Electron Beam Therapy: An Overview

Introduction to Electron Beam Therapy

• The most clinically useful energy range for electrons is 6 to 20 MeV.
• At 6-20 MeV the electron goes less than 5 cm deep.
• Electron beams with a sharp drop-off in dose beyond the tumor are used for treating superficial tumors at 6-20MeV.
• Principal applications include treatment of skin and nodes, head and neck lip cancers, chest wall irradiation for breast cancer, and administering boost dose to cancers.

Electron Interactions

• Coulomb force interactions include inelastic collisions with atomic electrons (ionization and excitation), inelastic collisions with nuclei (bremsstrahlung), elastic collisions with atomic electrons (electron-electron scattering), and elastic collisions with nuclei (nuclear scattering).
• Low atomic number (Z) tends towards ionization.
• High atomic number (Z) tends towards bremsstrahlung.

Collisional Losses

• Collisional losses depend on the electron density of the medium.
• As Z decreases then the mass stopping power (S/p, MeV-cm²/g) increases
• As Z increases the number of electrons per gram decreases.
• As Z increases the number of tightly bound electrons increases.
• For energy ≥1 MeV, the energy loss rate in water ≈ 2 MeV/cm.
• Mass stopping power is greater for low-atomic-number (Z) materials than for high-Z materials.
• High-Z materials have fewer electrons per gram than low-Z materials.
• High-Z materials have more tightly bound electrons which are not readily available for this type of interaction.
• The energy loss rate first decreases and then increases with increase in electron energy with a minimum occurring at about 1 MeV.
• Above 1 MeV the variation with energy is very gradual.
• The rate of energy loss of electrons of energy 1 MeV and above in water is roughly 2 MeV/cm.

Radiation Losses (Bremsstrahlung)

• Energy loss per cm is proportional to the electron energy and Z².
• The probability of radiation loss relative to the collisional loss is proportional to the electron energy and Z.
• X-ray production is more efficient for higher-energy electrons and higher-atomic-number absorbers.

Most Probable Energy

• The Nordic Association of Clinical Physics recommends the specification of most probable energy, (Ep)0, at the phantom surface.
• (Ep)0 is defined by the position of the spectral peak.
• The relationship: (Ep)0 = C₁ + C2Rp + C3Rp²
• (Ep)0 is the most probable energy at the phantom surface, equivalent to the position of the spectral peak.
• Rp is the practical range in centimeters.
• For water, C₁=0.22 MeV, C2=1.98 MeV cm-1, C3=0.0025 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.
• An electron beam is usually characterized by the energy at the body surface in clinical practice.

Energy Specification and Depth-Dose Curve

• Range measurements are usually made using the depth ionization curve.
• The practical range, Rp, is the depth of the point where the tangent to the descending linear portion of the curve (at the point of inflection) intersects the extrapolated background.

Mean Energy

• The mean energy of the electron beam, Ē0, at the phantom surface is related to R50.
• R50 is the depth at which the dose is 50% of the maximum dose.
• The relationship: Ē0 = C4R50
• For water, C4 = 2.33 MeV
• The most probable energy and the mean energy of the spectrum decreases linearly with depth.
• Harder's equation for energy at depth (z): Ez = E0 (1 - z/Rp)
• Harder's equation is important in dosimetry to know the mean electron energy at the location of the chamber.