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RAD 400 > Test 2 > Flashcards

Test 2 Flashcards

1
Q

Family of isodose curves usually drawn at equal increments of percent depth dose (PDD), depth dose values are usually normalized in reference to the prescription dose
Ex: 100%, 90%, 80%, etc.

A

Isodose chart

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2
Q

Isodose lines are usually normalized in reference to the prescription dose

A

Absolute dose

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3
Q

Isodose lines are given in percentages relative to the prescription dose; 105%, 100%, 90%, etc.

A

Relative dose

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4
Q

4 isodose line properties

A

Dose at any depth is greatest on central axis (CA) and decreases laterally away from the CA
Near beam edges the penumbra region exists
Near beam edges, the dose reduction is not only due to geometric penumbra but also from reduced side scatter
Outside the geometric limits of the beam and penumbra, dose is due to side scatter as well as leakage

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5
Q

Lateral distance between 90-20% isodose lines at a depth of Dmax
Rapid falloff region of dose
Scatter only coming from light side

A

Physical penumbra

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6
Q

Dose variation across the field while staying at a specified depth

A

Beam profile

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7
Q

Coincidence of the light field and the 50% isodose line of the radiation field
Verified with QA test: marking the light field on radiochromic film, then exposing the film

A

Beam alignment

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8
Q

Another way of depicting dose variation across a field is to plot isodose curves in a plane __________ to CA

A

Perpendicular

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9
Q

Most common tool to measure isodose curves

A

Ion chamber

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10
Q

6 parameters of isodose curves

A 
Beam quality/energy
Source size
Beam collimation
Field size
SSD
SDD
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11
Q

Higher energy carries dose deeper in a medium and is more ________ peaked
Lower energy has wider penumbra regions so isodose lines ________ out on the side

A

Forward, bulge

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12
Q

Source size, SDD, and SSD affect penumbra by virtue of __________ penumbra

A

Geometric

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13
Q

Increase source size = ________ geometric penumbra

A

Increase

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14
Q

Increase SSD = ________ geometric penumbra

A

Increase

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15
Q

Increase SDD = ________ geometric penumbra

A

Decrease

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16
Q

A smaller field size(FS)/collimation eliminates more scatter, so dose at depth ________

A

Decreases

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17
Q

Makes a more forward peaked beam and has a hardening effect

A

Flattening filter

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18
Q

Beam at a depth of 10 cm with flattening filter; beam is within 3% across 80% of the field or 1 cm from the field edge

A

Flat

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19
Q

3 accelerators that don’t need a flattening filter

A

Radiosurgery machines: very small field sizes
Tomotherapy
Modulated fields

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20
Q

3 advantages of flattening filter free (FFF)

A

Higher dose rate
Less side scatter outside the field
Shorter treatment times

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21
Q

Field size selection must always be made __________ rather than geometrically; a certain isodose should be selected to cover a field, rather than a predetermined __________

A

Dosimetrically, field border

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22
Q

Caution should be used with small field sizes as a large portion of the field will lie within the __________ region; isodose curves tend to be _______-shaped
Ex: if there is 1 cm of penumbra on a given field, this is much more pronounced in a 5x5 cm field compared to a 20x20 cm field

A

Penumbra, bell

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23
Q

2 types of wedge filters

A

Physical

Nonphysical

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24
Q

Wedge shaped absorber that causes a progressive decrease in beam intensity, resulting in a tilted isodose line
Has more scatter to patient because it’s mounted outside treatment head; forgetting to place this leads to over-treating a patient

A

Physical wedge

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25
Q

Single wedge serves for each beam width

60 degree wedge used with relative open field

A

Universal wedge

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26
Q

Electronic filter that generates a tilder isodose line by moving a collimator jaw
Superseded by IMRT technology (MLC) movement
Varian: Enhanced Dynamic Wedge (EDW); Siemens’: Virtual Wedge

A

Nonphysical wedge

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27
Q

3 advantages of nonphysical wedges

A

Automation of treatment delivery
Less chance of user error
Less scatter to patient: 15 cm minimum distance away from patient

