Test 3 Flashcards
4 ways electrons interact as they travel through matter
Inelastic collisions with atomic electrons
Inelastic collisions with atomic nuclei (bremsstrahlung)
Elastic collisions with atomic electrons (electron-electron scattering)
Elastic collisions with atomic nuclei
Some of kinetic energy (KE) is lost producing ionization and excitation or converted to other forms such as Bremsstrahlung
More common in low Z mediums like water or tissue
Inelastic collisions
KE is not lost, but it may be redistributed among particles emerging from collision
More common in higher Z mediums such as lead
Elastic collisions
Rate of energy loss depends on electron density of the medium
Collisional losses (ionization and excitation)
Rate of energy loss per gram per cm^2 is greater for low atomic (Z) number materials compared to high Z materials due to high Z materials having fewer electrons per gram compared to low Z materials
Also due to high Z materials having tighter bound electrons/higher BE
Mass stopping power
Rate of energy loss of electrons of 1MeV and above water is about ___MeV/cm
2Mev/cm
Probability of radiation loss relative to collisional loss _______ with electron energy and Z
Increases
Equation for 90%, 80%, 50%, and the practical range (Rp) electron isodose lines
90% = E/4 80% = E/3 50% = E/2.5 Rp = E/2
Increased field size (FS) leads to _________ scatter from collimator as well as the phantom
Increased
Increased FS = _______ PDD
Increase
Increase FS = depth of Dmax shifts toward the _________
Surface
After passing through vacuum window, bend magnet, scattering foil, monitor chamber and air column, the electron beam appears to diverge from a point
Point where electrons start to diverge
3 cm when they go through accelerator, point after scattering foil closer to patient
Close to patient and further from head of machine than photon source
Virtual source (VS)
3 things electron beam energy selection is dictated by
Depth of target volume
Minimum target dose required
Dose to normal tissue
Beam obliquity = ________ side scatter at Dmax depth
Increased
Beam obliquity = shift of Dmax towards the __________
Surface
Beam obliquity = ________ depth of penetration
Decreased
Electron correction factor/effective thickness for tissue inhomogeneities related to stopping power and depends on energy and depth
Coefficient equivalent thickness (CET)
Electron density
Effective dose (Deff) formula
Deff = d1(CET) + d2(CET) d3(CET)
d = measured depth
CET of spongy and compact bone and lung
Compact = 1.65 Spongy = 1 Lung = 0.2-0.33
3 purposes of bolus
Flatten out irregular surfaces
Reduce penetration
Increase surface dose
When an electron field is abutted to a photon field, a hot spot develops on the side of the _______ field and a cold spot develops on the side of the _______ field
Photon, electron
Rule of thumb for electron lead cutout field shaping devices
1/2 the energy + 1mm
3 situations the require internal shielding during electron treatments
Lip
Buccal mucosa
Eyelids
While lead can be a good stopping medium, it can cause backscatter; to eliminate the effect backscatter, a ____-Z absorber is placed between the lead and preceding tissue
Low-Z
3 total skin irradiation (TSI) techniques
Transitional
Large field/Stanford
Modified Stanford
Patient lies on a motor driven couch and is moved in a downward motion or the patient is stationary and the radiation source is translated horizontally
Transitional technique
Large electron fields can be produced by scattering electrons through wide angles and using large treatment distances
Patient is treated in a standing position with 4-6 fields equally spaced around the patient
X-ray contamination becomes a limiting factor
Large field technique
Uses 6 fields spaced 60 degrees apart (AP/PA and four obliques)
AP and two obliques, PA and two obliques
Stanford technique
2 treatment planning algorithms
Pencil beam
Monte Carlo
Algorithm assumes a collimated photon beam striking a patient is a collection of many smaller, narrow pencil beams
These pencil beams have a central axis where it deposits dose which varies with intensity and spectrum of beam
Pencil beam
Algorithm takes into account millions of interactions (Co, Pho, and Com) which lead to more electron interactions
Large statistical probability calculation
More accurate dose calculation algorithm, but very time consuming due to number of statistics it must consider
Monte Carlo
Gap calculation formula
(1/2)(L1)(d/SSD) + (1/2)(L2)(d/SSD)
Distance that’s equivalent to that measured in water
Distance x equivalent thickness
Equivalent thickness/path
Same tissue density
Homogeniety
Different tissue density
Heterogeneity
Maximum range obtained by electrons incident on the surface
Practical range (Rp)
Electrons have _________ block margins than photons because of scatter and penumbra
Wider
Increase electron energy = _________ skin dose and dose at depth
Increase
Electron Dmax is a __________
Range
4 electron PDD curve characteristics
Buildup
Range
Fall-off
Photon contamination tail
2 causes of photon contamination
Head of machine (majority from high Z material)
Patient
For head and neck (H&N) treatments; treat with photons until cord tolerance is reached, then treat with electrons of cord so they fall off before reaching cord depth and still get dose to LNs
Posterior triangles
What is a treatment that commonly uses a bolus?
Chest wall
Increasing or decreasing the dose at a given percentage because electrons are prescribed to certain isodose lines, usually 90%
Normalization/scaling
98% scaling means a ______ increase
2%
More oblique beam ________ skin dose
Increases
5 electron applicator/cone sizes
6x6 10x10 15x15 20x20 25x25
What are the different components in the linac in photon (2) versus electron (1) mode?
