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part 3: treatment dose and prescription > C. dose calculation > Flashcards

C. dose calculation Flashcards

1
Q

The beam energy that is used for treatment is determined by the

A

depth of the target volume, minimum target dose, and acceptable dose to organs at risk in path of beam

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2
Q

For dosimetric calculations, all rectangular treatment fields must be converted to

A

squares

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3
Q

What is equivalent square equation?

A

EQSQ= (2ab) / (a+B) or ( 4 x area)/ perimeter

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4
Q

_____fields are fields that use MLCs or blocks to change the shape of the treatment field

A

Blocked

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5
Q

A simpler approach is to take the square root of the open area minus the

A

blocked areaExample: the open area is 20 x 10 and the blocked area is 4x 6 First do the equation (20 x 10) — (4 x 6) Next, take the square root of the above answerClarkson integration can be used to find the area of irregularly shaped fields

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6
Q

The main cause of collimator scatter (sc) is from photons scattering within the

A

head of the gantry

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7
Q

Collimator scatter (Sc) Mostly occurs with the

A

collimating jaws and the flattening filter

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8
Q

Bigger field sizes creates ___ scatter

A

more

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9
Q

Smaller field size creates ___ scatter

A

less

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10
Q

Scatter that happens within the patient or phantom is called

A

phantom scatter (Sp)

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11
Q

phantom scatter (Sp) amount depends on the

A

volume of tissue within the treatment field

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12
Q

As the field size increases, the amount of phantom scatter ___

A

increases

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13
Q

Backscatter factor (BSF) and peak scatter factor (PSF) both compare the dose rate in free space to the dose rate within

A

a phantom at the dmax

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14
Q

Backscatter factor (BSF) is used for ___ beams

A

low-energy beams

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15
Q

Peak scatter factor (PSF) is used for ____ beams

A

high-energy beams

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16
Q

____ is the depth where electronic equilibrium is reached and 100 percent of the dose is delivered

A

dmax

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17
Q

The dmax for higher-energy beams is

A

deeper

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18
Q

What is dmax for 4MV beam

A

1 cm

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19
Q

What is dmax for 6 MV beam

A

1.5cm

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20
Q

What is dmax for 10MV beam

A

2.5 cm

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21
Q

What is dmax for 15MV beam

A

3 cm

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22
Q

What is dmax for 18MV beam

A

3.2cm

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23
Q

What is dmax for 20MV beam

A

3.5 cm

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24
Q

What is dmax for 25MV beam

A

5 cm

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25
Q

What is dmax for 6MeV beam

A

1.5cm

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26
Q

What is dmax for 9MeV beam

A

2.2cm

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27
Q

What is dmax for 12MeV beam

A

2.8cm

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28
Q

What is dmax for co-60

A

0.5cm

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29
Q

_____ is A ratio that compares the absorbed dose at a depth to the absorbed dose at a specific reference depth (dmax)

A

PDD= percent depth dose

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30
Q

What is equation for PDD?

A

[(Absorbed dose at depth) ÷ (absorbed dose at dmax)] x 100

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31
Q

PDD i only used to calculate MU in

A

SSD treatments

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32
Q

PDD changes depending on four components

A

energy, depth, field size, and SSD

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33
Q

An increase of ____ causes an increase of PDD because there is more scatter, which contributes to the dose

A

field size

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34
Q

As depth within the patient goes past Dmax then PDD

A

decreases

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35
Q

PDD and depth have a

A

inverse relationship

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36
Q

To determine the increase in PDD when there is a change in SSD, use

A

Mayneord F Factor

37
Q

As field size increases PDD will

A

increase

38
Q

As SSD increases PDD will

A

increase

39
Q

As energy increases PDD will

A

increase

40
Q

As Depth increases PDD will

A

decrease

41
Q

_______ is the ratio of an absorbed dose at a depth within the phantom at a given distance compared with the absorbed dose in free space at the same distance

A

Tissue air ratio (TAR)

42
Q

TAR is ____ of SSD

A

independent

43
Q

TAR is dependent on what?

