1. Principles and Practice of Radiation Therapy
Chapter 26 Electron Beams in Radiation Therapy
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2. Physics of Electron Beams
Electron (e-)
Negative charge
Mass is 2000 times smaller than proton
Proton
No charge
No mass
Electron beams
Interact differently with matter than photon beams do because of their mass and negative charge
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

3. Physics of Electron Beams
Mass stopping power (S/p)
Rate of energy loss per unit length (S) divided by density of the medium (p)
Total mass stopping power = Sum of all energy losses
(S/p)tot = (s/p)col + (S/p)rad
Includes losses caused by collisions of e- with atomic e-
Also includes radiation losses or bremsstrahlung production
Bremsstrahlung (Gr. braking radiation) is caused by e- decelerations when passing through the field of atomic nuclei
Contribution of each process is affected by energy of e- beam and atomic (Z) number of irradiated material
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

4. Physics of Electron Beams
Restricted mass collisional stopping power
Better describes absorbed dose than total mass stopping power because it accounts for energy transferred by delta rays
Delta rays – e- scattered with enough energy to cause further ionization and excitation in other atoms
Task Group 51 Protocol
Uses stopping power ratios for “realistic electron beams”
More precise representation of dose
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

5. Physics of Electron Beams
Collisional losses
Predominant interaction is that of incident e- interacting with e- of an atom
Low Z number materials have greater electron density than high Z number materials
Electron density
Number of e- per unit mass
Probability of occurrence of energy loss resulting from radiation process increases with increasing energy or increasing Z number of absorbing material
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

6. Physics of Electron Beams
Energy dependence of e- interactions
Radiation losses range from 0.01 to 0.4 MeV/cm in the 1- to 20-MeV range
In radiation therapy in the 1- to 20-MeV range, e- beam energy losses result from collisional interactions in tissues because of low Z number of tissue
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

7. Physics of Electron Beams
Electron beam energy spectrum dependence on depth
Beam moves through linear accelerator, accelerator window, scattering foils, ionization monitor chambers, and air to patient
When beam passes through patient, it undergoes a decrease in energy and broadening of energy spectrum
Energy spectrum of beam in patient depends on depth of patient and energy spectrum of surface of patient
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

8. Physics of Electron Beams
Production of clinically useful electron beams
Most commonly produced by linear accelerators
Modifications must be made
Remove “target” and flattening filter
Decrease “electron gun” current to lower dose rate
A “pencil beam” is produced
Beam can be widened for clinical use by scattering foil or by scanning electron beam
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9. Physics of Electron Beams
Scattering foil
Thin sheet of material with high Z number placed in path of pencil beam
“Dual scattering foil” arrangements
First widens beam
Second improves beam’s flatness
Most common method of producing a wide beam for clinical use
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10. Physics of Electron Beams
Scanning beams
Pencil beam is scanned by magnetic fields across treatment area
The constantly moving pencil beam distributes dose evenly
Especially useful with energies greater than 25 MeV
Thickness of required scattering foil would result in difficulties due to their size and would cause problems with e- contamination
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

11. Physics of Electron Beams
Scattering foils vs. scanning beams
Scattering foils cause bremsstrahlung contamination
+ Scattering foils are simple and reliable
+ Scanning beams have no bremsstrahlung contamination
Scanning beam requires maintenance of complex electronic system
Failure of electronic system could allow entire dose to be delivered in one small area of patient with disastrous results
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12. Characteristics of Therapeutic Electron Beams
Dosage gradients of clinically useful electron beams
Electron beam therapy allows superficial treatment of lesions with almost no dose to underlying deep tissues
Gradient – rate of change of a value (dose) with a change in position
High-energy beams approximate depth dose characteristics of low-energy photon beams with no “true” skin-sparing effect
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

