1. Chapter 14 Electron Beam Therapy (1)

2. Electron Interactions
Coulomb force interactions
Inelastic collisions with atomic electrons (ionization and excitation)
Inelastic collisions with nuclei (bremsstrahlung)
Elastic collisions with atomic electrons
Elastic collisions with nuclei
Low Z  ionization
High Z  bremsstrahlung
Ionization
Excitation
Bremsstrahlung

3. Delta-ray cutoff energy = D Bremsstrahlung (Radiation)
Stopping Power and LET
Delta-ray cutoff energy = D
Ionization
Bremsstrahlung (Radiation)
S =
= Scol + Srad
= collisional SP + radiative SP
Scol = Scol,<D + Scol,>D
LET = Scol,<D
LET (linear energy transfer)
= restricted collisional stopping power

4. Collisional Losses Depending on the electron density of the medium
ZMass stopping power (S/, MeVcm2/g)
Z  electrons/g 
Z  tightly bound electrons 
Fig 14.1
1 MeV,
the energy loss rate in water  2 MeV/cm

5. Radiation Losses Energy loss/cm  Ee, Z2
The probability of radiation loss relative to the collisional loss  Ee, Z

6. Polarization Polarization of the condensed medium
A high E electron loses more energy (J/g·cm2) in a gas than in a more dense medium.
Atoms close to the electron track screen those remote from the track.
The ratio of S/ of water to air varies with Ee
the dose conversion factor for an air ionization chamber in water varies with depth.

7. Absorbed Dose The “restricted” collision stopping power
= the linear energy transfer (LET)
=energy loss per unit path length in collisions in which energy is “locally” absorbed.

8. Energy Specification
Almost monoenergetic before striking the collimation
A spectrum of energies at the phantom surface
Usually characterized by the energy at the phantom surface
Measurement of threshold energy for nuclear reactions
Range measurements
The measurement of Cerenkov radiation threshold

9. Most Probable Energy (Ep)0 = C1 + C2Rp + C3Rp2
(Ep)0 the most probable energy at the phantom surface
Rp the practical range in centimeters
For water, C1=0.22 MeV, C2=1.98 MeV cm-1,
C3= MeV cm-2
Rp is the depth of the point where the tangent to the descending linear portion of the curve intersects the extrapolated background.

10. Mean Energy Energy at Depth for water, C4= 2.33 MeV
The most probable energy and the mean energy of the spectrum decreases linearly with depth.
It is important in dosimetry to know the mean electron energy at the location of the chamber.
Harder’s equation

11. Output Calibration (1) Ion Chamber Phantom
Plane-parallel ionization chambers for energies less than 10 MeV
Plane-parallel or cylindrical chambers for higher-energy beams
Phantom
Water, or plastic phantoms such as polystyrene and Lucite
Dimensions large enough to provide full scatter for all field sizes and energies

12. Output Calibration (2) Reference depth field size
The point of maximum dose on the central axis
The deepest part of the maximum
To avoid low-energy electron contamination problems close to the phantom
field size
1010 cm as reference field
The maximum dose at dmax set at 1 cGy/MU
Another cone is expressed as an output factor.

13. Output Calibration (3) Chamber voltage
Under saturation conditions with a known ion-collection efficiency
Less than 1% loss in charge by ion recombination
The collection efficiency
The ratio of current for the given voltage to that for the infinite voltage
The ion recombination correction (Pion)
Two sets of measurements

14. Output Calibration (4) Effects of chamber polarity
The difference between the charges measured at positive and negative polarity should be less than 5%.
Perturbation or replacement correction (Prepl)
Depending on the electron energy and the size of the cavity
The mean energy at the point of measurement calculated using Harder’s equation

15. Output Calibration (5) Displacement correction
The effective point of measurement upstream toward the source from the center of the chamber
For a thimble chamber
0.45r for 3-10 MeV electrons
0.6r for MeV electrons
AAPM0.5r for all electron energies and depths

16. Absorbed Dose Calculation
If the measurements are made in a medium other than water, then

17. Depth Dose Distribution (1)
Ionization chambers
Depth ionization curvesdepth dose curves
Silicon diodes
To give depth dose distribution directly
Film
Energy independence
Relative dosimetry
Sensitometric curve