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28
Q

1 disadvantage of nonphysical wedges

A

More effort for commissioning

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29
Q

Angle of isodose lines at CA at a reference depth of 10 cm
Isodose curve angle at the central axis at a specified depth, ICRU recommends this depth to be 10 cm
Dosimetrically a 45 degree angle
Angle of isodose lines

A

Wedge angle

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30
Q

Ratio of dose with and without wedge, always less than 1

A

Wedge factor (WF)

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31
Q

WF _________ MUs in proportion

A

Increases

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32
Q

Require a separate wedge for each beam width
Designed to minimize loss of beam output
Physics labor intensive; must measure beam data for every small change

A

Individualized wedge system

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33
Q

_______ of wedge should be at border; if center of wedge oriented at CA, MUs __________

A

Toe, increase

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34
Q

3 criteria for using a single field

A

Target uniformity is within 5%
Max dose to tissues in the beam is not excessive: over 110%
Normal critical structures don’t exceed tolerance dose

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35
Q

Simplest combination of two fields

A

Parallel opposed fields

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36
Q

3 advantages of parallel opposed fields

A

Simplicity and reproducibility of setup
Homogeneous target dose
Less chance of geometric miss

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37
Q

1 disadvantage of parallel opposed fields

A

Excessive dose to normal tissue above and below tumor

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38
Q

All doses close to prescription; depends on patient thickness and beam energy and flatness

A

Dose uniformity

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39
Q

Increase patient thickness/diameter = _________ uniformity

A

Decreased

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40
Q

Increase beam energy = _________ uniformity because higher energy pushes dose further
Lower energy has more entry and exit dose; higher energy carries dose through

A

Increase

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41
Q

Increase beam flatness in profile = _________ uniformity

A

Increase

42
Q

Dose closer to the surface relatively compared to the dose at the midpoint/isocenter

A

Peripheral dose

43
Q

The lower the peripheral dose/midpoint dose ratio (closest to 1), the ______ uniform dose distribution is
Higher energies have _________ peripheral dose/midpoint dose ratios

A

More, better

44
Q

Data shows that there is _________ biologic damage with using higher daily dose from one field, even though the total dose is the same

A

Greater

45
Q

6 ways dose uniformity and normal tissue sparing can be achieved; treatment planning seeks to deliver maximum target dose while preserving the function of normal tissues

A 
Appropriate FS
Increasing the number of fields
Beam directions
Beam weighting
Beam energy
Beam modifiers
46
Q

Distance from source to axis always remains the same

A

Isocentric techniques

47
Q

2 types of isocentric techniques

A

Static beams

Rotational arcs

48
Q

With static beams (IMRT uses computer-modulated MLCs), SAD of 100 remains constant but SSD varies with what?

A

SSD = SAD - depth

49
Q

Beam moves continuously around patient, best suited for deep seated tumors
Can be faster
Go all the way around patient, use computer-modulated MLCs

A

Rotational arcs

50
Q

3 contraindications for rotational arcs

A

Irradiated volume is too large
External surface differs too much from a cylinder
Tumor is far off center

51
Q

Partial arcs have hotspots displaced toward the surface, so they should be aimed at a distance just beyond the tumor

A

Past pointing

52
Q

Hot spot of up to ___% in the treatment volume is usually acceptable

A

10%

53
Q

Hot spots often occur under the ______ edge of the wedge; however _____________ can occur with large hotspots under the toe

A

Thin, over-wedging

54
Q

Wedges generally suitable when a tumor is ___-___ cm deep in tissue

A

0-7 cm

55
Q

Purpose is to modify the shape of the isodose curves by changing the beam intensity across the field
Most desirable feature is rapid dose falloff beyond overlap region
Do not always have to match
Used in breast treatments, larynx, etc.
Heels go together

A

Wedge (filters)

56
Q

Gross demonstrable extent and location of a tumor, delineation possible with imaging

A

Gross tumor volume (GTV)

57
Q

GTV plus presumed tumor/microscopic disease

A

Clinical target volume (CTV)

58
Q

Compensates for physiologic movements and CTV size, shape, and position variation

A

Internal margin (IM)

59
Q

CTV and IM

A

Internal target volume (ITV)

60
Q

Compensates for movement and setup uncertainties

A

Planning target volume (PTV)

