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RAD 400 > Test 3 > Flashcards

Test 3 Flashcards

1
Q

4 ways electrons interact as they travel through matter

A

Inelastic collisions with atomic electrons
Inelastic collisions with atomic nuclei (bremsstrahlung)
Elastic collisions with atomic electrons (electron-electron scattering)
Elastic collisions with atomic nuclei

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2
Q

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

A

Inelastic collisions

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3
Q

KE is not lost, but it may be redistributed among particles emerging from collision
More common in higher Z mediums such as lead

A

Elastic collisions

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4
Q

Rate of energy loss depends on electron density of the medium

A

Collisional losses (ionization and excitation)

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5
Q

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

A

Mass stopping power

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6
Q

Rate of energy loss of electrons of 1MeV and above water is about ___MeV/cm

A

2Mev/cm

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7
Q

Probability of radiation loss relative to collisional loss _______ with electron energy and Z

A

Increases

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8
Q

Equation for 90%, 80%, 50%, and the practical range (Rp) electron isodose lines

A 
90% = E/4
80% = E/3
50% = E/2.5
Rp = E/2
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9
Q

Increased field size (FS) leads to _________ scatter from collimator as well as the phantom

A

Increased

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10
Q

Increased FS = _______ PDD

A

Increase

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11
Q

Increase FS = depth of Dmax shifts toward the _________

A

Surface

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12
Q

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

A

Virtual source (VS)

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13
Q

3 things electron beam energy selection is dictated by

A

Depth of target volume
Minimum target dose required
Dose to normal tissue

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14
Q

Beam obliquity = ________ side scatter at Dmax depth

A

Increased

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15
Q

Beam obliquity = shift of Dmax towards the __________

A

Surface

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16
Q

Beam obliquity = ________ depth of penetration

A

Decreased

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17
Q

Electron correction factor/effective thickness for tissue inhomogeneities related to stopping power and depends on energy and depth

A

Coefficient equivalent thickness (CET)

Electron density

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18
Q

Effective dose (Deff) formula

A

Deff = d1(CET) + d2(CET) d3(CET)

d = measured depth

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19
Q

CET of spongy and compact bone and lung

A 
Compact = 1.65
Spongy = 1
Lung = 0.2-0.33
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20
Q

3 purposes of bolus

A

Flatten out irregular surfaces
Reduce penetration
Increase surface dose

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21
Q

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

A

Photon, electron

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22
Q

Rule of thumb for electron lead cutout field shaping devices

A

1/2 the energy + 1mm

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23
Q

3 situations the require internal shielding during electron treatments

A

Lip
Buccal mucosa
Eyelids

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24
Q

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

A

Low-Z

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25
Q

3 total skin irradiation (TSI) techniques

A

Transitional
Large field/Stanford
Modified Stanford

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26
Q

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

A

Transitional technique

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27
Q

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

A

Large field technique

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28
Q

Uses 6 fields spaced 60 degrees apart (AP/PA and four obliques)
AP and two obliques, PA and two obliques

A

Stanford technique

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29
Q

2 treatment planning algorithms

A

Pencil beam

Monte Carlo

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30
Q

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

A

Pencil beam

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31
Q

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

A

Monte Carlo

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32
Q

Gap calculation formula

A

(1/2)(L1)(d/SSD) + (1/2)(L2)(d/SSD)

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33
Q

Distance that’s equivalent to that measured in water

Distance x equivalent thickness

A

Equivalent thickness/path

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34
Q

Same tissue density

A

Homogeniety

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35
Q

Different tissue density

A

Heterogeneity

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36
Q

Maximum range obtained by electrons incident on the surface

A

Practical range (Rp)

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37
Q

Electrons have _________ block margins than photons because of scatter and penumbra

A

Wider

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38
Q

Increase electron energy = _________ skin dose and dose at depth

A

Increase

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39
Q

Electron Dmax is a __________

A

Range

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40
Q

4 electron PDD curve characteristics

A

Buildup
Range
Fall-off
Photon contamination tail

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41
Q

2 causes of photon contamination

A

Head of machine (majority from high Z material)

Patient

42
Q

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

A

Posterior triangles

43
Q

What is a treatment that commonly uses a bolus?

A

Chest wall

44
Q

Increasing or decreasing the dose at a given percentage because electrons are prescribed to certain isodose lines, usually 90%

A

Normalization/scaling

45
Q

98% scaling means a ______ increase

A

2%

46
Q

More oblique beam ________ skin dose

A

Increases

47
Q

5 electron applicator/cone sizes

A 
6x6
10x10
15x15
20x20
25x25
48
Q

What are the different components in the linac in photon (2) versus electron (1) mode?

A

Photon: target, flattening filter
Electron: scattering foil

49
Q

2 factors electron output factor varies with

A

SSD

Cone/applicator size

50
Q

Electron MU formula

A

TD/output

51
Q

What is the typical electron SSD and blocking tray distance, and why?

