Electrons Flashcards
How can electrons interact with matter?
Elastic scattering and soft collisions with atomic electrons (causes direction change and negligible energy loss)
Inelastic collisions with atomic electrons (ionisation and excitation)
Inelastic collisions with nuclei and electrons (brem, characteristic radiation)
What is range straggling?
Electrons with the same initial energy travel to different depths due to different interaction histories
Why is the electron PDD the shape it is?
Initial increase: excitation and ionisation, scattering results in increasing energy deposited in shallow layers of tissue, build up
Electrons lose energy after shallower depths and reach end of range, PDD decreases rapidly
Bremsstrahlung tail
Why are electrons clinically attractive?
High surface dose
Steep dose fall off
What is the 2, 3, 4, 5 rule?
d100 = 2 E0
d90 = 3 E0
d50 = 4 E0
Rp = 5 E0
in mm
What is the therapeutic range?
Depth of distal isodose suitable for treatment
What is the therapeutic interval?
Distance between isodoses suitable for treatment
What is surface dose dependent on?
Beam energy, distance from applicator, use of cutouts
What is shape of isodoses?
Low does isodoses bulge out (due to increased scatter at low energies)
High isodose levels are constricted laterally at depth
What happens to PDD with increasing energy?
Dmax increases (along with R50, Rp)
Gradient of dose fall off decreases
Surface dose increases
X ray contamination level increases
How does field size impact PDD?
Small fields: Dmax shifts towards the surface
Relative surface dose increases
Practical range remains unchanged
Scatter away from CAX reduces dose at depth
What are small field dimensions?
roughly E0 / 2.5
Why do inhomogeneities impact dose?
Scatter is dependent on energy electron, density, and atomic composition of inhomogeneity
Inhomogeneities impact scattering of electrons and change in scattering changes dose deposited
How does bone impact dose?
Higher density than soft tissue
Increased attenuation
Greater scattering per depth in medium
Increased dose in bone
Decreased dose beyond bone
Increased dose adjacent to bone
How does lung/ air impact PDD
Lower attenuation in lung
Lower scattering per linear depth in lung
How do high Z/ high density materials impact dose?
Electrons scattered away from high density/Z materials ot lower density/Z materials
Results in increased electron fluence and dose scatter lobes in lower density or Z materials
How does air cavity impact dose distribution?
Get hot spot immediately beyond cavity
Decrease in dose adjacent to cavity
Increased dose beyond inhomogeneity
How does an oblique beam impact the dose distribution?
The peak dose shifts close to the surface, density of dose deposition near surface increases
How do isodoses change with oblique beam and curved surface?
Oblique beam: isodoses run parallel to surface but direction tilted
Curved surface: isodose curves run parallel to skin surface
How does increasing SSD impact dose distribution?
Penumbra widens
Volume of high dose decreases
How could we deal with surface contour problems?
Bolus
How does bolus impact dose?
Dose delivered to surface is higher, PDD is effectively shifted close to surface
Therapeutic interval may decrease
Depth of treatment decreases
How can we alter the therapeutic interval?
Alter beam energy
Use thin foils to increase scattered radiation and hence dose
How does the use of scattering foils differ to bolus for changing surface dose?
High Z foils produce same angular scatter change but with less energy loss
Therefore have greater therapeutic interval
Surface dose increases but dose remains high at depth (less energy loss)
What advantages does the use of lead cut outs have?
Spares adjacent normal tissue
Dose homogeneity
Reduces penumbra width
Minimises effect of patient movement
How does using 2x2 cutout in 4x4 field differ to 2x2 field?
Using cutout provides wider more homogeneous field which is more clinically useful
What materials can be used for cutout and what thickness?
Lead, thickness (mm) should be at least = initial energy (MeV)/2
Alloy (eg Cerrobend), thickness (mm) = Lead thickness 1.2
Often use standard thickness of 1cm
When might distal shielding be used?
To protect underlying normal tissue
When treating lips, cheeks, ears..
What is a potential downside of using an internal shield?
Get backscatter upstream of the shielding, increases dose to tissue being treated close to shield
How can we decrease impact of scatter with internal shield?
Coat in wax: attenuates low energy photons and also protects patient tissue from Lead
What are issues of treating large areas with electrons?
Need to abut fields
Due to the shape of isodoses, this gives either hotspot at depth (if fields are abutting) or cold spot near surface (if gap is left)
Can move the gap on different fractions to smooth this out
Why is RT good for skin treatments?
Good cosmetic results
Short term side effects stop after treatment, long term ones aren’t major
Cost of treatment good
When would RT be used over surgery?
Older patients (less risk of late effects)
Medically inoperable or debilitated patient
Larger (>5mm) central face turmour, or eyelid tumour where margins not feasible
Larger tumour of ear, forehead, scalp
Upper lip tumour
What must be chosen when deciding on a treatment?
