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
describe what happens at interface of high-scattering region and low-scattering region
-high scattering region scatteres more dose out; therefore low scattering region gets more dose and high scattering region gets low dose
challenges with electron-electron field matching
large penumbra and bulging of low isodose lines
-Smaller gap means less cold spot at surface, but hotter hot spot at some depth within the phantom/patient
challenges with electron-photon field matching
If the fields are matched on the skin, then there will be a hot spot on the photon side due to electron low value isodose bulge.
There will be a cold spot on the electron side (due to large electron beam penumbra).
how do SSD and air gap affect matching
larger SSD means larger electron beam penumbra, more lateral constriction as well as more pronounced bulging (due to scatter spreading out) larger hotspot on photon side and larger cold spot on electron side
what hot spot would you expect for electron-photon abutting at 50% isodose at surface?
-120 to 130 % as electron lower isodose lines are the ones that bulge out
where are field matched photon-electron beams typically used?
H&N cancers: photon field treats anterior neck nodes; electron field treats posterior neck nodes while sparing cord
prescribing to 90% isodose results in what hot spot?
what happens if you prescribe to lower or higher isodose?
111%
lower-get hotter hot spot
higher- get lower hot spot
why might you need larger energy if using bolus?
-to still obtain coverage at distal end of lesion
why would you make the field size larger at surface?
-lateral constriction
-may need larger FS to get coverage at depth
-usually 1 cm margin (distance between 50 and 90% dose levels) is reasonable
-may need larger margin if FS < Rp
is there a PTV for electron treatments?
No, just CTV
what is energy degrader
when bolus is used to adjust range of electrons
what can bolus be used for
energy degrader
increase surface dose
reduce contour irregularity
purpose of cone/applicator
-remove scatter in air and patient, maintain beam shaping
-usually 10-20 cm away so bolus can fit but close enough to maintain shape
effective SSD dependence on FS, electron energy
-shorter for low energy (more scatter), smaller FS
cutout thickness dependence on FS
-required thickness increases with FS due to increased scatter
typical treatment depth
dmax
where do you measure output factor for cutout factor
dmax
margin for cutout size
1 cm to account for lateral constriction )50-90% isodose distance)
-add larger margin if using smaller FS as more lateral constriction
value of output factor if FS> Rp
should be about 1 because dose distribution is same for FS > Rp
can you just do output factor with different SSDs in numerator/denominator instead of air gap/effective SSD method?
yes, just need to tabulate more. Usually only tabulate 1 SSD
applicator factor
some centers multipl by applicator factor to convert from one FS to another
dmax on numerator and denominator of output factor is the same?
No, can be different for different FS or if there is a cutout
is outpout factor a measure of ion chamber readingd?
No it is a measure of dose
use ratio of readings times stopping power ratio since the stopping power ratios vary with depth and wont’ cancel out in the ratio
what can happen to dmax in blocked fields?
-may be closer to the surface than for the open field (due to loss of lateral scatter equilibrium along the CAX
equation for MU calcs
MU = dose/(dose rate * PDD *output factor)
-dose rate is 1 cGy/MU for 100 cm SSD with 10x10 FS at surface measured at dmax
-since we prescribe to 90% iso, PDD is 0.9
what do you consider when assising electron enegy and bolus choice?
target entrance dose, target exit dose, depth of R50, skin dose
what makes up initial phase space of electron monte carlo?
primary photons/electrons, scattered photons/electrons, photons transmitted through or generated in aperture, electrons scattered from aperture edge
briefly describe steps for electron monte carlo
local simulation: pre-calculations in spherical volumes of various sizes and materials, for different incident electron energies. These pre-calculations yield PDFs (probability density functions) describing the particles (i.e., their position, direction and energy) exiting the sphere. These PDFs are stored in a database which is later sampled during the global simulation
geometric pre-processing:each voxel is assigned a mass density based on HU value. Each voxel is also assigned a maximum sphere size. Smaller spheres near boundaries between media. Each sphere is assigned an average mass density based on which voxels it overlaps with.
