Electron Treatment Flashcards
Why is bolus used in electron therapy and what kind of material is bolus made from?
Bolus is used to: (1) reduce surface irregularities, (2) limit the penetration of the electron beam in certain places, and (3) increase the surface dose. Bolus is made of a tissue equivalent Z material. Commonly used bolus materials include wax, paraffin gauze, superflab, and other pliable plastics.
Electron beams are ideally treated en face, or perpendicular to a flat patient surface. What is the effect of treating on a sloped, or oblique, skin surface?
Treating on an oblique surface creates: increased surface dose, a higher maximum dose, a shift in R90 toward the surface, and an increase in Rp. These changes are most significant at angles of obliquity >30°. Beam obliquity increases side scatter at the point of maximum dose, increases the depth of penetration of the beam, and moves Dmax deeper into the patient.
What are the two primary considerations in designing internal collimation/shielding?
First, the collimator must be sufficiently thick to stop the radiation. Second, backscatter from the collimator can be significant and must be taken into account by coating the shield with plastic, wax, or ceramic.
Where does the X-ray contamination of an electron beam come from, and how does it vary with energy?
The X-ray contamination of an electron beam results from bremsstrahlung interactions within the linac head (scattering foils, ion chambers, collimator jaws, etc.) and in patient tissues. Contamination increases with increasing energy, typical values are: 0.5% to 1% for 6 to 12 MeV, 1% to 2% for 12 to 15 MeV, and 2% to 5% for 15 to 20 MeV.
How do electrons lose energy in a medium?
They lose energy through collisional interactions (ie, ionization and excitation) and radiative interactions (bremsstrahlung). In therapy, radiative losses only become significant at higher energies and high-Z materials.
How does electron backscatter vary with both atomic number and energy?
Backscatter increases with increasing atomic number and decreases with increasing electron energy.
What are the three significant regions in an electron depth dose distribution?
(1) Buildup region caused by side-scattered electrons, which increases with electron energy, (2) regions of sharp dose falloff, beginning around the 90% isodose, and (3) the tail due to bremsstrahlung, the magnitude of the tail increases with increasing energy.
What is the energy spectrum of an electron beam as it leaves the accelerator and reaches the patient?
As an electron beam leaves the accelerator, it is practically monoenergetic. On its way to the patient, it interacts with scattering foils, collimators, air, and other structures, resulting in a broadening of the energy spectrum. The mean energy at the patient’s surface is lower than that of the initial beam created inside the linear accelerator.
