Electrons Flashcards
choose e- to give ? DD
90% DD to desired depth, which is 1/2 of the probable e- E
e- used to treat lesions at what depth
superficial lesions < 7cm in depth
e- advantages over superficial regarding bone dose
no increase dose to bone
e- beam calc algorithm
pencil beams
e- are scattered outward by
steep projection
e- are scatter inward by
steep depression
e- max range dependent on
E
e- fall off is
rapid dose fall off
e- skin dose is
high
tx head component for e-
- scattering foil
- no target
e- significant impact on tissue ?
inhomogeneities
brems tail is what on a beam curve
slowly decreasing portion of an e- beam curve
Rp stands for
practical range
e- block thickness formula
MeV/2 + 1
R90 is
MeV/4
R80 is
MeV/3
ballooning effect on e- requires you need to adequately
cover target volume
in field blocking, block thickness must be adequate to reduce transmission to
< 5%
e- cone size affects
beam output
e- cone distance from pt
at least 5cm from skin surface
uniformity of e- beams are better than
photon beams
if air gap increases dose ?
dose decreases
TBI uses
spoilers
spoiler scatters
e-
TBI total dose
12 Gy
TBI dose rate
10-15 cGy/min
TBI frequency
bid
dose rate of e- varies with
field size & E
%DD e- depends on
E
to calc e- E you take ? of tumor
3x max depth of tumor
field margin for e- must be selected so target lies within?
90% line
e- range in lung is increased by a factor of
3
CET stands for
coefficient of equivalent thickness
in CET, the attenuation by a given thickness of inhomogeneity is equivalent to
attenuation by a certain thickness of water
CET for compact bone
1.5 g/cm^3
CET for spongy bone
1.1 g/cm^3
e- density of lung tissue varies depending on
depth
dose in tissue from of a less dense area (air/lung) is ?
decreased b/c scattering less
e- range is larger in lung (less dense tissue) making lung dose ?
increase
therapeutic range of e-
90% iso line (E/4)
when CW tx w/e-, beam E is often chosen so what line is at interface?
80% line
e- matching at surface causes ? at depth
hot spot
e- advantages over superficial regarding skin sparing
small amount of skin sparing
e- advantages over superficial regarding underlying tissue
greater sparing of underlying tissue
e- advantages over superficial regarding output
greater output & faster treatment
brems tail is generated where
scattering foil
brems tail represents
photon contamination
Rp is where e- have actually
stopped
Rp formula
MeV/2
e- block thickness below 10 MeV is
3mm block
ballooning effect occurs with what radiation
e-
what do higher isolines look like on ballooning effect
taper
ballooning effect requires larger than expected
field size to cover entire volume
ballooning effect aka
flowering pot effect
isolines appear ? in ballooning effect
flat
e- blocking affects
beam output
e- SSD affects
beam output
uniformity of e- beams have smaller ? than photon beams
horns at beam edge
if air gap increases dose profile is
more rounded
in TBI, surface dose increases with ? spoiler to surface distance
decreased
in TBI, point of max dose build up moves closer to surface with ? spoiler to surface distance
decreased
in TBI, ? becomes principle source of e- contamination of beam
spoiler
what increases dose to tissue in buildup region for TBI
spoiler
only 20-50% isoline into penumbra region causes difficulty in what
matching fields
CET can be used to correct
e- change in dose
e- density of lung tissue average is
0.5 g/cm^3
Lateral scatter equilibrium exists when the field size is ? Of the e- energy
Of the order of the e- energy
