Electron Beam Therapy

INTRODUCTION The most clinically useful energy range for electrons is 6 to 20 MeV. At these energies, the electron (less than 5 cm deep) with a characteristically sharp drop-off in dose beyond the tumor beams can be used for treating superficial tumors. The principal applications are (a) the treatment of skin and nodes (b) the treatment of head and neck lip cancers. (c) chest wall irradiation for breast cancer (d) administering boost dose to cancers.

Electron Interactions

Electron Interactions Coulomb force interactions Inelastic collisions with atomic electrons (ionization and excitation) Inelastic collisions with nuclei (bremsstrahlung) Elastic collisions with atomic electrons Elastic collisions with nuclei Low Z  ionization High Z  bremsstrahlung Ionization Excitation Bremsstrahlung

Collisional Losses Depending on the electron density of the medium ZMass stopping power (S/, MeV-cm2/g) Z  electrons/g  Z  tightly bound electrons  Fig 14.1 1 MeV, the energy loss rate in water  2 MeV/cm

Rate of energy loss in MeV per g/cm2 as a function of electron energy for water and lead

Radiation Losses Energy loss/cm  electron energy & Z2 The probability of radiation loss relative to the collisional loss  electron energy & Z

Energy Specification

Depth-dose curve of electron beam

Most Probable Energy (Ep)0 = C1 + C2Rp + C3Rp2 (Ep)0 the most probable energy at the phantom surface (defined by the position of the spectral peak) Rp 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.

Mean Energy Energy at Depth(z) for water, C4= 2.33 MeV The most probable energy and the mean energy of the spectrum decreases linearly with depth. It is important in dosimetry to know the mean electron energy at the location of the chamber. Harder’s equation

Characteristics of Clinical Electron Beams

Central Axis Depth Dose Curves A rapid dropoff of dose X-ray contamination 90%E/4 cm 80%E/3 cm Dmax does not follow a linear relationship with energy. The percent surface dose for electrons increases with energy. In clinical practice, isodose distributions for an individual machine, cone, and/or field size is required.

Isodose Curves Depending on the energy, field size, and collimation For the low-energy beams All the isodose curves show some expansion For the higher energies Only the low dose levels bulge out Higher isodose levels tend to lateral constriction, which becomes worse with decreasing field size.

Isodose Curves

Field Flatness The flatness changes with depth AAPM At the depth of the 95% isodose beyond the depth of dose maximum Not exceed 5% (3%) over an area confined within lines 2 cm inside the geometric edge of fields

Field Symmetry The AAPM recommends that 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 the increase scatter from the collimator and phantom. Various size cone with a fixed jaw opening minimizes the variation of collimator scatter. If the x-ray jaw setting changed with the field, the output would vary widely, especially for lower-energy beam.

X-ray Contamination The tail of the depth dose curve Bresstrahlung interactions of electrons with the collimation system and the body tissues In a modern Linac 6-12 MeV 0.5-1% 12-15 MeV 1-2% 15-20 MeV 2-5% Critical for total body electron irradiation

Treatment Planning

Choice of Energy and Field Size EnergyThe target volume lies entirely within the 90% isodose curve. Field size A significant tapering of the 80% isodose curve at energies above 7 MeV The constriction of the useful treatment is worse for the smaller fields. A larger field at the surface may be necessary to cover a target area adequately.

Corrections for Beam Obliquity Increase side scatter at the depth of dmax. Shift dmax toward the surface. Decrease the depth of penetration

Corrections for Beam Obliquity Pencil or slit beams Obliquely incident beam The point at the shallow depth receives greater side scatter from the adjacent pencil beam. The point at the greater depth receives less scatter.

Use of Bolus and Absorbers Bolus is used to Flatten out an irregular surface Reduce the penetration of the electrons in parts of the field Increase the surface dose Equivalent to tissue in stopping power and scattering power

Field Shaping Lead cutouts To give shape to the treatment area To protect the surrounding normal tissue or critical organs Placed on the skin or at the end of the treatment cone For lower-energy electrons (<10 MeV), less than 5 mm thickness of lead is required (5%).

External Shielding Allowable transmission – 5% If the lead is too thin, the transmitted dose may be enhanced directly behind the shield. FS, the desired thickness of lead

Electron Arc Therapy For superficial tumors along curved surfaces

The six-field Stanford technique TOTAL SKIN IRRADIATION The six-field Stanford technique An acrylic scatter plate (1 cm) to provide additional scatter