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Radiation Dosimetry > Midterm Exam > Flashcards

Midterm Exam Flashcards

1
Q

What is Roentgen?

A

Unit of exposure in air

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2
Q

What is a Rad?

A

Radiation Absorbed Dose

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3
Q

What is an f factor?

A

Roentgen to cGy conversion factor

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4
Q

What factors affect the f factor?

A

Machine Energy and Density of Material

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5
Q

1 Rad =

A

1 cGy

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6
Q

1 Roentgen (R) =

A

2.58 x 10^-4 Coulombs/kilogram of air

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7
Q

D/max

A

Depth of maximum absorbed dose and ionization

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8
Q

What is the Dmax of a 6 Mv beam?

A

1.5 cm below the skin

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9
Q

What is the Dmax of a Co-60 beam?

A

0.5 cm below the skin

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10
Q

What is the Dmax of a 10 Mv beam?

A

2.5 cm below the skin

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11
Q

What is the Dmax of a 18 Mv beam?

A

3.5 below the skin

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12
Q

What is the Dmax of a Orthovoltage machine?

A

On the surface of the skin

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13
Q

What affects Dmax? Explain how.

A

Beam Energy, as your increase in beam energy then Dmax goes deeper in the patient

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14
Q

What is a Half Value Layer?

A

Some thickness of a material that decrease the beam intensity by half its value

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15
Q

How much of the beam remains after 1 HVL?

A

50%

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16
Q

How much of the beam remains after 2 HVL?

A

25%

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17
Q

How much of the beam remains after 3 HVL?

A

12.5%

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18
Q

How much of the beam remains after 4 HVL?

A

6.25%

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19
Q

What is the Isocenter?

A

A point of intersection of the collimator axis and gantry axis rotation

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20
Q

What is erythema?

A

Skin reddening

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21
Q

What is an Erythema dose?

A

The dose amount, which when given with cause skin reddening

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22
Q

What is bolus?

A

tissue equivalent material utilized during treatments to raise the dose higher on the skin

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23
Q

What is the requirement to be considered bolus?

A

have the same density as tissue

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24
Q

What are the 5 common materials utilized as bolus?

A

Rice, Superflab, Water, superstuff, Wax

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25
Q

What is SSD?

A

Source to Skin distance

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26
Q

What is SAD? Explain its relationship to SSD.

A

Source to Axis Distance. SAD is the distance that includes the SSD and Depth of isocenter.

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27
Q

What is the simple equation involving SSD used to find SAD?

A

SSD + Depth = SAD

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28
Q

What are the 6 machine types? (Remember mnemonic)

A

Grenz, Contact, Superficial, Orthovoltage, Supervoltage, Megavoltage

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29
Q

What is the range of voltage for Grenz Ray Therapy?

A

Greater than or equal to 10-15 KvP

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30
Q

What is the range of voltage for Contact Therapy?

A

40-50 KvP

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31
Q

What is the range of voltage for Superficial Therapy?

A

50-150 KvP

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32
Q

What is the range of voltage for Orthovoltage Therapy?

A

150-500 KvP

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33
Q

What is the range of voltage for Supervoltage Therapy?

A

500-1000 KvP

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34
Q

What is the range of voltage for Megavoltage Therapy?

A

Greater than 1 Mv

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35
Q

What machines have their Dmax on the Skin?

A

All except for Megavoltage, Mv is beneath the patients skin.

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36
Q

What is the name of the orthovoltage filter and what are the elements in the correct order from patient to end of the filter?

A

Thoreaus Filter, Aluminum, Copper and Tin

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37
Q

Which machines measure their HVL in Aluminum?

A

Grenz, Contact, and Superficial

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38
Q

Which machines measure their HVL in Copper?

A

Orthovoltage

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39
Q

Which machines measure their HVL in Lead?

A

Megavoltage

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40
Q

When is the Grenz ray beam almost fully absorbed?

A

2mm

41
Q

When is the Contact ray beam almost fully absorbed?

A

2 cm

42
Q

At what distance is the orthovoltage beam 90%?

A

2 cm

43
Q

What depth of tumors is superficial therapy common use for?

A

no greater than 5 mm

44
Q

What are some examples of Megavoltage machines?

A

Linac, Co-60, Cyclotron, Betatron,

45
Q

What is Inverse Square Law?

A

Law that supports the intensity of a beam is inversely proportional to the square of the distance`

46
Q

What is Divergence?

A

As beam travel, they begin to spread out or “Diverge”

47
Q

Whats is the relation behind Divergence?

A

Directly Proportional

48
Q

What is the inverse square law formula used to calculate?

A

the intensity of the beam

49
Q

What is the divergence formula used to calculate?

A

the size of the radiation field

50
Q

What is an equivalent square?