Photon: target, flattening filter
Electron: scattering foil
2 factors electron output factor varies with
SSD
Cone/applicator size
Electron MU formula
TD/output
What is the typical electron SSD and blocking tray distance, and why?
SSD: 105 cm
Blocking tray: 95 cm
Since there’s only 5 cm between patient and cone, extend to 105 cm so patient doesn’t get hit
Do electrons follow the inverse square law (ISL) and why?
They don’t follow the ISL because they repel each other
Most useful electron energies are between ___ and ___ MeV
6 and 20 MeV
The short, well-defined range of electrons makes them advantageous for treating superficial tumors at a depth of _____ cm or less and if we tried to treat past this, we’d burn the skin to get that deep
5 cm
Are electrons mono- or polyenergetic?
Monoenergetic (MeV)
Are electrons treated SSD or SAD?
SSD
Setup by looking at skin surface/scar wire; don’t use imaging (IGRT) because electrons are superfiecial
Clinical setup
Small blocks put into end of applicator that shapes electron field ports
Electron cutouts
Increase cone size = _______ output factor
Increase
Relationship necessary block thickness formula; lead sufficient to completely stop electrons but some x-ray contamination may penetrate the cutout
tPb(mm) = 0.5E0(MeV) + 1
For the same transmission as lead, cerrobend cutouts needs to be a little bit thicker; thickness of cerrobend in millimeters (tC[mm]) formula
tC(mm) = 1.2tPb(mm)
What is the purpose of internal shielding?
Protect internal structures with lead and wax
Electron beams bow _______ because they’re negatively charged and scatter more
Outward
Provides communication standards for sharing image information
Digital imaging and communications in medicine (DICOM)
Describe formats for the exchange of image or textual information
Information object definitions
6 information object definitions
Radiation therapy (RT) image RT dose RT structure set RT plan RT beams and brachytherapy RT treatment summary
Conventional and virtual simulation images, DRRs, and ports
RT image
Dose distributions, isodose lines, and DVHs
RT dose
Volumetric contours drawn from CT images
RT structure set
Text information that describes treatment plans, including prescriptions and fractionation, beam definitions, etc.
RT plan
Treatment session reports for EBRT or brachytherapy, may be used as part of a record and verify (V&R) system
RT beams and brachytherapy
Cumulative summary information, may be used after treatment to send information to hospital EMR
RT treatment summary
Match divergence from PA spine field (SSD)
Collimator angle
Accounts for divergence from lateral cranial fields (SAD)
Couch kick
Inverse tangent (tan^-1) formula
tan^-1 = opposite (o)/adjacent (a)
Measured depth
Physical depth
Effective depth formula
(d1)(Pe1) + (d2)(Pe2) + (d3)(Pe3)
TAR method correction factor (CF)
CF = TAR(effD,FS)/TAR(physical D,FS) CF = hetero dose/homo dose
Therapy that delivers non-uniform exposure across the radiation field using a variety of techniques and equipment; CT and tell computer treatment goals with DVH
Intensity modulated RT (IMRT)
IMRT has _____ MUs than 3D treatment planning because it modulates the whole time while 3D field is open the whole time but IMRT is more ________
More, conformal
Five or less beams per fraction
Stereotactic
Cranial treatment has less fractions, delivers a large dose of radiation on a single day
Stereotactic radiosurgery (SRS)
Body treatment from cranium down
Stereotactic body radiotherapy (SBRT)
Delivers a large dose of radiation on a fractionated treatment schedule
Stereotactic radiotherapy (SRT)
Sequence of leaves moving for repositioning, then coming to rest while beam’s delivered in multiple segments at each gantry angle
Step and shoot/segmental MLC (SMLC)
MLC moves from one side of field to another within a narrow opening while beam’s on, more MUs because beam’s staying on the whole time
Sliding window (IMRT)
Rule of thumb for wedge placement
15-20 cm away from patient or they’ll get too much scatter
Scatter comes off wedge/compensator, closer to patient ______ skin dose
Increases
2 dose at tissue interfaces
Re-dmaxing
Bone and tissue
When going through lung, why would you rather use a 6X than 18X?
Redmaxing effect (scatter in = scatter out) 6X used for lungs because their Dmax is shallower, builds up in tumor since there's not scatter equilibrium in lung/air The smaller the lung tumor, the more important it is to use a lower energy
Until you get to water, don’t have backscatter to build-up to Dmax; nothing to build-up against in air
Goes through air without interacting and has to build back up
Re-dmaxing
Why is 18X not used for IMRT?
Neutron contamination begins to occur at 10 MV and IMRT uses more MUs, which increases the chance of neutron contamination
Neutrons want to combine with patient/hydrogenous material; weighting factor = 10, more biologically damaging
Point through tissue/water before bone would have higher dose because backscatter increases; point in tissue/water after bone would have less dose because of shielding effect
Bone and tissue interface
Beam goes through bone and has to re-dmax in water/tissue so it has less dose
Shielding effect
CT number/Hounsfield Unit (HU) formula
HU = 1000(ut-uw/uw)
ut = linear attenuation coefficient (LAC) of tissue under analysis uw = LAC of water = 1
Intensity after half value layer (HVL) formula
Ix = Ioe^-ux
Ix = intensity after filtration Io = original intensity u = LAC per unit length x = filter thickness
Mass attenuation coefficient formula
u/P
P = density
Percent transmitted formula
Ix/o = e^-ux
HVL as a thickness formula
0.693/u
Number of HVLs formula
Ix/Io = (1/2)^n