Field size
Shielding/collimation
Bolus
SSD
Foils
Set up
What are superficial treatment options?
Brachy
kV photons
Electrons
What kind of questions might you have when deciding how to treat?
What dose is being prescribed?
What % dose is treatment?
What depth is treatment required over?
Is marked area the field or the disease, and then what margin if disease
What dose is required at surface
Are there any critical structures nearby
Has patient had previous RT?
When use orthovoltage x rays vs electrons?
Both are comparably effective
Physical properties of orthovoltage dose distribution allow tighter margins
Orthovoltage not appropriate if tumour thickness >1cm (increased skin dose)
Not appropriate if bone invasion (increased absorption)
Delivers more dose to cartilage and bone
When would brachy be used?
Daily fractionation is inconvenient for patient
Tumour has more convex/irregular surface
What is changed in the linac to produce an electron beam?
Gun current is reduced (in photon beam losing a lot of energy to target, no longer happening)
Target replaced with scattering foil
FF replaced with second scattering foil
Why is an applicator used in electron beams?
Using jaws alone would not give a usable penumbra due to the amount electrons scatter
What are the properties of materials used to make foils?
High Z to broaden beam
Thin to minimise attenuation and production of bremstrahlung
How does energy of electron beam change with depth?
Mean energy decreases, most probable energy Ep decreases
Energy spread increases
How does PDD vary with field size if field sizes are not considered small?
PDD doesn’t really change, dose at surface slightly lower
Why is there a build up region in electron beams?
Average path of individual electrons become more oblique, so there are more interactions per unit depth
At deeper depth, no overall direction of electrons
What typical rules of thumb are there for electron beams?
2,3,4,5 rule
Treatment volume typically 10mm smaller on all sides than light patient
Central part of treatment beam will be flat, but this will fall off at distance form light field edge equivalent to twice the energy in mm
(for 6MeV beam, dose begins to fall of at 12mm from light field and is clinically unusable at 10mm)
What is VSD and why does it exist?
Virtual source distance
Electrons scatter so much that they appear to come from a different position than the photon beam
Can’t use typical corrections, need to correct for distance using VSD
How can we measure VSD?
Using measurements at a range of SSDs and different methods:
- ISL equation (only uses two measurements, less robust)
- Plotting cm distance from 100cm SSD vs (I0/Ig)^1/2, taking slope. f = 1/slope - d (Depth in phantom)
- Taking measurements in air and plotting distance from 100cm SSD vs ionisation current ^-1/2, crosses x axis at VSD
What is beam quality specification?
R 50,D
Depth on central axis in water at which absorbed dose is 50% of maximum at 100cm SSD in large field size (large enough when electron from edge cannot travel to CAX)
Define at large field sizes so beam quality specification is not function of field size. Still parameter defining beam in small fields
Why don’t we used R100 as specification for beam energy?
At higher energies, the differences between linac manufacturers become more significant than differences between higher and lower energies
Also at higher energy, top of PDD is very flat, can make repeated measurements of dmax variable
How is zref defined and why was it chosen?
z ref = 0.6 R50,D - 0.1 (cm)
At this depth in any electron beam you get consistency between stopping power ratios between different manufacturers and therefore across range of clinical beams. Allows correction factor given by NPL to be used in clinical beams
What do you need to consider when setting up chamber to zref?
Effective point of measurement of chamber
Apply water equivalent depth of chamber window
How do we determine beam quality?
Measure R50,I (depth at which measured ionisation is 50% of maximum)
As long as it is between 2cm and 10cm and beam energy is between 5 MeV and 24 MeV:
R50,D = 1.029 R50,I - 0.063 (cm)
When are R50,D and R50,I indistiguishable?
Below 4cm and 9.5 MeV
What methods do we have for measuring depth dose curves?
Ionisation chamber
P-type diode
Film measurements
How do we measure PDD using ionisation chamber?
Measure curve, this gives ionisation curve
At each depth, correct for:
-stopping power ratios
-perturbation (unity for recommended parallel plate chambers)
-ion recombination
T/P corrections unnecessary if phantom has sufficient time to attain equilibrium
Account for EPOM
How do we measure PDD using p-type diode?
Depth dose curve can be measured directly (because stopping power ratios of silicon to water vary very little with electron energy unlike those of air and water)
Diode must be validated against an ionisation chamber and you must not use a shielded diode - must use electron diode
How does corrected PDI curve vary from diode curve?
In shallow depths, stopping power ratios are dependent on fine details of linac/applicator etc, so published values are uncertain below zref
For best PDD, use diode below zref and match to ion chamber above this
How do we measure PDD with film?
Take film measurements
Must validate against an ionisation chamber
Must account for scanner issues, need a very good knowledge of processing
What phantom options are there and how big should phantom be?
Water vs non-water
Water is ideal.
