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ABR Oral JSJ > Linac Shielding NCRP 151 > Flashcards

Linac Shielding NCRP 151 Flashcards

1
Q

Define occupancy factor (T)

A

The average fraction of time that the maximally exposed individual is present while the beam is on.

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2
Q

What areas are covered by a occupancy factor (T) of 1

A 
Full occupancy. 
administrative offices
treatment control areas
treatment planning areas
nurse stations
attended waiting rooms
receptionist areas
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3
Q

What areas are covered by a occupancy factor (T) of 1/2

A

Adjacent treatment room

patient exam room adjacent to vault

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4
Q

What areas are covered by a occupancy factor (T) of 1/5

A

Corridors
employee lounge
staff restroom

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5
Q

What areas are covered by a occupancy factor (T) of 1/8

A

treatment vault doors

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6
Q

What areas are covered by a occupancy factor (T) of 1/20

A 
Public toilets
unattended vending rooms
storage areas
outdoor area with seating
patient holding area
attics
janitor's closet
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7
Q

What areas are covered by a occupancy factor (T) of 1/40

A

outdoor areas, parking lots, vehicular drop off areas, stairways, unattended elevators

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8
Q

Define Controlled area

A

a limited access area in which the occupational exposure is under supervision. Usually in the immediate area that radiation is being used.

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9
Q

What are the shielding design goals for Controlled Areas

A

0.1 mSv/wk

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10
Q

What are the shielding design goals for Uncontrolled Areas

A

0.02 mSv/wk

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11
Q

What are the NCRP annual limits for radiation workers

A

50 mSv/yr

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12
Q

What are the NCRP annual limits for non-radiation workers

A

1 mSv/yr

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13
Q

What conservative assumptions are made in NCRP 151

A

Neglect Primary beam attenuation by patient. Perpendicular incidence of radiation. Leakage assumed max value. Occupancy factors are high. Minimum distance of 0.3m from barrier is low. If data is estimated safety factors (x1.5e.g.)are used. The two source rule is used.

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14
Q

Define Controlled area

A

a limited access area in which the occupational exposure is under supervision. Usually in the immediate area that radiation is being used.

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15
Q

What are the shielding design goals for Controlled Areas

A

0.1 mSv/wk

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16
Q

What are the shielding design goals for Uncontrolled Areas

A

0.02 mSv/wk

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17
Q

What are the NCRP annual limits for radiation workers

A

50 mSv/yr

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18
Q

What are the NCRP annual limits for non-radiation workers

A

1 mSv/yr

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19
Q

What conservative assumptions are made in NCRP 151

A

Neglect Primary beam attenuation by patient. Perpendicular incidence of radiation. Leakage assumed max value. Occupancy factors are high. Minimum distance of 0.3m from barrier is low. If data is estimated safety factors (x1.5e.g.)are used. The two source rule is used.

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20
Q

What is the two source rule

A

When a location is to be shielded from two different sources of radiation, the thickness of the barrier should be equal to the greater thickness if they differ by more than a TVL or else equal to the greater thickness plus one HVL of the more penetrating source.

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21
Q

Define Workload (W)

A

Time integral of the absorbed dose rate determined at dmax 1m from source.

22
Q

Define Use factor (U)

A

Fraction of a primary beam workload that is directed toward a primary barrier

23
Q

What is the shielding equation for primary barriers

A

Bpri=Pd^2pri/WUT

24
Q

What is the equation for number of TVL from Bpri

A

n=-log(Bpri)

25
Q

What is the equation for the barrier thickness from #TVL

A

tbarrier=TVL1+(n-1)TVLe

26
Q

Typical thickness for 18x primary barrier

A

5-6 feet (2 meters)

27
Q

Relationship between Concrete Steel and Lead 18X

A

Concrete TVL = 0.47m
Steel TVL = 0.11m (1/4 of Concrete)
Lead TVL = 0.06m (1/8 of Concrete)

28
Q

Would steel and or lead alone be good enough for 18X Linac room?

A

FAST neutrons produced above 10X

Fast Neutrons moderated with hydrogenated materials (Borated polyethylene)

29
Q

How much additional shielding if you go from control to uncontrolled and occupancy from 0.5 to 1.0

A 
P increases by a factor of 50
Use factor from 0.5 to 1 therefore factor of 2
B pri increased by 25
To bring it down to below 0.2
(1TVL = 2.5, 2TVL = .2)  So add 2 TVL
30
Q

For higher energy linacs. (over 20 MV) rule of thumb for neutron dose at the outer maze entrance.

