Radiotherapy with Electron Beams Quiz

Electron Beam Dosimetry

• Megavoltage electron beams are a significant treatment modality in modern radiotherapy, particularly for superficial tumors.
• Electrons have been utilized in radiotherapy since the early 1950s.
• Modern linear accelerators (linacs) produce several electron beam energies ranging from 4 MeV to 25 MeV, in addition to photon energies.
• In electron mode, the x-ray target is removed, scattering foils are placed, and an applicator is used to restrict electron leakage outside the treatment field.
• Electron beams are collimated by an applicator (e.g., cone) and cutout (insert). Collimation is energy-dependent and uses thin, low atomic number materials (avoiding bremsstrahlung production).
• The insert is placed close to the skin to minimize the large spot size associated with geometric spread.
• The general shape of the depth dose curve for electron beams differs from that of photon beams, exhibiting a more rapid falloff.
• Electron beam central axis percentage depth dose curves display uniform dose with a rapid falloff.
• Surface dose is high (typically 80-100%).
• Maximum dose occurs at a specific depth (depth of dose maximum, Zmax).
• Beyond Zmax, the dose drops off rapidly to a low level; this low-level dose is the bremsstrahlung tail. This tail is a small percentage of the maximum dose.
• Electron beams are nearly monoenergetic as they leave the linac accelerating waveguide.
• Interactions within the body include interactions through the waveguide exit window, scattering foils, transmission ionization chambers, and air. Bremsstrahlung radiation is produced during these interactions and forms a tail on the PDD curve.
• Interactions with the absorbing medium involve elastic and inelastic Coulomb force interactions between the incident electrons and the atoms of the medium.
• Inelastic collisions lead to energy loss through ionization and excitation of absorber atoms. These collisions can be hard or soft.
• Hard collisions result in the ejection of orbital electrons with sufficient energy to ionize other atoms; these ejected electrons are called delta rays.
• Soft collisions result in the ejection of orbital electrons with insufficient energy to ionize other atoms.
• Elastic collisions primarily change the direction of motion of the incident electron (elastic scattering), causing a small energy loss.
• The cumulative effect of multiple scattering events causes the incident electron to lose kinetic energy.
• The rate of energy loss per gram and per square centimeter is mass stopping power, comprised of mass collision stopping power and mass radiation stopping power.
• High-energy electrons lose energy at a rate of 2 MeV/cm in water.
• Depth of penetration is approximately 0.5 cm/MeV.
• In contrast to photon beams, electron beams do not have a fixed focal point. Rather, they appear to originate from a virtual source.
• Effective source-surface distance (SSD_eff) is measured from the virtual source to the edge of the cone applicator.
• The inverse square law can be used to correct for small differences (air gaps) between the actual patient and the nominal SSD.
• Data, normalized to the applicator (cone) edge, is used to determine SSD_eff. A typical example is a graph versus air gap g, with a slope reflecting the effective SSD formula.
• The nominal SSD is usually a fixed distance from the distal edge of the cone and coincides with the linac isocenter, for practical reasons.
• Effective SSD is determined from measurements at Zmax in a phantom. The value does not change with depth of measurement.
• Measures of effective SSD depends on the energy of electron beams, and must be measured for every electron beam energy used in the clinic.
• Key dosimetric parameters include R90 (90% depth dose), R80 (80% depth dose), R50 (50% depth dose), Rp (practical range), E/4, E/2, and surface dose.
• Typical electron beam energies frequently used, with their corresponding values for depth-dose parameters, are identified.
• Shielding can be calculated using Rp, and thicknesses of various materials are presented for different scenarios.

Howe Work

• Questions about the best thickness of Tungsten shield for eye shielding from 6 MeV electrons are asked.
• Also inquired regarding the preferential use of Lead over Tungsten.

Clinical Considerations

• Electron beam therapy is often used for superficial or subcutaneous diseases.
• Treatment is usually with a single, directly positioned field at a 100 cm nominal SSD.
• Dose is typically prescribed at the depth (that lies at, or beyond, the target distal margin): Zmax, R90, or R80
• Healthy tissue sparing is maximized by prescribing the dose at these depths
• The maximum dose within the target may be 20% higher than the prescribed dose, and therefore should be reported.

Isodose Distributions

• Isodose curves connect points of equal dose, within the medium. Curves are usually drawn at dose intervals.
• Isodose curves give the distribution in the medium, at a given dose.
• Isodose curves often show how the dose changes due to a variety of factors (energy, field size, beam collimation).

Electron MU Calculation

• Dose depends on time, field size, depth, and distance.
• A formula exists for Electron MU calculation (MU = Dose/(D' × OF × COF × IDL × PDD × ISL)), incorporating several factors like dose, cutout factor, field-size correction, distance correction, and isodose line.
• The prescribed dose is typically 90% or 80% of the dose at Dmax.
• If calibration is performed at dref, (which is usually estimated as 0.6R50 – 0.1), then, MU can be calculated.

Electron Beam Flatness and Symmetry

• IEC standards specify measurements of beam flatness (and symmetry) at Zmax.
• Distances between the 90% dose level and the geometrical beam edge are measured, using major and minor axes.
• A maximum dose value within the 90% isodose contour should not exceed 1.05 times the central-axis dose at the same depth.
• Cross-beam profile symmetry is measured and cannot deviate more than 3% for symmetric points with respect to the central ray.

Dosimetric Parameters of Electron Beams

• Electron beams (unlike photons) show an increase in percentage surface dose with increasing beam energy.
• Zmax does not follow a clear trend with electron beam energies.
• Depth dose curves are measured at specific reference distances (SSD).
• The output factor is defined as the ratio of the dose at the field size, to reference field size. Ratio is calculated at Zmax, at a nominal 100 cm SSD.
• Beam obliquity (a) is defined by the angle between the beam axis and the normal to the phantom. PDD measurements for oblique incidences deviate from normal incidences at high angles.

Test your knowledge on the use of megavoltage electron beams in radiotherapy. This quiz includes questions about electron beam dose distribution, setup components, and dosimetric parameters. Sharpen your understanding of the intricacies involved in electron beam therapy.