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Rules of Thumb > Rules of thumb > Flashcards

Rules of thumb Flashcards

1
Q

Bremsstrahlung max and mean photon energy

A 
max = designated energy,
mean = 1/3 designated energy
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2
Q 
PDD for 10x10 field @ 10 cm depth
co-60
6MeV 
10MeV
18MeV
A 
co-60 = 55%
6MeV = 65%
10MeV = 75%
18MeV = 80%
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3
Q

TMR @ 10cm depth
6MeV
10MeV
18MeV

A 
6MeV = 0.78
10MeV = 0.85
18MeV = 0.90
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4
Q

Approximate Attenuation rates
6MeV
10MeV
18MeV

A 
6MeV = 3%/cm
10MeV = 2.5%/cm
18MeV = 2%/cm
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5
Q 
Depth of Maximum dose (Dmax) for a 10x10 field size
Co-60
6MeV
10MeV
18MeV
A 
Co-60 = 0.5cm
6MeV = 1.5 cm
10MeV = 2.4 cm
18MeV = 3.3 cm
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6
Q 
Surface Dose for photons
Co-60 
6MeV
10MeV
18MeV
A 
Co-60 = 50%
6MeV = 25%
10MeV = 23%
18MeV = 20%
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7
Q

Neutron Contamination in an 18MeV beam

A
  1. 5% on the central axis

0. 15 outside of the bem

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8
Q

Neutron head leakage
6MeV
10MeV
18MeV

A 
6MeV = none
10MeV = 0.01%
18MeV = 0.15%
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9
Q

Percentage depth dose dependencies

A

Field size - as field size increases, the %DD increases due to increased scatter
SSD - %DD increases with increasing SSD due to the inverse square law

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10
Q

Scattering at 1m from phantom

A

1/1000 of the primary beam

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11
Q

Lateral scatter energy

A

Maximum energy of about 0.511MeV

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12
Q

Backscatter energy

A

maximum energy of 0.255MeV

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13
Q

Photon Interaction proportionalities per unit mass

A

Photo Electric = Z^3/E^3
Compton scattering = Z/sqrt(E)
Pair Production = Z^2/log(E)

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14
Q

Conversion factor for Roentgens to cGY

A

in air = 0.876cGY/R

in tissue = 0.97 cGY/R

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15
Q

Conversion factor for Roentgens to cGY in tissue for I-125

A

0.886 cGY/R

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16
Q

SRS therapy

A

LINAC based is prescribed 70% isodose line

Gamma Knife is prescribed 50% isodse line

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17
Q

Scatter contributions outside the field

A

Physical wedges - scatter 2.5% the central axis dose
Dynamic wedges - scatter about 1% the central axis dose
** dynamic wedges reduce contralateral bereast dose over physical wedges

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18
Q

Outside of the beam there is a quantifiable amount of dose, extending perpendicular from the beam in the plane of Dmax

A

10cm away receives approximately 1% of the central axis dose at Dmax
30cm away receives approximately 0.2% of the central axis dose at Dmax

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19
Q 
Tissue weighting factors for effective dose equivalent
Gonads
Red bone marrow
Bladder
Skin
A

Gonads = 0.2
Red bone marrow = 0.12
Bladder = 0.05
Skin = 0.01

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20
Q

Electron beams are nearly monoenergetic with average energy equal to that specified
– Electron Range

A

the 4, 3, 2, 2.33 rule
Depth max Dose R100 (cm) = E(MeV)/4
Therapeutic depth (80%) = R80(cm) = E(Mev)/3
Electron range in water Rp(cm) = E(MeV)/2
Depth of 50% isodose line R50(cm) = E(MeV)/2.33

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21
Q 
Depth of Maximum dose (Dmax) for 10x10 field size of electrons
6E
9E
12E 
20E
A 
6E = 1.2 cm
9E = 1.9 cm
12E = 2.1 cm
20E = 2.1 cm
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22
Q

Electron Surface Dose (in %)
6E
12E
20E

A 
6E = 80%
12E = 90%
20E = 95%
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23
Q

Electron X-ray contamination in %
6E
12E
20E

A 
6E = 1%
12E = 2%
20E = 5%
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24
Q

Amount of lead shielding needed in mm to stop an electron beam

A

Eo(MeV)/2

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25
Q

Occupational limit

A

50 mSv/year (5mSv if pregnant)

26
Q

Public limit in Sv

A

1mSv/year

27
Q

The average worker in the USA receives X from occupational exposures

A

2 mSv/year

28
Q

Neutrons need to be taken into account for energies above?

