrons?

A

d(80) = E(0) / 3

66
Q

What is the rule of thumb to calculate the depth at dose 50% for electrons?

A

d(50) = E(0) / 2.5

67
Q

What is the rule of thumb to calculate the practical range of electrons?

A

Rp = E(0) / 2

68
Q

What is the equation to find the energy at depth of electrons?

A

E¬(d) = E¬(0) * ( 1 - ( d / R(p) ) )
where
E¬(d) is mean energy at depth d
E¬(0) is mean initial energy

69
Q

Describe early TPS models for electrons.

A
  • Split broad beam into series of pencil beam kernels
  • Correctly predicted existence and position of perturbations in dose distributions resulting from inhomogeneities
  • Not necessarily magnitude of perturbation
  • Fermi-Eyges theorem predicted probability of electrons being found at a certain position in a material.
  • Dependent on the linear scattering power of the medium which is dependent on the mean energy of the electrons at each depth.
  • A Gaussian distribution of the electron fluence is modelled
  • It assumes infinite slabs of material
70
Q

Describe Fermi-Eyges theorem.

A
  • Probability of an electron being located at a depth z with displacement between x and x+dx and y + dy.
  • A Gaussian distribution describing the spatial distribution of the planar fluence of the electrons. (AKA A Gaussian that changes with depth)
  • Describes the increase in width of the distribution as z increases.
  • Model cannot account for electron loss
  • To obtain dose distribution need to use an empirical correction factor or weighting factor to weight the Gaussian distribution
  • Only valid for inhomogeneities of infinite width, therefore of limited clinical use.

THEREFORE USE MONTE CARLO!

71
Q

What information is required to create the correct electron therapy?

A
  • Depth required to cover
  • Margin required
  • Immobilisation
  • Previous RT
  • Dose prescription
  • Minimum dose to target

Y3: Superficial Radiotherapy (5 decks)

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