18. Depth Dose Distribution (2)
Phantoms
Water is the standard phantom
Polystyrene and electron solid water
A depth dose distribution measured in a nonwater phantom
The water equivalent or the effective density

19. Central Axis Depth Dose Curves
A rapid dropoff of dose
X-ray contamination
90%E/4 cm %E/3 cm
Dmax does not follow a linear relationship with energy.
The percent surface dose for electrons increases with energy.
In clinical practice, isodose distributions for an individual machine, cone, and/or field size is required.

21. Surface Irregularities: the ear is taped as flat as possible, a wax ear-plug is required to prevent electrons tunnelling down the external auditory meatus. Choice of Energy. The depth of the planning target volume is 49 mm, which will be reached by an 18 MeV beam, for which the 90% isodose is at 55 mm deep with a 200 mm!200 mm applicator (see Table 34.1) and 53 mm for a 100 mm!100 mm applicator. The surface dose of an 18 MeV beam is 88%, from Table However, it is not the intention to treat to the skin surface, and the target volume does not come closer to the skin surface than 4 mm, which, from Table 34.1, is more than the 1 mm required to raise the dose to 90%. Field Size. Using the criterion of a 10 mm margin around the planning target volume, the field size should be 100 mm!100 mm. However, because the target volume has been drawn with rounded corners at depth, it may be treated with a 90 mm!90 mm field in this case. Calculation of Monitor Units. The dose prescription point is on the central axis of the treatment beam, at the depth of dmax, i.e. 22 mm for this linear accelerator. This means that a dose of 2.75 Gy will be given to 100% on the central axis, 22 mm deep. If a 90 mm!90 mm insert is not available, one may be produced by attaching lead strips onto the edges of a 100 mm!100 mm insert to reduce the field size appropriately. The output (in Gy per monitor unit) of the newly fashioned insert should be measured, although experience shows that where the field edges are brought in only minimally, the measured output may not show significant change from that for the unmodified applicator. Vulnerable Organs. The ear plug mentioned above is used to protect the external auditory meatus. The dose to the spinal cord is clinically acceptable.

22. Surface Irregularities: the ear is taped as flat as possible, a wax ear-plug is required to prevent electrons tunnelling down the external auditory meatus. Choice of Energy. The depth of the planning target volume is 49 mm, which will be reached by an 18 MeV beam, for which the 90% isodose is at 55 mm deep with a 200 mm!200 mm applicator (see Table 34.1) and 53 mm for a 100 mm!100 mm applicator. The surface dose of an 18 MeV beam is 88%, from Table However, it is not the intention to treat to the skin surface, and the target volume does not come closer to the skin surface than 4 mm, which, from Table 34.1, is more than the 1 mm required to raise the dose to 90%. Field Size. Using the criterion of a 10 mm margin around the planning target volume, the field size should be 100 mm!100 mm. However, because the target volume has been drawn with rounded corners at depth, it may be treated with a 90 mm!90 mm field in this case. Calculation of Monitor Units. The dose prescription point is on the central axis of the treatment beam, at the depth of dmax, i.e. 22 mm for this linear accelerator. This means that a dose of 2.75 Gy will be given to 100% on the central axis, 22 mm deep. If a 90 mm!90 mm insert is not available, one may be produced by attaching lead strips onto the edges of a 100 mm!100 mm insert to reduce the field size appropriately. The output (in Gy per monitor unit) of the newly fashioned insert should be measured, although experience shows that where the field edges are brought in only minimally, the measured output may not show significant change from that for the unmodified applicator. Vulnerable Organs. The ear plug mentioned above is used to protect the external auditory meatus. The dose to the spinal cord is clinically acceptable.

24. Isodose Curves Depending on the energy, field size, and collimation
For the low-energy beams
All the isodose curves show some expansion
For the higher energies
Only the low dose levels bulge out
Higher isodose levels tend to lateral constriction, which becomes worse with decreasing field size.