Non-water options include WT1, solid water, plastic water. These are preferred as they are less likely to have batch to batch variations and have charge store effects.
Could use Polystyrene, PMMA, but these have greater uncertainties and charge store effects
For simplicity, use WTe if non-water phantom
Must be thick enough that you can measure the brem. tail. t > 5 + 1.25*R50,D (cm)
What do we need to account for in non water phantoms?
Scaling for fluence and equivalent depth
Stopping power ratios are the same at appropriately scaled depths. Scaling factors are energy independent
If phantom density differs from quoted standard density then multiple Cpl by density/standard density
What are steps that NPL take to give correction factor?
- Define reference depth in water for beams at NPL (z ref,w)
- Use range scaling to get depth in graphite (z ref,g)
- Calibrate chamber against calorimeter at NPL in graphite at z ref,g
- Theoretical conversion from graphite to water (correction factor in graphite to correction factor in water)
- Compare user and reference chambers at NPL at z ref,w in water
What are some preferred properties of designated chambers?
Front window less than or equal to 1mm
Collecting electrode diameter is less than or equal to 20mm
Cavity heigh less than or equal to 2mm
Perturbation/polarity/leakage effects should be small
What do NPL provide wrt calibration coefficients?
A range of calibration coefficients N D,w for different R50,D
What do you do if you use beams beyond NPL values?
Extrapolate using tabulated stopping power ratios. This is not ideal
How do you determine dose at Dmax?
Set up phantom, designated chamber, and field size as per standard output. Chamber should be at EPOM and set up at zref.
Take a measurement and correct for T,P, polarity, ion recombination, electrometer
Convert reading to dose:
Dw(zref,w) = Mch,w ND,w(R50,D)
Transfer to dose at dmax using depth dose curve
How do we calibrate field instrument?
Set up field instrument for standard output check and take measurement
Cf field = Dose/Mf, where Mf is corrected field reading (Mf = Mraw .f T,P)
If using same field instrument, don’t need to correct for polarity and ion recombtination
How can we measure ion recombination?
Jaffe plot
Dual voltage
Boag formualism
How would we measure ion recombination using Jaffe plot?
Take readings at several voltages
Plot 1/V vs 1/M
Graph intersects y axis at 1/M for infinate voltage: ‘true’ reading if all ions were collected
Only valid if plot is linear, discard any non linear data. Linear when in saturated region, curved when in charge multiplication.
How can we measure ion recombination using dual voltage method?
Take two measurements
fion - 1 = [(M1/M2)-1] / [(V1/V2)-1]
Accurate if fion < 1.03 and if V1 > 3 x V2 and 1/M vs 1/V is linear in range considered
Why don’t we just use high V to minimise ion recombination?
Charge multiplication occurs
Insulator charging occurs
Changes in sensitive volume
Introduces errors which are more difficult to correct for than ion recombination
How would we use Boag formulism to measure ion recombination?
NPL gives formula for fixed V:
fion = c + md
Where c and m are given by NPL and d is dose per pulse
Measure dose per pulse with an oscilloscope by measuring pulse interval and length, measure dose for set number of MU
This is circular, measuring f ion so we can measure dose but to get f ion we need dose.
Typically used if voltage adjustment is unachievable or as a sanity check
How do we do T,P correction?
1013.25/P x (273.15+T)/293.15
Temperature is temperature of phantom, non-water phantom will reach temperature of room but water phantom will be degree or so cooler
How do we correct for polarity?
Take reading with bias voltage M-
Reverse polarity and wait 10 minutes
Take reading with reverse polarity
fpol = |M+|+|M-| / 2M-
Polarity should be less than 0.5% for parallel plate chambers and up to 1% for farmer chambers, if it is more than a few percent then not suitable
When would you use Jaffe vs dual voltage method vs Boag’s formulism?
Boag’s formulism: change of voltage is not possible
Jaffe plot is then preferable because you are taking a range of measurements instead of just 2
When determining R50 using an ionisation chamber, what is the most significant correction factor and why?
Stopping power ratios
Stopping power is energy loss per unit path length in material
Water to air stopping power ratios for electron beams vary with energy and therefore with depth
Why is it not necessary to apply stopping power ratio corrections when measuring a PDD with a p type diode?
The variation of stopping power ratio for silicon to water with energy is quite flat, unlike air to water. Readings taken with the diode are directly proportional to dose.
What correction factors convert Mraw to Mch,w in the CoP and how much will they vary from unity?
f T,P: corrects for variation in density of air in chamber cavity and number of therefore ionisations per unit dose. Corrects to standard temp and pressure to allow N to be used
f pol corrects for differences between having voltage applied positively or negatively. Reversing field could affect collection of ions.
f ion corrects for ions that recombine before being collected.
f elect is electrometer correction factor. Converts to true coloumbs and allows chamber to be used with different electrometers. Usually very close to 1.000
Factors likely to be within 0.5%: pol and elec