A

a method of finding an approximate equivalent field size size

51
Q

What is Back Scatter Factor?

A

Amount of radiation scattered “back” adding to the skin dose, its a ratio of dose with a scattering medium to dose at the same distance from the radiation without a scattering medium

52
Q

What is the other name for Back Scatter factor for Linacs?

A

Peak Scatter Factor

53
Q

What is the relation between Back Scatter Factor and Tissue Air Ratio (TAR?

A

Back Scatter factor is equivalent to the TAR at Dmax

54
Q

What is BSF affected by?

A

Beam Energy, Field size,

55
Q

As Beam energy increases, what happens to BSF?

A

Decreases

56
Q

As Field Size increases, what happens to BSF?

A

Increase

57
Q

What is BSF NOT affected by?

A

SSD

58
Q

How do you calculate BSF at a different distance?

A

Use the inverse square to calculate the change in dose,

59
Q

What is Percent Depth Dose or PDD?

A

The ratio of dose at depth compared to the depth at Dmax

60
Q

What is the PDD for any field size, SDD or Beam energy at Dmax?

A

100%

61
Q

What factors affect PDD?

A

Beam Energy, Field Size, SSD, Treatment Depth

62
Q

What happens to PDD, when SSD increases?

A

PDD Increases

63
Q

What happens to PDD, when Beam Energy decreases?

A

PDD decreases (Less Penetration)

64
Q

What happens to PDD, when Field Size increases?

A

PDD increase (More Scatter)

65
Q

What happens to PDD, when treatment depth increases?

A

PDD Increases (Going deeper into patient)

66
Q

What is Mayneord’s F Factor?

A

the formula used to adjust the PDD when there is a change in the SSD. ITS AN APPLICATION OF INVERSE SQUARE LAW

67
Q

What is an application of Inverse Square Law?

A

Mayneord’s F Factor

68
Q

Where should Machine output be measured for PDD MU calculations?

A

Treatment SSD + D/max

69
Q

Where is the field size measured at for PDD MU calculations?

A

On the skin

70
Q

What does Sp stand for?

A

Phantom/Patient scatter

71
Q

What does Sc stand for?

A

Collimator Scatter

72
Q

What happens to Output factors when the field size is greater than 10x10?

A

Factor increases (More Scatter)

73
Q

What happens to Output factors when the field size is less than 10x10?

A

Factor Decreases (Less Scatter)

74
Q

What are some other names for Total dose at Dmax?

A

Applied Dose, Given Dose, Incident Dose

75
Q

How is Low Energy machine’s output calculated?

A

Low energy machine utilize time in mins and seconds

76
Q

How is High Energy machine’s output calculated?

A

Using Monitor Units

77
Q

What is a monitor unit?

A

A unit of output measure

78
Q

What is machine output?

A

dose from a machine, can be measured in air or in phantom using reference field and known distance.

79
Q

What is the common reference field size used?

A

10x10

80
Q

What is the common reference output used for Linacs?

A

1.0 cGy/MU

81
Q

What is output factor?

A

a factor that account for a change in scatter when the field size is different than reference field size.

82
Q

What is described as the ratio of dose of a given field size to dose of the reference field size?

A

Output factor

83
Q

What is the total D/max dose?

A

Entrance dose + Exit dose

84
Q

What is Tissue Air Ratio (TAR)?

A

Ratio of absorbed dose in a given depth to the absorbed dose at the same distance in air.

85
Q

Who developed TAR?

A

Johns for use in rotational therapy

86
Q

What does equally weighted fields mean?

A

same dose from each field (Ex: 100 cGy tx, with 2 Equally weighted fields = 2x 50 cGy fields)

87
Q

What does unequal weighted fields mean?

A

Different doses from each field (Ex: 100 cGy tx, with 2 UNequally weighted fields = 1 x 60 cGy field + 1 x 40 cGy field)

88
Q

What is the simple equation for TAR?

A

Dose (tissue) / Dose (air)

89
Q

What factors affect TAR?

A

Beam Energy, Field Size, Treatment Depth

90
Q

What factors DO NOT affect TAR?

A

SSD

91
Q

As Beam energy increases, what happens to TAR?

A

Increases

92
Q

As field size increases, what happens to TAR?

A

Increases

93
Q

As Treatment depth increases, what happens to TAR?

A

decrease (deeper into patient)

94
Q

For TAR MU calculations, where should machine output be measured?

A

Treatment SAD

95
Q

For TAR MU calculations, where should field size be measured?

A

Treatment SAD

96
Q

What is any treatment less than 360 degrees?

A

Arc

97
Q

What is any treatment that is 360 degrees?

A

Rotational

98
Q

Why is Sc only used in the TAR MU calculations?

A

Sp is built into the TAR table

Radiation Dosimetry (2 decks)

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