27. Field Flatness The flatness changes with depth ICRU AAPM
Uniformity index
at the depth of half the therapeutic range
AAPM
At the depth of the 95% isodose beyond the depth of dose maximum
Not exceed 5% (3%) over an area confined within lines 2 cm inside the geometric edge of fields

28. Field Symmetry
The AAPM recommends that the cross-beam profile in the reference plane should not differ more than 2% at any pair of points located symmetrically on opposite sides of the central axis.

29. Beam Collimation The dual-foil system The beam-defining collimators
To widen the beam by multiple scatter
To make the beam uniform in cross-section
The beam-defining collimators
A primary collimation close to the source that defines the maximum field size.
A secondary collimation close to the patient to define the treatment field

30. Principle of dual-foil system

31. Field Size Dependence (1)
The dose increases with field size because of the increase scatter from the collimator and phantom.
Various size cone with a fixed jaw opening minimizes the variation of collimator scatter.
If the x-ray jaw setting changed with the field, the output would vary widely, especially for lower-energy beam.

32. If the x-ray jaw setting changed with the field, the output would vary widely, especially for lower-energy beam.

34. Field Size Dependence (2)
If the distance between the point of measurement and the edge of the field is shorter than the range of the laterally scattered electrons
phantom scatter
The effects of field size on output and the central axis depth dose curve is significant.

37. Electron Source (1) Virtual source
An intersection point of the backprojections along the most probable directions of electron motion at the patient surface.
Pohlit’s method-a grid of copper wires
A multipinhole technique-double conical holes
Meyer method-FS magnification on film
The use of the virtual SSD to predict dose variation with distance requires the inverse square law factor and correction factors as a function of FS and energy.

38. Electron Source (2) Effective SSD To correct air gap
A function of energy and FS

39. X-ray Contamination The tail of the depth dose curve
Bresstrahlung interactions of electrons with the collimation system and the body tissues
In a modern Linac
6-12 MeV %
12-15 MeV %
15-20 MeV %
Critical for total body electron irradiation

42. Chapter 14 Electron Beam Therapy (2)

43. Choice of Energy and Field Size
EnergyThe target volume lies entirely
within the 90% isodose curve.
Field size
A significant tapering of the 80% isodose curve at energies above 7 MeV
The constriction of the useful treatment is worse for the smaller fields.
A larger field at the surface may be necessary to cover a target area adequately.

44. Corrections for Air Gaps
Beam divergence
Inverse square law correction from a virtual or effective SSD
The relative depth dose distribution is unchanged.
The absolute value of the dose is decreased at all depths.

45. Corrections for Beam Obliquity
Increase side scatter at the depth of dmax.
Shift dmax toward the surface.
Decrease the depth of penetration

46. Corrections for Beam Obliquity
Pencil or slit beams
Obliquely incident beam
The point at the shallow depth receives greater side scatter from the adjacent pencil beam.
The point at the greater depth receives less scatter.

47. Corrections for Beam Obliquity
The depth dose in an obliquely incident beam is affected both by the ”pencil scatter effect” and the “beam divergence”.

48. , OF(,d)
Rp  E (MeV)/2

49. the calculated distribution using a pencil beam algorithm v. s
the calculated distribution using a pencil beam algorithm v.s. the measured distribution obtained in a cylindrical polystyrene phantom

50. Sharp Surface Irregularities
Electrons are predominantly scattered outward by steep projections and inward by steep depressions.

51. Tissue Inhomogeneities
It is difficult to determine dose distribution within or around small inhomogeneities becausfe of enhanced scattering effects
For large and uniform slabs, dose distribution can be corrected by using the of equivalent thickness (CET) method.
deff = d-z(1-CET)

52. Tissue Inhomogeneities
Bone
CET of a compact bone = 1.65
CET of a spongy bone = 1
Lung
CET=0.5
Relative e =
A water-cork system
Reduced scatter from the low-density cork
The increased penetration overtakes the reduced scatter

54. Small Inhomogeneities
A material M’ of a higher mass scattering power
A decrease in the electron fluence behind the slab
Increase the dose in the medium M

55. Small Inhomogeneities
Pohlit and Manegold
To evaluate the maximum values for increase and decrease of dose behind the inhomogeneities
-the position of the maxima of reduction and of increase of dose
-the mean angle at which the effect is negligible

