Chapter 14 Kahn: The Physics of Radiotherapy
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What is the most clinically useful energy range for electrons?
- At these energies,the electron beams can be used for treating superficial tumors (less than 5 cm deep) with a characteristically sharp drop-off in dose beyond the tumor
What are the principal applications of electron beam therapy?
- (a) the treatment of skin and lip cancers,
- (b) chest wall irradiation for breast cancer,
- (c) administering boost dose to nodes,
- (d) the treatment of head and neck cancers.
What are the main advantages to electron therapy over superficial x-rays, brachytherapy and tangential photon beams?
Dose uniformity in the target volume and in minimizing dose to deeper tissues.
What are the four general processes of interaction for electrons traveling through a medium?
- (a) inelastic collisions with atomic electrons (ionization and excitation),
- (b) inelastic collisions with nuclei (bremsstrahlung),
- (c) elastic collisions with atomic electrons,
- (d) elastic collisions with nuclei.
What is the difference between elastic and inelastic collisions for electron therapy?
In inelastic collisions, some of the kinetic energy is lost as it is used in producing ionization or converted to other forms of energy such as photon energy and excitation energy. In elastic collisions, kinetic energy is not lost although it may be redistributed among the particles emerging from the collision.
What is the primary way that electrons lose energy in low Z media?
In low atomic number media such aswater or tissues, electrons lose energy predominantly through ionizing events with atomicelectrons.
What is the primary way that electrons lose energy in high Z media?
In higher atomic number materials, such as lead, bremsstrahlung production is more important.
In the collision process with the atomic electrons, if the kinetic energy acquired by the stripped electron is large enough for it to cause further ionization, the electron is known as a _______.
Secondary electron or delta ray.
As a beam of electrons travels through a medium, the energy is continually degraded until the electrons reach thermal energies and are captured by the surrounding atoms.
The rate of energy loss for electrons depends on what property of the medium?
The electron density.
The rate of energy loss per gram per centimeter squared (mass stopping power) is greater for low or high Z materials?
- Low Z materials.
- High Z materials have fewer electrons per gram than low Z materials have and, high Z materials have more tightly bound electrons, which are not as available for this type of interaction.
As seen in Fig. 14.1, 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.
What is the approximate energy loss rate of electrons of energy 1MeV and greater in water?
Roughly 2 MeV/cm.
The rate of energy loss per centimeter is approximately proportional to the electron energy and to the ______.
- Square of the atomic number (Z^2).
- The probability of radiation loss relative to the collisional loss increases with the electron kinetic energy and with Z. That means that x-ray production is more efficient for higher energy electrons and higher atomic number absorbers.
A high-energy electron loses more energy per gram per square centimeter in a ___ than in traversing a more dense medium.
- , This is because of appreciable polarization of the condensed medium (5-7). Atoms close to the electron track screen those remote from the track. This phenomenon is particularly important in dosimetry with ionization chambers when energy deposition in a medium and a gas cavity are compared.
The ratio of mass stopping powerof water to air varies with what?
- Electron energy.
- Consequently, the dose conversion factor for an air ionization chamber in water (or another condensed medium) varies with depth.
Define total mass stopping power of a material for charged particles.
The quotient of dE by pdl, where dE is the total energy lost by the particle in traversing a path length dl in the material of density p.
What is needed in order to calculate the energy absorbed per unit mass (absorbed dose)?
The electron fluence and the "restricted" collision stopping power.
Define the restricted mass stopping pwer of a material for charged particles.
The quotient of dE by pdl,where dE is the energy lost by a charged particle in traversing a distance dl as a result of locally absorbed energy collisions with atomic electrons in which the energy loss is less than delta.
When a beam of electrons passes through a medium, the electrons suffer multiple scattering due to _____ _______ interactions between the incident electrons and, predominantly, the nuclei of the medium.
- Coulomb Force.
- As a result, the electrons acquire velocity components and displacements transverse to their original direction of motion. For most practical applications, the angular and spatial spread of a narrow, collimated beam of electrons can be approximatedby a Gaussian distribution
How does scattering power vary with respect to atomic number and kinetic energy?
The scattering power varies approximately as the square of the atomic number and inversely as the square of the kinetic energy. For this reason, high Z materials are used in the construction of scattering foils. Scattering foils spread out the electron beam that emerges from the accelerator tube and are made thin to minimize x-ray contamination oft he electron beam.
In clinical practice, where is an electron beam usually characterized by the energy?
at the body surface. There are several methods that can be used to determine this energy: measurement of threshold energy for nuclear reactions; range measurements; and the measurement of Cerenkov radiation threshold. Of these, the range method is the most practical and convenient for clinical use.
Each point on the depth ionization curve should be corrected for beam divergence before the range is determined. What is the equation for the correction factor?
, where f is the effective source-tosurface distance and z is the depth.
The most probable energy and, approximately, the mean energy of the spectrum decreases linearly with depth. What is the equation?
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