A

10MV

29
Q

Energy of neutrons produced in photonuclear reactions

A

2MeV

30
Q

Energy of neutrons at shielded door to the vault

A

100 keV

* most dangerous with radiation weighting factor = 20

31
Q

Leakage limit

A

0.1% of the central axis value

32
Q

Concrete TVLs for shielding 6 and 18 MV beams

A 
6MeV = 37cm
18MeV = 45 cm
33
Q

Amount of lead in mm to shield an electron beam (to less than 5%)

A

Rp/10 - in other words

E(MeV)/20

34
Q 
Approximate shielding materials and thickness
X-ray 
CT
PET
Cs-137
HDR
Co-60 
LINAC/Cyberknife
A 
X-ray - 1 mm lead
CT - 2 mm lead
PET - 1 cm lead
Cs-137 - 3 cm lead
HDR - 50 cm concrete
Co-60 - 70 cm concrete
LINAC/Cyberknife - 170cm concrete
35
Q

Sources of radiation exposure for the average person

A 
Radon - 1mSv/year
Ingested isotopes - 0.4mSv/year
Isotopes occurring naturally - 0.4 mSv/yr
Cosmic rays - 0.4 mSv/yr
Medical exposures - 2.0 mSv/yr
36
Q

Lead and Iodine’s k-edge

A

Lead - 88 keV

Iodine - 33 keV

37
Q

Increase in mortality risk due to acute radiation

A

8% per Sv

38
Q

Increase in mortality risk due to chronic radiation exposure

A

4% per Sv

39
Q

Radiation quality factors (from 10CFR20)

A 
Photons = 1
Electrons = 1
Protons = 10
Neutrons = 5-20 (10 for unknown energy)
Alpha = 20
40
Q

Package receiving wipe test limits for 300cm^2 area

A

Alpha - 22 dpm/cm^2

Beta/gamma - 220 dpm/cm^2

41
Q

ALARA levels

A

ALARA level 1 - 10% of the allowed limit per quarter

ALARA level 2 - 30% the allowed limit per quarter

42
Q

Limit for removing sealed source from service due to leaking

A

185Bq

*sealed sources need to be checked every 6 months

43
Q

Patient exposure release levels
I-125
Pd-103
I-131

A

I-125: <1mR/hr at 1 meter
Pd-103: <3mR/hr at 1 meter
I-131: <7mR/hr at 1 meter

44
Q

Brachytherapy Medical Events

A

Wrong patient, side or nuclide
Total dose differs by 20%
Single fraction differs by 50%

45
Q

Teletherapy Medical Events

A

Wrong patient or site
Total dose differs by 20% (for treatments> 3 fractions)
Total dose differs by 10% (for treatments<= 3 fractions)

46
Q

LDR dose rate at prescription point

A

0.2 - 2Gy/hr

47
Q

HDR dose rate at prescription point

A

> 12Gy/hr

48
Q

Exposure time to achieve occupational limit while 1 meter away from HDR source

A

1 hour

49
Q

Ir-192 HDR decayment rate

A

1% per day

50
Q

Dose from Cs-137 LDR to Ir-192 HDR

A

D(Ir-192) = D(Cs-137)*0.65

* due to radiobiological effects related to the differing dose rates

51
Q

Spine chords division

A

7 - cervical
12 - thoracic
5 - lumbar
5 - sacral

52
Q

Spinal cord ends

A

L1/L2 and terminates in cauda equina

53
Q 
Sample CT numbers - relative to water
Air
Lung
Soft tissue
Fat
Water
CSF
Blood and Muscle
Spongy Bone
Cortical Bone
A 
Air: -1000
Lung: -700
Soft tissue: -300 to -100
Fat: -80
Water: 0
CSF: 15
Blood and Muscle: 40
Spongy Bone: 400 and up
Cortical Bone: up to 3000
54
Q

Ultrasound - velocities of sounds in different media

A

Air: 330 m/s
Water: 1497 m/s
Bone: 3500 m/s
Soft Tissue: 1540 m/s

55
Q

Typical dose associated with CT

A

50-75 mGy

56
Q

TG-21 to TG-51 output level

A

1% higher

57
Q

MLC leakage

A

Intraleaf - 2%
Interleaf - 3%
End leakage - 15%

58
Q

Pacemaker tolerance

A

2 Gy

59
Q

Approximate Z of materials

A

Air - 7.6
Tissue - 7.4
Bone - 14.0
Prostheses - 20-25

60
Q 
density of various materials
Air
Lung
Bone
Concrete 
Steel
Lead
Tungsten
A 
Air - 0.0012 g/cm3
Lung - 0.33 g/cm3
Bone - 1.8 g/cm3
Concrete - 2.35 g/cm3 (can vary significantly with composition)
Steel - 7.85 g/cm3
Lead - 11.34 g/cm3
Tungsten = 19.25 g/cm3
61
Q

Predominance and occurrence energies
Photoelectric
Compton
Pair Production

A 
Predominance - Occurrence
P.E: 
10-30 Kev - 10-70Kev
Compton
30Kev - 25Mev - Occurs everywhere
P.P.
>25 Mev - >10Mev

Rules of Thumb (1 decks)

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