56. Pohlit and Manegold Method
Under the inhomogeneity but outside angle   the regular CET method
The maxima and minima of dose along the boundaries of angle  estimated by a maximum change, Pmax

57. Pmax
The influence of an edge increases with increasing Ee.

58. Use of Bolus and Absorbers
Bolus is used to
Flatten out an irregular surface
Reduce the penetration of the electrons in parts of the field
Increase the surface dose
Equivalent to tissue in stopping power and scattering power
Decelerators
Low Z
To reduce the energy of an electron beam

59. Adjacent Fields
The decision based on the uniformity of the combined dose distribution across the target volume

60. electron field v.s. photon field
The electron beam SSD=100 cm
A hot spot on the side of the photon field
A cold spot on the side of the electron field
Outscattering of electrons from the electron field
The electron beam SSD=120 cm

61. Field Shaping Lead cutouts
To give shape to the treatment area
To protect the surrounding normal tissue or critical organs
Placed on the skin or at the end of the treatment cone
For lower-energy electrons (<10 MeV), less than 5 mm thickness of lead is required (5%).

62. External Shielding Allowable transmission – 5%
If the lead is too thin, the transmitted dose may be enhanced directly behind the shield.
FS, the desired thickness of lead

63. Measurement of Transmission Curves
A parallel-plate ion chamber
A polystyrene phantom
The measurement depth  5 cm
Broad beam
Bremsstrahlung  the transmission dose
E (MeV)/2=the minimum thickness of lead
The required thickness of cerrobend is 20% greater than that of pure lead.

64. Effect of Blocking on Dose Rate
The magnitude of the effect depends on
The extent of blocking
The thickness of lead
The energy
The reduction in dose also depends on the depth of measurement.

65. Effect of Blocking on Dose Rate
If a field produced by a lead cutout is smaller than the minimum size required for maximum lateral dose buildup, the dose in the open portion is reduced.
Rp as the lower limit for field diameter

66. Internal Shielding
For the treatment of lip, buccal mucosa, and eyelid lesions
The electron backscatter from lead enhances the dose to the tissue near the shield
30% - 70% in the range of 1 – 20 MeV, having a higher value for the lower-energy beams
For the polystyrene-lead interface
ESF = e( Ez)

67. Electron Backscatter v.s. Z

68. The dose enhancement drops off exponentially with the distance from the interface on the entrance side of the beam.

69. The relative backscatter intensity v. s
The relative backscatter intensity v.s. the thickness of absorber (polystyrene)
To dissipate the effect of electron backscatter, a suitable thickness of low Z absorber may be placed between the lead shield and the preceding tissue surface.

70. Electron Arc Therapy For superficial tumors along curved surfaces
“Pseudoarc technique”
Field is defined by the x-ray jaws
The electron collimation is provided on the patient’s skin surface
large number of overlapping fields
Directed isocentrically

71. Calibration of Arc Therapy Beam
Dose per arc can be determined
Integration of the stationary beam profiles
An isodose distribution as well as the dose rate calibration of the field (under stationary beam conditions) for arc
The dose per arc at P is given by
Direct measurement
A cylindrical phantom

72. Treatment Planning Beam energy
For small scanning field width, the depth dose curve shifts slightly and the beam appears to penetrate somewhat farther than for a stationary beam.
The surface dose is reduced.
The bremsstrahlung dose at isocenter is increased.

73. Treatment Planning Scanning field width Location of isocenter
Small scanning fields give lower dose rate and greater x-ray contamination.
Simplifying dosimetry
4-8 cm
Location of isocenter
Equidistant from the surface contour for all beam angles
Greater than the maximum range of electrons

74. Treatment Planning
Field shaping

75. Total Skin Irradiation
2-9 MeV
Translational technique
A horizontal patient is translated relative to a beam of electrons.
Large field technique
A standing patient is treated with a combination of broad beams produced by electron scattering and large SSDs (2-6 m).

76. The six-field Stanford technique
An acrylic scatter plate (1 cm) to provide additional scatter