13. Characteristics of Therapeutic Electron Beams
Shape of plot of percent depth dose vs. depth
Surface dose is approximately 85% of maximum
Builds up to 100% in the first few centimeters below the surface
Beyond the 80% to 90% depth dose, there is rapid fall-off
The curve does not reach 0 but “flattens out” at a value of a few percent
Result of bremsstrahlung-produced x-ray contamination
See Figure 26-3 on page 554 of the textbook
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14. Characteristics of Therapeutic Electron Beams
Shape of electron beam isodose curves
Lateral bulge or ballooning of isodose curves
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15. Treatment Planning of Electron Beam Therapy
Electron beam rules of thumb
Depth of 50% dose in cm (R50) is multiplied by a constant (C4). Product is mean energy of the electron (Eo) beam stated in MeV at phantom surface
Eo = C4R50
The value of C4 has varied
2.33 MeV in American Association of Physicists in Medicine (AAPM) Task Group 21
2.4 MeV in AAPM Task Group 25
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16. Treatment Planning of Electron Beam Therapy
Electron beam rules of thumb
Reduction of energy of an electron beam as it moves through matter
Mean energy at surface/2
Practical range (Er) in cm of an electron beam in tissue
Er = MeV/2
Depth of the 80% isodose line in cm in tissue
80% isodose = MeV/3
Depth of the 90% isodose line in cm in tissue
90% isodose = MeV/4
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17. Treatment Planning of Electron Beam Therapy
Electron beam characteristics at surface
Factors affecting surface dose
Scattering system
Atomic number of absorber
Beam energy
Field size
Beam collimation
Beam energy, field size, and beam collimation may be manipulated for treatment planning
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

18. Treatment Planning of Electron Beam Therapy
Electron beam characteristics in the buildup region
The buildup region is an underdose risk in many clinical situations
Most true for electron beams less than 12 MeV
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19. Treatment Planning of Electron Beam Therapy
Use of bolus
Partial bolus should never be used
“Edge effect”
Areas under the bolus have decreased dose
Unbolused areas have increased dose
Tissue compensator for irregular surfaces or air cavities
Shape isodose distributions
Better conform to the treatment volume
Decrease the dose to critical structures at depth
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20. Treatment Planning of Electron Beam Therapy
Energy dependence of the width of the 80% isodose curve
With increasing electron beam energy, the ballooning of the isodose lines decreases
To cover an area at depth with the 80% isodose line, a larger area must be treated on the skin surface
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

21. Treatment Planning of Electron Beam Therapy
Distance correction factors for electron beam treatments
The use of extended source-skin distances (SSDs) has minimal effect on the central-axis depth dose and the off-axis ratios
Recommended that standard depth dose curves be used
Output and beam penumbra change dramatically with a change in treatment distance
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

22. Treatment Planning of Electron Beam Therapy
Effective point source method
D’max = Dmax (SSDeff + dmax)2/(SSDeff + g + dmax)2
D’max = The dose to Dmax at extended distance
Dmax = The dose to Dmax at normal distance
SSD = Normal SSD
SSD’ = Extended SSD
SSDeff = Effective SSD
dmax = Depth of maximum dose on central axis
g = Difference between the extended SSD and the nominal SSD (SSD’ – SSD)
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23. Treatment Planning of Electron Beam Therapy
Virtual source method
D’max = (Dmax)[(SSDvir + dmax)2/(fair)(SSDvir + g + dmax)2]
SSDvir = Virtual SSD for calibration
fair = Air gap correction factor
All other parameters are identical to those of the effective point source method
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24. Irregular Fields and Electron Beams
Shielding dependence on electron beam energy
Various methods have been used
Lead strips, cutouts, or masks placed directly on the patient’s skin or at the end of the treatment cone or collimator
Approximate shielding thickness rule of thumb
MeV/2 = Shield thickness in millimeters of lead
The thickness of Lipowitz alloy required may be obtained by multiplying the thickness of lead indicated by 1.2
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25. Irregular Fields and Electron Beams
Internal shielding
Tissues directly in front of the shield may receive 30% to 70% higher dose
Results from electron backscatter from the shield
May be minimized by placing a low Z number material between the shield and the tissue
Dental acrylic
Wax
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26. Irregular Fields and Electron Beams
Effects of irregularly shaped electron fields on dose
Variables
Include field size, thickness of shielding material, amount of blocking, and treatment distance
Almost all the absorbed dose results from electrons that have been scattered
Lateral equilibrium
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27. Irregular Fields and Electron Beams
Tissue heterogeneities and their effects on electron beams
Small heterogeneities
Changes in dose range from a 20% underdose behind bone to a 15% to 35% overdose behind air cavities
Large heterogeneities
The major factor responsible for changes in the dose distribution of large tissue heterogeneities of uniform density is absorption
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

28. Irregular Fields and Electron Beams
Gaps in electron beam therapy
Because of the lateral bulge of the isodose curves, electron fields with adjacent treatment areas that abut each other on the surface will result in an overlap at depth.
Placing a “gap” on the patient’s surface results in underdosing on the surface.
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.