PHYSICS: Week 10

What is electron beam therapy and what is it used for?

• External beam radiotherapy using high-energy electrons (6–20 MeV).

• Treats superficial tumours (less than 5 cm deep).

• Delivers relatively uniform dose followed by rapid fall-off.

• Uses: Skin/lip cancer, breast chest wall, lymph node boosts, head and neck cancer.

How are electron beams produced in a LINAC?

• Electron gun: Produces and injects electrons.

• Waveguide: Accelerates electrons using microwaves.

• Magnetron/Klystron: Generates/amplifies microwaves.

• Electrons accelerated to MeV energies.

How are electron beams delivered compared with photon beams?

• Photons: Electrons hit tungsten target → X-rays; flattening filter makes beam uniform.

• Electrons: Scattering foil spreads electrons to create uniform beam.

• Electron cones/applicators: Collimate beam close to patient (less than 5 cm).

• Cones reduce penumbra caused by electron scattering.

• Typical fields: 5 × 5 to 25 × 25 cm.

What are the four main electron interactions?

• Inelastic collisions with electrons: Ionisation/excitation.

• Inelastic collisions with nuclei: Bremsstrahlung.

• Elastic collisions with nuclei: Nuclear Coulomb scattering.

• Elastic collisions with electrons: Electron-electron scattering.

• Inelastic = energy lost; elastic = direction changes with minimal energy loss.

What is stopping power?

• Rate at which charged particles lose energy travelling through matter.

• Collisional losses: Ionisation/excitation; depend on electron density.

• Radiative losses: Bremsstrahlung; depend on \(Z^2\).

• Total mass stopping power = collisional + radiative stopping power.

• Electrons lose approximately 2 MeV/cm in water/soft tissue.

How is electron beam energy specified?

• Initially narrow, almost monoenergetic beam (~3 mm).

• Scattering and energy loss create a range of energies at the patient surface.

• R₅₀: Depth in water where dose falls to 50% of maximum.

• Mean surface energy: \(E_0=2.33R_{50}\) (MeV, with R₅₀ in cm).

What is the electron percentage depth dose (PDD) distribution?

• Relatively high, uniform dose near the surface.

• Rapid dose fall-off beyond therapeutic depth.

• Small residual dose from bremsstrahlung contamination.

• Advantage: Treats superficial tumours while sparing deeper tissues.

How is electron energy selected for tumour depth?

• Choose energy to deliver 80–90% dose at tumour depth.

• Approximate treatment depth (cm) = energy (MeV) ÷ 3–4.

• Example: Tumour at 4 cm → approximately 12–16 MeV.

• Higher energy = greater penetration.

How does increasing electron energy affect dose distribution?

• Greater penetration depth.

• Modest skin-sparing effect, which decreases with increasing energy.

• Less rapid distal dose fall-off at higher energies.

• Energies above 20 MeV generally less clinically useful.

What is the virtual source of an electron beam?

• Electrons do not originate from one physical point like X-rays.

• Scattered electrons appear to diverge from an imaginary virtual source.

• Defined by back-projecting electron directions to their apparent origin.

• Used to describe beam divergence and virtual SSD.

What factors affect electron treatment planning?

• Usually uses a single field.

• Energy/field size selected based on tumour depth and required dose.

• Tissue inhomogeneities: Bone, lung and air alter dose distribution.

• Oblique incidence/air gaps: Affect beam distribution.

• Bolus: Tissue-equivalent material used to:

  • Increase surface dose.

  • Reduce penetration depth.

  • Flatten irregular surfaces.

What is total skin electron beam therapy (TSEBT)?

• Irradiates the entire skin surface using low-energy electrons.

• Commonly treats mycosis fungoides (skin lymphoma).

• Patient usually stands during treatment.

• Four or six fields from different angles provide body coverage.

• Requires more complex dosimetry.

What is proton therapy and the Bragg peak?

• External beam therapy using high-energy protons.

• Bragg peak: Sharp maximum energy deposition at a specific depth.

• Peak depth depends on proton energy.

• Almost no dose deposited beyond the peak.

• Allows tumour irradiation while sparing deeper healthy tissues.

What are the advantages of proton therapy?

• Higher tumour dose with reduced normal tissue irradiation.

• Lower integral dose than conventional X-ray therapy.

• Reduced risk of radiation toxicity and secondary cancers.

• Particularly useful for paediatric and complex head/neck tumours.

How are proton beams produced and delivered?

• Clinical energies: 60–300 MeV.

• Accelerated using cyclotrons or synchrotrons.

• Pencil beam scanning:

  • Narrow proton beams (~3 mm).

  • Sweeper magnets scan across tumour.

  • Beam energy changes to control treatment depth.

  • Treatment planning system controls individual spot positions/doses.

What are the disadvantages of proton therapy?

• Very expensive equipment and treatment facilities.

• Large accelerators, beam transport systems and gantries.

• Uncertainties in proton range.

• Uncertainties in relative biological effectiveness (RBE).

How does carbon ion therapy compare with proton therapy?

• Both use the Bragg peak for targeted dose delivery.

• Carbon ions have higher linear energy transfer (LET).

• Higher LET → denser ionisation → more severe DNA damage.

• Potentially more effective for radio-resistant tumours.

• Disadvantages:

  • More expensive than proton therapy.

  • Complex radiobiology and RBE uncertainties.

  • Range uncertainties and limited clinical trial evidence.