global simulation: PDFs from the spheres are sampled
dose deposition:energy from primary electron is deposited within sphere evenly along a straight line between the entry and exit points on the sphere of the primary electron. Secondary electrons may also deposit energy within the sphere. Secondary photons may exit the sphere without interacting at all within it.
how do calculation time and statistical accuracy change with number of histories in electron monte carlo
calc time increases linearly with number of histories
statistical accuracy decreases as square root of number of histories
describe pencil beam method
-based on multiple scatterng theory
-assumes lateral spread is Gaussian
-convolve single pencil beam over entire field and sum to get total dose to field
-incorporate measured PDDs and inhomogeneities vis use of effective depth
are there point spread functions for electrons?
No because everything is scattered dose
when would you use electron arc therapy?
treating superficial tumors along curved surface like chest wall
-isocenter must be deeper than max range of electrons so there is no dose buildup at isocenter
explain total skin electron irradiation
-treats mycosis fungoides and other cutaneous lymphomas
-treat up to 1 cm depth to avoid bone marrow toxicty
-can use translational technique with patient lying down- treat both AP and PA
-or make patient standa and treat with large open fields including oblique fields
what does PDD of single electron look like?
straight line with Bragg peak
where is lateral constriction more apparent?
E > 15 MV- higher energy electrons travel further and produce more scatter per electron; lateral constriction is more apparent as you go deeper and beam spreads out more; lower energies don’t penetrate as far
smaller field size- loss of lateral CPE
deeper depths- more scatter
larger SSD- more divergence, scatter in air
when does lateral scatter equilibrium occur?
-FS > Rp
-CAX is receiving as much lateral scatter dose contribution as possible (making field larger will not have any effect on CAX dose)
what is Rmax?
depth at which bremstrahlung tail of PDD meets bremstrahlung background
rule of thumb for ideal electron energy
depth x 3.2
definition of virtual source
intersection point of the back projections along the most probable directions of electron motion at the patient surface
ideal angle for photon electron junction of IMN nodes with breast tangents
want the electron beam angle to be somewhere in between being parallel with photon field divergence and perpendicular to the patient. There is a trade-off between hotspot at the junction due to electron isodose distribution bulge when the electron beam is too close to being parallel with the photon medial field, and cold spot at the junction when the electron beam is too perpendicular to the body
reverse hocky stick method for chest wall
-photon POP is used to treat SCN and area of chest wall that isn;’t over lung
-for area over lung, electrons are used (to spare the lung)
-feather the junction (consider uncertainty in marking the location on the skin and in lgiht field-coincidence)
surface dose rule of thumb
76 + E
dmax for 6, 9, 12, 16 MeV beams
1.2, 1.95, 2.8, 3.3
In contrast to the behaviour of megavoltage photon beams, the depth of
maximum dose in electron beams zmax does not follow a specific trend with
electron beam energy; rather, it is a result of the machine design and
accessories used.
benefit of skin collimation compared to cutout
sharper penumbra
x-ray contamination of electron beam from Bremstahlung in linac head and patient tissue as function of energy
6 to 12 MeV- 0.5-1%
12 to 15MeV- 1-2%
15 to 20 MeV- 2 to 5%
How would the depth dose of rectangular field of width LXW be determined if the dpeth doses for square fields LXL and WXW are known?
PDD (WXL) = square root (PDD(LXL)XPDD(WXW))
Effects of oblique treatment
-increased surface dose
-higher max dose
-shift in R90 toward surface
-increase in Rp
what is the effect of treating through an air cavity> What should be done to mitigate this effect when treating through nasal passages?
-dose distal to air cavity is increased
-dose lateral and distal to air cavity is decreased (loss of side scatter equilibrium)
-range of electrons is increased due to lost attenuation
-nose plugs should be used to replace lost scatter equilibrium
what happens when treating through bone?
