Electron Beam Therapy Flashcards
What is the therapeutic range usually?
80-90% isodose line
How does Electron %DD curves vary with energy?
More surface dose as energy increases because less scatter.
How does Electron %DD Energy vary with slope?
Slope decrease as energy increase.
How does energy of beam and brehms vary?
As the energy of the electron beam increases, the amount of x-ray contamination increases due to the higher likelihood of bremsstrahlung interactions (proportional to energy).
Electron %DD curves also vary with field size. How?
A smaller field size results in the depth of dmax shifting shallower and a decrease in the slope of the falloff (notice the practical range does not really change as the energy has not changed).
angle of incidence?
An oblique angle will shift dmax shallower along with the 80% isodose curve depth (effective treatment depth).
On a LINAC when delivering electron fields, the jaw size is fixed at a constant value. Why?
his is due to the huge variation in output with jaw size especially at lower energies (see left figure above) which can vary as much as 200%.
What is Uniformity Index?
The uniformity index is a way to quantify the amount of penumbra in an electron beam.
It is defined as the ratio of the area of 90% isodose curve to that of the 50% isodose curve.
Formula for VSD
Check notes
The thickness of shielding ?
Rp/density of material
The most probable (average) energy of an electron beam is equal to that specified by the selected energy at the specified output SAD. (That is, a 6 MeV electron beam has an average electron energy equal to 6 MeV at the output SAD).
False. Actually, the most probable electron energy is equal to the selected energy for the electrons as they exit the bend magnet. It is here that they have a very narrow energy window which will broaden out significantly as the electrons reach greater distances.
As the field size of an electron field approaches the practical range of the electrons the %DD remains relatively constant. For 9 MeV electrons, what field size would this require?
MeV/2 is the practical range. so 4.5*4.5
When electron beams are incident at an oblique angle, the depth of dmax shifts ______ while the practical range shifts ________?
Shallower/Deeper
As electron beam energy increases, the percentage depth dose _______ and the percent surface dose _______.
increases / increases
