Flashcards in Test 3 Deck (100)
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1
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
2
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
3
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
4
Rate of energy loss depends on electron density of the medium
Collisional losses (ionization and excitation)
5
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
6
Rate of energy loss of electrons of 1MeV and above water is about ___MeV/cm
2Mev/cm
7
Probability of radiation loss relative to collisional loss _______ with electron energy and Z
Increases
8
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
9
Increased field size (FS) leads to _________ scatter from collimator as well as the phantom
Increased
10
Increased FS = _______ PDD
Increase
11
Increase FS = depth of Dmax shifts toward the _________
Surface
12
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)
13
3 things electron beam energy selection is dictated by
Depth of target volume
Minimum target dose required
Dose to normal tissue
14
Beam obliquity = ________ side scatter at Dmax depth
Increased
15
Beam obliquity = shift of Dmax towards the __________
Surface
16
Beam obliquity = ________ depth of penetration
Decreased
17
Electron correction factor/effective thickness for tissue inhomogeneities related to stopping power and depends on energy and depth
Coefficient equivalent thickness (CET)
Electron density
18
Effective dose (Deff) formula
Deff = d1(CET) + d2(CET) d3(CET)
d = measured depth
19
CET of spongy and compact bone and lung
Compact = 1.65
Spongy = 1
Lung = 0.2-0.33
20
3 purposes of bolus
Flatten out irregular surfaces
Reduce penetration
Increase surface dose
21
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
22
Rule of thumb for electron lead cutout field shaping devices
1/2 the energy + 1mm
23
3 situations the require internal shielding during electron treatments
Lip
Buccal mucosa
Eyelids
24
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
25
3 total skin irradiation (TSI) techniques
Transitional
Large field/Stanford
Modified Stanford
26
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
27
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
28
Uses 6 fields spaced 60 degrees apart (AP/PA and four obliques)
AP and two obliques, PA and two obliques
Stanford technique
29
2 treatment planning algorithms
Pencil beam
Monte Carlo
30
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
31
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
32
Gap calculation formula
(1/2)(L1)(d/SSD) + (1/2)(L2)(d/SSD)
33
Distance that's equivalent to that measured in water
Distance x equivalent thickness
Equivalent thickness/path
34
Same tissue density
Homogeniety
35
Different tissue density
Heterogeneity
36
Maximum range obtained by electrons incident on the surface
Practical range (Rp)
37
Electrons have _________ block margins than photons because of scatter and penumbra
Wider
38
Increase electron energy = _________ skin dose and dose at depth
Increase
39
Electron Dmax is a __________
Range
40
4 electron PDD curve characteristics
Buildup
Range
Fall-off
Photon contamination tail
41
2 causes of photon contamination
Head of machine (majority from high Z material)
Patient
42
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
43
What is a treatment that commonly uses a bolus?
Chest wall
44
Increasing or decreasing the dose at a given percentage because electrons are prescribed to certain isodose lines, usually 90%
Normalization/scaling
45
98% scaling means a ______ increase
2%
46
More oblique beam ________ skin dose
Increases
47
5 electron applicator/cone sizes
6x6
10x10
15x15
20x20
25x25
48
What are the different components in the linac in photon (2) versus electron (1) mode?
Photon: target, flattening filter
Electron: scattering foil
49
2 factors electron output factor varies with
SSD
Cone/applicator size
50
Electron MU formula
TD/output
51
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
52
Do electrons follow the inverse square law (ISL) and why?
They don't follow the ISL because they repel each other
53
Most useful electron energies are between ___ and ___ MeV
6 and 20 MeV
54
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
55
Are electrons mono- or polyenergetic?
Monoenergetic (MeV)
56
Are electrons treated SSD or SAD?
SSD
57
Setup by looking at skin surface/scar wire; don't use imaging (IGRT) because electrons are superfiecial
Clinical setup
58
Small blocks put into end of applicator that shapes electron field ports
Electron cutouts
59
Increase cone size = _______ output factor
Increase
60
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
61
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)
62
What is the purpose of internal shielding?
Protect internal structures with lead and wax
63
Electron beams bow _______ because they're negatively charged and scatter more
Outward
64
Provides communication standards for sharing image information
Digital imaging and communications in medicine (DICOM)
65
Describe formats for the exchange of image or textual information
Information object definitions
66
6 information object definitions
Radiation therapy (RT) image
RT dose
RT structure set
RT plan
RT beams and brachytherapy
RT treatment summary
67
Conventional and virtual simulation images, DRRs, and ports
RT image
68
Dose distributions, isodose lines, and DVHs
RT dose
69
Volumetric contours drawn from CT images
RT structure set
70
Text information that describes treatment plans, including prescriptions and fractionation, beam definitions, etc.
RT plan
71
Treatment session reports for EBRT or brachytherapy, may be used as part of a record and verify (V&R) system
RT beams and brachytherapy
72
Cumulative summary information, may be used after treatment to send information to hospital EMR
RT treatment summary
73
Match divergence from PA spine field (SSD)
Collimator angle
74
Accounts for divergence from lateral cranial fields (SAD)
Couch kick
75
Inverse tangent (tan^-1) formula
tan^-1 = opposite (o)/adjacent (a)
76
Measured depth
Physical depth
77
Effective depth formula
(d1)(Pe1) + (d2)(Pe2) + (d3)(Pe3)
78
TAR method correction factor (CF)
CF = TAR(effD,FS)/TAR(physical D,FS)
CF = hetero dose/homo dose
79
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)
80
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
81
Five or less beams per fraction
Stereotactic
82
Cranial treatment has less fractions, delivers a large dose of radiation on a single day
Stereotactic radiosurgery (SRS)
83
Body treatment from cranium down
Stereotactic body radiotherapy (SBRT)
84
Delivers a large dose of radiation on a fractionated treatment schedule
Stereotactic radiotherapy (SRT)
85
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)
86
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)
87
Rule of thumb for wedge placement
15-20 cm away from patient or they'll get too much scatter
88
Scatter comes off wedge/compensator, closer to patient ______ skin dose
Increases
89
2 dose at tissue interfaces
Re-dmaxing
Bone and tissue
90
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
91
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
92
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
93
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
94
Beam goes through bone and has to re-dmax in water/tissue so it has less dose
Shielding effect
95
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
96
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
97
Mass attenuation coefficient formula
u/P
P = density
98
Percent transmitted formula
Ix/o = e^-ux
99
HVL as a thickness formula
0.693/u
100
