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
2,3,4,5 rule for electrons
multiplt E by 2,3,4,5 to get
R100
R90
R50
Rp
do electron PDDs have bragg peak?
yes but not apparent due to scatter
-also electron beam is not monoenergetic - many overlapping PDDs for different energies smear out the Bragg peak
PDD for an electron pencial beam
straight line with negative slope, surface dose is 100%, Rp is smae regardless of field size
No in-scatter hence no build-up
shouldn’t it be bragg peak?
Brems tail dependence on Z, E
increases with Z, E
describe electron depth profiles
low value isodoses (< 20%) bulge out more due to increasing scattering angle as electron energy decreases
-Above 15 MeV, isodose lines > 80% show lateral constriction due to more out scatter than in scatter at field edges- this effect is bigger for smaller fields
-90% is a closed, elliptical shape (close it at surface)
difference between 7.5 MeV and 17 MeV electron profiles
17 MeV has larger Rp
more lateral constriction for 17 MeV
how do electron penumbra behave
decrease with increasing energy (opposite of photons) due to decrease in scattering angle with increasing energy
At dmax, 20-80 % is 10-12 mm for electrons below 10 MeV and 8 to 10 mm for electrons above 10 MeV
how does SSD affect penumbra?
-prnumbra increase with SSD due to scatter in air
how do isodose lines behave with increasing SSD?
-isodoses > 50 % bulge out more due to scatter in air and more lateral constriction
how does SSD change surface dose?
o Larger SSD also corresponds to a higher surface dose (due to increased scatter in air) and a less steep fall-off region (i.e. flatter top portion lasts deeper, due to ISL being a relatively smaller effect – HOWEVER, this is not a very strong effect for electrons in practice since they don’t penetrate very far and changes in SSD are not usually drastic). As a result, the depth of the therapeutic range R90 is increased. The Rp is the same though.
-unlike photons extending SSD doesn’t allow larger areas to be treated uniformly
-ONLINE SAYS RELATIVE SURFACE DOSE DECREASES WITH INCREASING SSD
AAPM flatness and symmetry requirments
o Flatness within 3% - measured as the max variation in dose relative to CAX within lines 2 cm from field edge (defined by 50% isodose, normalized to CAX for particular depth; not normalized to Dmax) for field greater than or equal to 10x10 cm2. Measured at depth of 95% isodose beyond dmax (reference plane).
o Symmetry within 2% - measured as max difference between symmetric points on opposite sides of the central axis (within same region as defined above for flatness). Measured at the reference plane.
dual foil scattering system
first foil widens the beam by scattering, and the second foil makes the beam uniform in cross-section by having a variable thickness across the beam
field size effects
o For field sizes larger than the practical range (Rp) of the electron beam in width (this is approximate rule of thumb):
PDD curve remains constant with increasing field size.
o When the field is reduced below that required for lateral scatter equilibrium (field size < Rp):
Output/dose rate decreases.
dmax moves closer to the surface.
• R90 also moves closer to the surface (this is relevant for choosing an energy needed to cover the target).
PDD curve becomes less steep in fall off region (ie flat top portion becomes smaller).
• This will potentially result in loss of coverage at the distal and lateral ends of the target!
o Can compensate for this by using a larger field; however, this will result in additional normal tissue exposure.
o OR can compensate for this by prescribing to a lower value isodose; however, this will result in a hotter hot spot.
• Important to point this out to the RO.
Flatness of beam profile is compromised
Surface dose increases
Note that Rp is always in the same place, regardless of field size.
what is field size equivalence
for same incident fluence and cross sectional beam profile the equivalent feilds have same depth dose distribution along central ray
-for example if FS> Rp, all those FSs are equivalent
-for rectangular field size, equivalent side length = square root (A *B)
what happens if beam incident on oblique surface
shift of dmax and R90 toward surface
• Consider dose along beam CAX (not a line perpendicular to surface):
o Dmax increases due to obliquity due to scatter from upstream excess of tissue adjacent.
o Also, the “missing” adjacent tissue on the other side leads to reduced attenuation and increased in-scatter at deeper depths along the CAX, beyond the practical range, leading to increased residual dose.
-fall off is not as steep (ie flat portion around dmax is smaller)
-surface dose tends to increase with increased obliquity
obliquity factor
dose (oblique case)/ dose (beam perpendicular to surface) at some depth along CAX
-increases with increasing angle of obliquity, increases more for lower energies due to more scatter and large scattering angles
OBF reaches a higher maximum for higher energy beams.
OBF reaches a max around 75 degrees (this is the angle between incident beam and a line perpendicular to the surface), then starts decreasing again [I think because some of the lateral scatter starts exiting the patient at a certain point, leading to a decreased surface dose].
-get multiplied to output factor in electron MU calcs
issue with radclac and obliquity
RadCalc does not account for obliquity, so although you specify the correct depth, the result will be wrong
Consider a curved surface with an electron beam incident, and a point of interest which is off-axis. Actual dose will be lower than RadCalc expects due to excess adjacent tissue which provides additional in-scatter close to the surface, but which is attenuated closer to the source than it would in non-oblique case, resulting in less in-scatter deeper in the patient. Furthermore, beam profile intensity generally drops off as you move toward the periphery of the field; therefore, the in-scatter contribution from the missing adjacent tissue on the other side (which is attenuated less, and therefore contributes more in-scatter at deeper depths than it would in non-oblique case) may not be enough to compensate for lack of in-scatter from other side, and resulting dose will be lower.
how can we account for obliquity effect?
-isodose shift
-accounts for lack of attenuation due to obliquity but not for changes in scatter
-also use ISL to account for changes in SSD (isodose shift only account for more/less depth)
describe how isodose lines look at an oblique surface
generally roughly follow surface shape
how do contour irregularities affect dose?
o Result in both hot and cold spots due to importance of lateral scatter contributions
Get increased dose (hot spot) where there is reduced attenuation due to missing tissue, and/or scatter contribution from adjacent tissue that isn’t missing or is extra.
-isodose contours <90% somewhat follow the surface contour)
Get reduced dose (cold spot) where there is more attenuation due to additional tissue, and/or less scatter contribution due to missing adjacent tissue.
-see examples page 8 of electron notes
internal shielding
-used for oral cavity, eyelid, and lip treatment
-required thickness of lead in mm = E/2
-however backscatter from Pb enhances dose to tissue near the shield. Worse for low energies. More backscatter with higher Z shields
-For lower energies, the backscatter on the entrance side decays exponentially upstream faster than for higher energies.
-coat shields in low Z material like wax to absorb the backscattered electrons
o Example: 9 MeV treatment to buccal mucosa; cheek thickness including lesion = 2 cm. The thickness of lead required to shield oral structures beyond the cheek
-2 MeV/cm rule of thumb yields mean energy at depth of 5 MeV
-Pb thickness ~ 5/2 = 2.5mm or 2.5 * 1,3 = 3 mm cerrobend
-for internal shield, must use electron energy at depth
0.5 mm/MeV for lead, Alasdair says 1 mm/MeV
simplest correction for inhomogeneity
-use coefficient of equivalent thickness (CET) to scale electron density relative to water
deff = d - z(1-CET)
• Where d is the actual depth, z is the inhomogeneity thickness, and CET is the ratio of electron density of material to that of water. Examples: compact bone CET = 1.65; lung CET = 0.25
-method doesn;t consider position of inhomogeneity relative to point of interest
