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Dosimetry > Exam 1 > Flashcards

Exam 1 Flashcards

1
Q

What is exposure? What are its units?

A

measurement of radiation in air

measured in C/kg

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2
Q

What is air kerma?

A

kinetic energy released in a given medium

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3
Q

What is absorbed dose? What are its units?

A

radiation dose absorbed by the given material.

Measured in Gray

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4
Q

What is the formula for absorbed dose?

A

(exposure rate) x (time of exposure)

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5
Q

What is equivalent dose? What are its units?

A

the absorbed dose when the type of radiation is taken into account.

Measured in Sievert

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6
Q

What is effective dose? What are its units? Formula?

A

the equivalent dose when the tissue type is taken into account.

Its units are Sv

(D) x (Wr) x (Wt)

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7
Q

What is the SI unit of exposure?

A

C/kg

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8
Q

What is the SI unit of Air Kerma?

A

Gy

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9
Q

What is the SI unit of Equivalent Dose?

A

Sv

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10
Q

What is the SI unit of Effective Dose?

A

Sv

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11
Q

What is the formula for determining EqD?

A

(absorbed dose) x (weighting factor)

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12
Q

What is the formula for effective dose?

A

(D) x (Wr) x (Wt)

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13
Q

The distance-dose rate calculation can be calculated the same as the _____________.

A

Inverse Square Law

(dose rate 1)=(distance 2)^2
—————— —————–
dose rate 2 (distance 1)^2

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14
Q

What is the formula for BSF?

A

(Dose surface)/ (Dose air)

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15
Q

What is the formula for MU?

A

MU= (Dose)/(doserate)

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16
Q

What dose the radiation therapy prescription define?

A

treatment site

tumor dose

of fractions

dose per treatment

frequency of treatments

type and energy of treatment

DOSE NOT INCLUDE MONITOR UNITS

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17
Q

What does the isodose plan include?

A

field sizes

machine angles

doses

beam weighting

wedges

compensators

blocks

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18
Q

True or false: the isodose plan is considered part of the radiation therapy prescription

A

True

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19
Q

__________ is measured at a specific point in a medium (typically a patient) and refers to the energy deposited at that point.

A

Absorbed dose

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20
Q

_______ is the distance beneath the skin surface where the prescribed dose is to be delivered. The radiation oncologist will state an exact point or depth of treatment.

A

depth

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21
Q

Electrons use ______ or the _______% isodose line for depth.

A

Dmax

905m

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22
Q

multiple fields use ______ for calculation of depth.

A

isocenter

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23
Q

opposed fields like AP/PA use __________ for depth planning.

A

mid plane

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24
Q

_____________ is the measurement of the patient’s thickness from the point of beam entry to the point of the beam exit and is also known as ________.

A

Separation

IFD (intrafield distance)

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25
Q

What does ODI stand for?

A

optical distance indicator

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26
Q

What is the SAD of a cobalt 60 machine?

A

80 cm

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27
Q

What is the SAD of a linear accelerator?

A

100 cm

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28
Q

Field size is defined at _________.

A

machine’s isocenter (100 cm)

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29
Q

What is backscatter?

A

radiation that is scattered back toward the surface of the patient

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30
Q

________ is the depth of the maximum equilibrium, the point where the maximum absorbed dose occurs for single field photon beams and chiefly depends on the energy of the bream.

A

Dmax

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31
Q

As beam energy increases, depth of Dmax _________.

A

increases

32
Q

Dose at Dmax will be _________ than the prescribed dose for single and parallel-opposing fields.

A

higher

33
Q

using many multi fields, the dose at Dmax will normally be ________ than the prescribed dose.

A

less

34
Q

__________ is the amount of radiation exposure produced by a treatment machine or source as specified at a reference FS and distance.

A

Output

35
Q

Increased FS ______ dose rate.

A

increases

36
Q

increased distance from source _________ dose rate.