A

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
Q

Do electrons follow the inverse square law (ISL) and why?

A

They don’t follow the ISL because they repel each other

53
Q

Most useful electron energies are between ___ and ___ MeV

A

6 and 20 MeV

54
Q

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

A

5 cm

55
Q

Are electrons mono- or polyenergetic?

A

Monoenergetic (MeV)

56
Q

Are electrons treated SSD or SAD?

A

SSD

57
Q

Setup by looking at skin surface/scar wire; don’t use imaging (IGRT) because electrons are superfiecial

A

Clinical setup

58
Q

Small blocks put into end of applicator that shapes electron field ports

A

Electron cutouts

59
Q

Increase cone size = _______ output factor

A

Increase

60
Q

Relationship necessary block thickness formula; lead sufficient to completely stop electrons but some x-ray contamination may penetrate the cutout

A

tPb(mm) = 0.5E0(MeV) + 1

61
Q

For the same transmission as lead, cerrobend cutouts needs to be a little bit thicker; thickness of cerrobend in millimeters (tC[mm]) formula

A

tC(mm) = 1.2tPb(mm)

62
Q

What is the purpose of internal shielding?

A

Protect internal structures with lead and wax

63
Q

Electron beams bow _______ because they’re negatively charged and scatter more

A

Outward

64
Q

Provides communication standards for sharing image information

A

Digital imaging and communications in medicine (DICOM)

65
Q

Describe formats for the exchange of image or textual information

A

Information object definitions

66
Q

6 information object definitions

A 
Radiation therapy (RT) image
RT dose
RT structure set
RT plan
RT beams and brachytherapy
RT treatment summary
67
Q

Conventional and virtual simulation images, DRRs, and ports

A

RT image

68
Q

Dose distributions, isodose lines, and DVHs

A

RT dose

69
Q

Volumetric contours drawn from CT images

A

RT structure set

70
Q

Text information that describes treatment plans, including prescriptions and fractionation, beam definitions, etc.

A

RT plan

71
Q

Treatment session reports for EBRT or brachytherapy, may be used as part of a record and verify (V&R) system

A

RT beams and brachytherapy

72
Q

Cumulative summary information, may be used after treatment to send information to hospital EMR

A

RT treatment summary

73
Q

Match divergence from PA spine field (SSD)

A

Collimator angle

74
Q

Accounts for divergence from lateral cranial fields (SAD)

A

Couch kick

75
Q

Inverse tangent (tan^-1) formula

A

tan^-1 = opposite (o)/adjacent (a)

76
Q

Measured depth

A

Physical depth

77
Q

Effective depth formula

A

(d1)(Pe1) + (d2)(Pe2) + (d3)(Pe3)

78
Q

TAR method correction factor (CF)

A 
CF = TAR(effD,FS)/TAR(physical D,FS)
CF = hetero dose/homo dose
79
Q

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

A

Intensity modulated RT (IMRT)

80
Q

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 ________

A

More, conformal

81
Q

Five or less beams per fraction

A

Stereotactic

82
Q

Cranial treatment has less fractions, delivers a large dose of radiation on a single day

A

Stereotactic radiosurgery (SRS)

83
Q

Body treatment from cranium down

A

Stereotactic body radiotherapy (SBRT)

84
Q

Delivers a large dose of radiation on a fractionated treatment schedule

A

Stereotactic radiotherapy (SRT)

85
Q

Sequence of leaves moving for repositioning, then coming to rest while beam’s delivered in multiple segments at each gantry angle

A

Step and shoot/segmental MLC (SMLC)

86
Q

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

A

Sliding window (IMRT)

87
Q

Rule of thumb for wedge placement

A

15-20 cm away from patient or they’ll get too much scatter

88
Q

Scatter comes off wedge/compensator, closer to patient ______ skin dose

A

Increases

89
Q

2 dose at tissue interfaces

A

Re-dmaxing

Bone and tissue

90
Q

When going through lung, why would you rather use a 6X than 18X?

A 
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
Q

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

A

Re-dmaxing

92
Q

Why is 18X not used for IMRT?

A

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
Q

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

A

Bone and tissue interface

94
Q

Beam goes through bone and has to re-dmax in water/tissue so it has less dose

A

Shielding effect

95
Q

CT number/Hounsfield Unit (HU) formula

A

HU = 1000(ut-uw/uw)

ut = linear attenuation coefficient (LAC) of tissue under analysis
uw = LAC of water = 1
96
Q

Intensity after half value layer (HVL) formula

A

Ix = Ioe^-ux

Ix = intensity after filtration
Io = original intensity
u = LAC per unit length
x = filter thickness
97
Q

Mass attenuation coefficient formula

A

u/P

P = density

98
Q

Percent transmitted formula

A

Ix/o = e^-ux

99
Q

HVL as a thickness formula

A

0.693/u

100
Q

Number of HVLs formula

A

Ix/Io = (1/2)^n

RAD 400 (4 decks)

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