electrons Flashcards
electron output factor
output for FS/output for 10x10 field
at dmax
dmax may be different for each scenario
equation for MUs at standard SSD
MU = D * 100 %/(D’o * PDD(d, ra, SSDo) * Sc(ra, SSDo)
D’o = (D/MU)dm(ro),ro,SSDo i..e dose rate at nominal condition
electron calcs at extended SSDs
output factor usually tabulated for one SSD only. Effect of treatment distance can be handled using:
-effective SSD- multiple output factor by ((SSDeff+do)/(SSDeff+do+g))^2, g is difference btween treatment SSD and calibration SSD, and SSDeff is effective source to surface distance for the given field size
-air gap- multiply output factor by ((SSDo+do)/(SSDo+do+g))^2 * fair, fair is air-gap correction factor for given field size and SSD
why do electrons have effective SSD?
Potential dose-delivering electrons near the central axis are scattered out of the field and not fully replaced by electrons originating peripheral to the central axis. The net loss of scatter to the central axis causes the fluence to decrease with SSD more rapidly than the inverse-square law predicts.
-different for each applicator
how to determine effective SSD?
measure dose rate at zmax in phantom for various air gaps g
Plot square root of (dose for g=0 over dose for g=x) vs air gap x;
effective SSD = 1/slope - dmax
what does SSDeff change with?
-smallest for low energy and small fields
-low energy = more outward scatter
SSDeff for rectangular field size?
geometric mean of SSDeff for each side
common electron sites
skin lesions, boost fields
-breast/chest wall, superficial nodes, H/N superficial lesions
-superfifical tumors where distal sparing is important and lateral fall-off not primaru concern
why electrons vs ortho?
-faster fall off depth dose
-less dose enhancement in bone
-for large fields, more uniform across field area
-near dmax there is region of uniform dose
why not electrons? (vs ortho)
-may need bolus (dmax not at surface)
-RBE of ortho photons is 10% higher than electrons- less dose required
-MV linac more complex than ortho
what material is used in scattering foil?
high Z-scattering is proportional to Z^2
-scattering incareses with decreasing energy
-scattering increases with increasing density
-lower enrgies also scatter to larger angles
are isodose curves the same for different machines with same energy?
• Significant differences exist among shapes of isodose curves for different machines but for the same nominal energy due to the important role played by the beam collimation system in affecting scatter conditions (e.g., scattering foil, monitor chambers, primary and secondary collimators, cones)
mean energy of incident electrons
2.33R50
3 types of interactions for electrons
-inelastic collisions with atomic electrons
-inelastic collisions with nuclei (bremstrahlung)
-elastic collisions with atomic electrons and nuclei
probability of bremstrahlung
Z^2 * E
efficiency of bremstrahlung
9*10^-10 * Z *V
v is proportional to E
-efficiency = (energy of output x-rays) / (electron energy input into target)
what happens when electrons finally reach thermal energies
captured by surrounding atoms
how do collision stopping powers change with Z, E?
• Collision stopping powers are larger for lower Z; decreasing with increasing energies < ~1 MeV, increasing slightly with increasing energies above this threshold
how do radiative powers change with Z, E
increase with Z, E
inflection point around 1 MeV (similar area to min point for collision stopping power)
profile of higher energy electron beams
bulgier
-deeper Rp
CSDA range
continuous slowing down approximation
range increases with increasing energy
range is integral from 0 to E of 1/total stopping power with respect to dE
therapeutic range for electrons
R90
why does electron surface dose increase with energy?
at low energy electrons scatter more and at larger angles, causing buildup to happen over shorter distance.
Therefore dmax dose is bigger so surface dose to dmax is smaller for low energy electrons
why do electron beams have buildup region?
The electron paths are deflected through increasing mean angles from the original incident direction.
continues until mean scattering angle does not increase further; at this point the depth dose becomes flat
At increasing depths, it continues until electrons begin to be lost from the beam, in which case the depth-dose curve begins to fall