-dose distal to bone is decreased
-dose distal and lateral to bone is increased
-more scatter out of bone than into bone
in electron arc therapy, the air gap from the secondary colli to the patient is larger than normal. What issue does this cause and how is it remedied?
-larger penumbra, use skin collimation to sharpen the penumbra
how does dmax depth vary with energy?
irergularly with energy and machine type
how does electron backscatter vary with atomic number and energy?
-increases with Z
-decreases with E
how deep will a 6 MeV beam reach into the lung if there is 1 cm f tissue, 0.5 cm of compact bone, and then 10 cm of lung?
range in water is 3 cm
cone is0.5 cm * 1.6 = 0.8 cm
so 1 cm + 0.8 cm = 1.8 cm is gone
1.2 cm remains (in water)
in lung this is about 1.2 cm/0.2 g/cm3 = 6 cm
thickness of Pb required
0.5 mm/ MeV
cerrobend is 1.2 X Pb thickness
Alasdair says 1 mm/MeV
what is the energy spectrum of an electron beam as it leaves the accelerator and reaches the patient?
as electron beam leaves the accelerator it is typically monoenergetic. On it way to the patient, it interacts with scattering foils, collimators etc, resulting in boradening of energy spectrum and lower mean energy at patient’s surface than that of initial beam created in linac
common use for electrons in breast
IMN nodes
tumor bed boosts
what happens if there is gap between bolus and skin?
bolus scatters the electrons, which can then tralve out of field in air gap
-decreased dose and increased penumbra
what do applicator factors depend on?
energy, size
changing applicator also changes jaw size, scatter
what happens to slope off of electron PDD as energy increases?
slope gets shallower
ortho penumbra
1.5 mm
what is the concern with backscatter from cutouts?
-ortho: photoelectrons from PE effect backscatter. Use wax to absorb them
-electrons- electrons backscatter- use aluminum cap on shield to absorb (thinner than wax)
reasons for virtual source point in electron beams
-scatter along trajectory
-scattering foil lower down than target in linac
how are electron plans evaluated?
Rad Calc
OR:
calculate dose for prescription condition (considering applicator factor, cutout, SSD) and relate to dose at nominal condition
-remember to search for dmax in prescription condition as its depth could change especially for small fields
-also stopping power ratio at prescription dmax is different than that at nominal dmax…
what happens to electron PDD for 3% isodose line of a 22 MeV beam?
- get bremstrahlung- isodose line caves in towards field edges and then dips down into a triangle because Brems continues distal
If using bolus with higher MeV beam to get coverage for a surface target, what is the issue?
-now have more distal dose
-use thicker bolus because chances are the suface dose for high MeV beam is > 90 % and now want to minimize dose to tissue distal of target
If you use a high enough energy such that target is fully covered by 90% iso line, what is still the concern if the FS is still smaller than Rp?
-lateral constriction
-can use larger FS but then irradiate more tissue
thickest part of bulge of electron beam (distance between 30% isodose bulge and 50% isodose line) for 16 MeV
7-8 mm for 16 MeV for 6x6 FS
isodose line where you start to see the bulge for electron beam profiles for 16 MeV beam (6x6 FS)
20 %
bulge is about 13 mm from 50% iso line at its thickest\
bulging starts at depth of about R99
remember to consider Rp, R50, R90 etc when drawing isodose lines
also consider penumbra width for electrons
where do you start to see the constriction?
90% iso line
isodose line where you start to see the bulge for electron beam profiles for 9 MeV beam (6x6 FS)
60 % iso line
20-80% including bulge of 20% is about 13 mm
10-20% including bulge is about 5 mm
isodose line where you start to see the bulge for electron beam profiles for 16 MeV beam (10x10 FS)
20 % iso
20-80% diagonal line including bulge of 20% is about 25 mm
10-20% at bulge is about 8 mm
