Electron Beams Flashcards
What is the relationship between path length and range of electrons?
Path length is always > range. Each electron in a beam takes a unique path, so range will vary from electron to electron
How is the ‘practical range’ identified?
On an x-y plot of depth vs. PDD, a curve can be plotted.
-The R50 represents the depth of the 50% IDL
-The R90 represents the depth of the 90% IDL
If you were to draw a straight line between the two, the X-intercept would be the Rp (practical range)
Is the dose at a surface 100% for electrons? why/not?
While there is no charged particle buildup, multiple scattering can increase the dose in tissue. For lower energy beams, electrons will scatter sideways, giving rise to ionizations that reflect dose sideways (once coming, once going). The influence of this can be 5-20%.
How do the dose curves of high vs. low energy electrons differ?
The dmax for a high energy electron is closer to the surface, whereas due to multiple scattering, the dmax for a low energy beam may be deeper.
The range of a high energy beam will be deeper
The falloff of a low energy beam will be sharper
How is the energy of an electron beam described? (what is E, E0)
The nominal energy of an electron beam is equivalent to the accelerator. Thus a 15MeV electron in the accelerator is a 15 MeV when it exits the window.
AFTER this however, the electrons lose energy as they pass through the scattering foil, monitor chamber, and air.
The mean energy of a beam (E0) by the time it reaches a patient surface is = 2.33 * R50
What are the rules of thumb for electron beam:
1) R80
2) R90
3) Rp
1) R80 = E/2.8 cm
2) R90 = E/3.2 cm
3) Rp = E/2cm
What is a key consideration for electrons (vs photons) with regards to physical penumbra?
As electrons exit the window, they repel one another and spread out as they pass through air. Thus, collimator jaws (very close to the window) are somewhat useless for shaping a beam. Getting as close to the patient as possible (cones, blocks) will have the tightest physical penumbra.
How does the field size affect dose in an electron beam?
It depends. For a large field size (where radius > practical range of electrons), the field size is negligible as electrons from the edge will never deposit dose in the center.
For r
What is the ISF equation for an electron beam?
ISF = [SSDeff/(SSDeff + SSDchange)]^2
What happens if an electron field is matched at the surface?
Due to the ‘bowing’ of dose at depth, you’ll get hotspots. Never match electron fields at the surface (if at all)
How do you measure electron range in inhomogeneities?
Each tissue is given a “coefficient of equivalent thickness” (CET, to water) that depends on electron density.
Ex: lung is 0.25 CET
Normalize the thickness to this value
What happens to electron beams when they encounter metal or other high Z material?
The electrons take the path of least resistance (least density). Thus minimal dose is deposited into the metal and more electrons will reflect back into the tissue (backscatter) or around the metal (sidescatter).
What is the role of a beam spoiler?
The beam spoiler degrades the energy of the electrons and generates scatter –> decreases tissue penetration and adds dose to surface
What is the beta angle in electron therapy and its utility?
It is defined by the amount of arc to make two abutting but non-overlapping fields at the surface. A larger beta value means less overlapping tissue at depth, which is more desirable in TSEs
How does electron beam energy affect surface dose?
As beam energy increases, surface dose increases. Low E beams have multiple scatter, which deposits relatively more of its dose at depth.
