Clinical Electron Beams Characteristics

• Clinical electron beams are characterized

Central Axis Depth Dose Curves

• Rapid dropoff of dose
• X-ray contamination is present
• 90% of the dose occurs at E/4 cm
• 80% of the dose occurs at E/3 cm
• Dmax does not follow a linear relationship with energy
• As energy increases, the percent surface dose for electrons increases
• Isodose distributions for the machine, cone, and field size is required in clinical practice

Isodose Curves

• Dependant on energy, field size and collimation
• Low-energy beams: all isodose curves show some expansion
• Higher energy beams: only the low dose levels bulge out
• Higher isodose levels tend to lateral constriction, which is worsened with decreasing field size

Field Flatness

• Flatness changes with depth
• AAPM recommends that at the depth of the 95% isodose beyond the depth of dose maximum, it should not exceed ±5% (±3%)
• This is measured over an area confined within lines 2 cm inside the geometric edge of the fields

Field Symmetry

• AAPM recommends 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

Field Size Dependence

• The dose increases with field size because of increased scatter from the collimator and phantom
• Using various size cones with a fixed jaw opening minimizes collimator scatter variation
• If the x-ray setting is changed with the field, the output would vary widely, especially for lower-energy beams

X-Ray Contamination

• The tail of the depth dose curve is due to it
• It is caused by the Bresstrahlung interactions of electrons with the collimation system and body tissues
• In a modern Linac:
  • 6-12 MeV has 0.5%-1%
  • 12-15 MeV has 1-2%
  • 15-20 MeV has 2-5%
• It is critical for total body electron irradiation

Choice of Energy and Field Size

• The target volume lies entirely within the 90% isodose curve
• A significant tapering of the 80% isodose curve occurs at energies above 7 MeV
• The constriction of useful treatment worsens for smaller fields
• A larger field at the surface may be necessary to adequately cover a target area

Corrections for Beam Obliquity

• Increased side scatter at the depth of dmax
• dmax shifts toward the surface
• The depth of penetration decreases
• For obliquely incident beams, the point at the shallow depth receives greater side scatter from the adjacent pencil beam
• The point at the greater depth receives less scatter

Bolus and Absorbers

• Bolus is used to flatten out an irregular surface
• It also reduces the penetration of electrons in parts of the field and increases the surface dose
• Bolus is equivalent to tissue in stopping power and scattering power

Field Shaping

• Lead cutouts are used to give shape to the treatment area
• They protect surround normal issue or critical organs
• For lower-energy electrons (<10 MeV), less than 5 mm thickness of lead is required (≤5%)