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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

Number of HVLs formula

Ix/Io = (1/2)^n