A

decreases

37
Q

____________ is the ratio of dose rate of a given FS to dose rate of the reference FS (10 x 10).

What is the typical?

A

output factor

10 X 10 FS= 1.00 cGY/MU

38
Q

_________ is the ratio, expressed as a percentage, of the absorbed dose at a given depth to the absorbed dose at a fixed reference depth usually Dmax.

A

Percent Depth Dose

absorbed dose at depth//absorbed dose at Dmax

39
Q

What is PDD dependent on?

A

energy

FS

SSD

40
Q

PDD ________ as energy, FS, and SSD _________ increase.

A

increases

41
Q

PDD ________ as the depth increases..

A

decreases

42
Q

__________ is the ratio of the absorbed dose at a given depth in a phantom to the absorbed dose at the same point in free space.

Formula?

A

Tissue air ratio

TAR= (dose in tissue)//(dose in air)

43
Q

Tray transmission, wedge, and compensating filters are examples of _______.

A

dose rate modification factors

44
Q

the electron beam diverges from a point called the _________.

A

virtual source point

45
Q

For electron treatments, is SSD is increased, there is a __________ in overall energy of the beam.

A

decrease

46
Q

What’re the reasons for the biggest differences between electrons and photon beams?

A

particulate vs non particulate nature

charge vs uncharged nature

47
Q

__________ collisions with the nucleus result in scattering of the electron with no loss of energy. Higher atomic numbers cause more scattering. ______ and _______ are conserved.

A

elastic

momentum and kinetic energy

48
Q

_________ collisions result in bremsstrahlung.

A

inelastic

49
Q

True or false: Bremsstrahlung is clinically desired.

A

False

50
Q

What is the useful range for electron beam therapy?

A

6-20 MeV

51
Q

Superficial tumors are less than ________ cm deep.

A

7

52
Q

Electrons treat areas such as:

A

skin

boost for nodes

head and neck cancers

chest wall

53
Q

The collimator setting must be _________ than the cone size.

A

larger

54
Q

The energy of the beam leaving the __________ is the highest and is nearly mono energetic.

A

waveguide

55
Q

What causes lowering of energy on average and a mix of energies in an electron beam?

A

hitting the scattering foil and bouncing off portions of the collimator

56
Q

The energy of the beam at the surface of the patient is called the _________ energy. E0

A

Mean energy E0

57
Q

What is the practical range?

A

the depth at which the average energy reaches zero

58
Q

The electron beam loses ______ per 1 cm of travel through soft tissue.

A

2 MeV

59
Q

For electron beams, _________ are used instead of Dmax.

A

penetration depths

60
Q

What is E/2?

A

Practical range (Rp) 2 MeV/cm lost

61
Q

What is E/2.33?

A

50% range

62
Q

What is E/3?

E/2.8??

A

80% isodose line

63
Q

What is E/4?

A

therapeutic range (90%) isodose

64
Q

What is E/5?

A

100% Dmax

65
Q

What is the formula for mean or average energy?

A

E0=2.33 d50

66
Q

Skin dose ______ with increasing energy of the electron beams.

A

increases

67
Q

Electron dose uniformity depends on:

A

field size

beam energy

beam collimation

SSD

68
Q

Shielding thickness rule of thumb for lead:

A

MeV/2= shield thickness in millimeters of lead

69
Q

Shielding thickness rule of thumb for alloy:

A

multiply thickness of lead indicated by 1.2

70
Q

Field flatness is _______ related to beam energy.

A

inversely

71
Q

field flatness is _________ directly related to field size.

A

directly

72
Q

the correction factor for bone is ______.

A

1.65

73
Q

the correction factor for air cavity is ______.

A

.5

74
Q

Reasons for bolus in electron treatment:

A

increase skin dose

fill in for missing tissue

flatten out an irregular surface

decrease electron penetration

75
Q
A

Dosimetry (1 decks)

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