A

energy, field size, depth

44
Q

As the energy increases, the beam becomes more penetrating and TAR ______

A

increases

45
Q

_____is the ratio of an absorbed dose at a specified depth in a Phantom to the absorbed dose at dmax at the same distance from the radiation source

A

Tissue maximum ratio (TMR)

46
Q

TMR is used to measure dose of

A

high energy beams

47
Q

the value of TMR is usually

A

less than 1

48
Q

SSD technique is when the ____is located on the skin surface

A

isocenter

49
Q

SAD, or isocentric, technique is when the isocenter is located within the

A

target volume

50
Q

____ Describes the correlation between the distance from the source of radiation and the intensity of the beam

A

Inverse square

51
Q

The _____of the radiation beam is inversely proportional to the square of the distance from the source of radiation

A

intensity

52
Q

What is the inverse square equation?

A

I1 / I2 = (D2 / D1 )^2

53
Q

When the distance is increased, the same amount of radiation spreads to a larger area and the intensity is

A

reduced

54
Q

if the distance is increased by two, the intensity of the beam is decreased by

A

one fourth of the original amount

55
Q

Extended distances have SSDs that are greater than the

A

isocenter (100 cm)

56
Q

____ are necessary when larger field sizes are required

A

Extended distances

57
Q

_____change the shape of the isodose curve and dose distribution within patient

A

Wedges

58
Q

Isodose curves are angled due to the unequal ____

A

absorption of a wedge

59
Q

The “heel” of the wedge absorbs more of the beam than the

A

toe” of the wedge

60
Q

The heel brings isodose curves up closer to the

A

skin surface

61
Q

____wedge angles create a steeper isodose curve

A

Higher

62
Q

Wedge angle formula

A

90-(hinge angle/2)

63
Q

The angle of the wedge is not determined by the angle of the actual physical wedge, but by the angle between an isodose curve and a line perpendicular to the

A

central axis or at a specific depth

64
Q

Field sizes cannot be larger than the

A

wedge itself

65
Q

______ is added to the calculation of monitor units when a wedge is used

A

Wedge transmission factor, or wedge factor

66
Q

______“harden” the beam, meaning the low-energy photons are absorbed by the wedge, so a lower number of photons are available for dose

A

Wedges

67
Q

______ Calculate the dose rate at a point in the field away from the central axis

A

Off-axis calculations

68
Q

A change in dose rate is expected toward the field edge due to the

A

penumbra region

69
Q

___ can be used when the central axis is blocked by MLCs, blocks, or jaws for half-beam blocks

A

OARS

70
Q

_____are a demonstration of the dose distribution within a field in directions parallel and perpendicular to the direction of the radiation beam

A

Isodose curves

71
Q

Factors that can change the _____ are beam energy, beam type, SOUrce.to. surface distance, field weighting, beam modifiers, and field size

A

isodose distribution

72
Q

A higher-energy beam’s isodose curves are more spread out than lower-energy beams because the higher-energy beams are

A

more penetrating

73
Q

_____ can change the shape of the isodose curves

A

Wedges and patient contours

74
Q

_____ show the doses that normal tissue and tumor will receive during the treatment

A

Dose-volume histograms (DVH)

75
Q

When ____are needed for treatment, a tray made of some kind of plastic is used.

A

blocks

76
Q

The tray attenuates some of the beam when used with blocks. Usually less than

A

5 percent

77
Q

The _______, is a ratio of a dose with the tray in Place to the dose without the tray and js found by a physicist

A

tray factor (TF), or tray transmission factor

78
Q
  • Bring dose closer to surface - will attenuate some of the beam
A

bolus

79
Q

Wedges and compensators are used to alter the isodose distribution based on the

A

patient’s skin contour

80
Q

The transmission factor shows how much Of the beam is transmitted through the

A

wedge or compensator

81
Q

The transmission factor will change with changing beam

A

energies

82
Q

____ can change with tissue inhomogeneity

A

Dose distribution

83
Q

Tissue inhomogeneities may cause _____that can affect dose to other organs nearby

A

scatter

84
Q

What is lung correction factor

A

0.25 - 0.33 g/cm3

85
Q

What is bone correction factor?

A

1.8 g/cm3

86
Q

What is water correction factor?

A

1.0 g/cm3

87
Q

Correction factors are applied to dose calculations to adjust for the

A

dose distribution in different tissues

88
Q

The ___factor compares the dose rate in a known field size to the dose rate in a reference field size

A

output

89
Q

The output factor for the reference field size of 10 x 10 cm is

A

1

part 3: treatment dose and prescription (3 decks)

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