61
Q

Includes organs at risk plus a margin for movement

A

Planning organs at risk (PRV)

62
Q

Represents the volume enclosed by the isodose line that covers the PTV adequately

A

Treated volume (TV)

63
Q

Corresponds to the 50% isodose volume

A

Irradiated volume (IV)

64
Q

Highest dose in the target volume that covers 2 cm^3

A

Maximum target dose

65
Q

Lowest dose in the target volume

A

Minimum target dose

66
Q

Value between minimum and maximum dose values in the target

A

Mean target dose

67
Q

Most frequent dose that occurs in the target volume

A

Modal target dose

68
Q

Area outside target that covers a volume of 2 cm^3

A

Hot spots

69
Q

4 criteria for reference point that target dose should be specified and recorded at

A

Clinically relevant and representative of dose throughout the PTV
Easy to define in a clear way
Selected where dose can be accurately calculated
Not in penumbra region or within a steep gradient

70
Q

Lines passing through points of equal dose

A

Isodose curves

71
Q

2 things physical penumbra is a function of

A

Geometric penumbra

Lateral scatter

72
Q

Distance between the 50% isodose lines at Dmax

A

Field size (FS)

73
Q

Field defining light should coincide with 50% isodose lines within 2 mm (+/- 2 cm)

A

Alignment

74
Q

Hinge angle (HA) formula

A

HA = 180 - 2WA

WA = wedge angle

75
Q

2 components 2 beams have

A

Entry

Exit

76
Q

The calculation point is at the _________ part because you want it to get the whole prescription

A

Thickest

77
Q

3 beam modifiers

A

Wedges
Dynamic wedges by jaws
MLCs

78
Q

Angle between two beams

A

Hinge angle (HA)

79
Q

Increase HA = ________ WA; when beams spread out, overlap is not as bad

A

Decrease

80
Q

Wedge over attenuates and now apex is cold

A

Over-wedge

81
Q

Wedge ________ treatment time/MUs

A

Increases

82
Q

SAD MU formula with WF

A

MU = TD / (Dfs x INV^2 x TAR x WF)

83
Q

SAD POI formula with WF

A

POI = MU x Dfs x INV^2 x TAR x WF

84
Q

Modulated fields (IMRT, blocking field in segments, etc.) __________ MUs

A

Increase

85
Q

Increase number of fields = ________ uniformity in target

A

Increase

86
Q

Uniform dose formula

A

Peripheral dose/midpoint dose = 1

87
Q

WF affects _____, not doses relative to each other

A

MUs

88
Q

Dose is hotter on ________ weighted side

A

Higher

89
Q

Object has a very irregular shape, not square or circle, and amount of scatter is unknown; ex: mantle field

A

Irregular field size

90
Q

Calculates irregular field sizes

A

Clarkson algorithm

91
Q

Ratio of the scattered dose at a given point to the dose in free space at the same point

A

Scatter air ratio (SAR)

92
Q

Total radiation formula used to find scatter

A

TARd,fs = TARd + SARd,fs

TARd,fs = total
TARd = primary, no FS just Dfs
SARd,fs = scatter
93
Q

4 benefits of low MUs

A

Decreased treatment time
Patient doesn’t have to lay on table as long (mets)
Uncertainty in dose plans with more MUs because of more leakage and scatter
More economical

94
Q

Peripheral versus midpoint dose (max/Rx) usually _______ 1 with one and two fields (uniformity)

A

Greater than

95
Q

Increase energy and number of fields = _______ (max/Rx) = ________ uniformity

A

Decrease, increase

96
Q

Increase patient thickness = _______ (max/Rx) = _______ uniformity

A

Increase, decrease

97
Q

Sum of primary and scatter radiation, can measure total and primary radiation with ion chamber

A

Total radiation

98
Q

Cumulative histogram, plot of target or normal structure volume as a function of dose
Percent of volume dose, or percent of prescribed dose, or above
Tells if organ is failing
Want 100% of target to get all of dose without exposing other organs

A

Dose volume histogram (DVH)

99
Q

Increased number of fields = _________________ dose

A

Spread-out

100
Q

Wedge factor (WF) formula

A

Dose with wedge/dose without wedge

RAD 400 (4 decks)

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