A

1 Gy photons @ isocenter = 1 uSv neutron dose equivalent at the outer maze entrance.

31
Q

Neutron dose increases by ? With jaws closed vs open

A

Neutron dose increases by 75% with the jaws closed vs open

32
Q

How can you lower neutron dose rate at the maze entrance?

A

Increase length of the maze
Each 5m of maze decreases neutron dose by factor of 10
Decrease inner cross sec of maze (w*h)
Increase number of turns in the maze (1 turn decreases factor 3)
Sharper turns

33
Q

Ct shielding survey phantom in scan plane?

A

Survey should be done with phantom in scan plane
20cm for head and 40 cm for pelvis
Scanner auto shields for primary radiation so SCATTER is the main source of radiation
Scatter is max with phantom in scan plane

34
Q

CT survey with thin vs thick slices? High vs low kvp?

A

Max scatter and exposure from :
Largest slice thickness
Highest kvp
Typical - 140 kvp, 200 mA, long exposure time to use survey meter

35
Q

Ct shielding survey what instruments are appropriate?

A

Initial shielding integrity gaps, missing walls with Gieger muller
GM (high sensitivity)
After shielding integrity has been verified…ion chamber survey meter to measure instantaneous exposure!

36
Q

How to determine total weekly exposure for CT shielding?

A

X= 60XiWT
X total weekly exposure mR/week
Xi instantaneous exposure divided by mA (mR/{hrmA})
W weekly workload mA-min/ week AAPM report # 39
T occupancy factor NCRP # 49

37
Q

If tumor visible on PET why do we need CT?

A

We need CT to correct PET image for differential attenuation
PET only metabolic activity
CT for anatomical overlay
CT will give you heterogeneity due to electron density info

38
Q

If you replace CT with PET CT, Do you need more shielding?

A

0.511MeV annihilation photons w positron decay
Photon from CT = 120-140 keV
Shielding for CT alone 1.6 mm (only 20% of PET will be atten)
HVL for annihilation radiation from PET-CT greater than CT alone

39
Q

What are the limits for radiation worker and shielding goals for HDR?

A

NCRP 116
Radiation worker = 50 mSv/ year & cumulative <10mSv* age
Public = 1mSv/ year and not exceed 0.02mSv in any hour
NCRP 151 struc shielding MV X-ray
Control = 5mSv/year = 0.1 mSv/wk= 10 mrem/wk
Uncontrol = 1mSv/year = 0.02 mSv/wk= 2 mrem/wk
Shielding goal for control is lower than dose limit for rad wrker to limit pregnant worker to 0.5 mSv / month

40
Q

Typical room shielding for HDR?

A

30 - 60 cm of concrete
5 cm of lead
Patent attenuation is commonly neglected in HDR shielding calcs
TVL for HDR i192 is ~ 15cm concrete

41
Q

What are the typical properties of the HDR source i192?

A

Typical 10 Ci
1/2 life 74 days
Specific activity ( activity per mass) 450 Ci/g
Effective gamma energy = 0.38 MeV or 380 keV
TVL= 15 cm concrete
Clinical dose rate up to 700cGy / min @ 1 cm from source compared to linac

42
Q

Typical workload for HDR room?

A

W = ¥ fA t
¥ exposure rate constant 0.469R/Ci/hr@1m. i192
f exposure to dose conv. Factor 0.971cGy /R
A source activity 10-15 Ci
Typical patients 5/day 25/ week
10 minutes each ~ t = 250 minutes / week ~ 4 hr / wk
Therefore W = 20 - 30 cGy/ wk

43
Q

What factors affect photon sky shine?

A

Sky shine:
Increases with field size ( solid angle defined by Collimator)
Increases with dose rate
Increases with roof transmission ( minimal roof shielding)
Decreases with height of roof ( higher less sky shine)
Increases as lateral distance from iso ( if beam gets wider)
Max at some some distance away from wall due to geometric shadowing

44
Q

What factors affect neutron sky shine?

A

Solid angle defined by wall at gantry 0 degrees
Neutron production threshold
Increases with dose rate
Increases with roof transmission ( minimal roof shielding)
Decreases with height of roof ( higher less sky shine)
Increases as lateral distance from iso ( depends on largest solid angle)
Max at some some distance away from wall due to geometric shadowing but fall off is slower and less geometric shadowing

45
Q

What is the typical magnitude of sky shine?

A 
18X and 400 MU/ min McGinley's data
Dose rate at Iso = 400cGy / min or 6.7 cGy/s
Max photon sky shine = 0.16 mSv / hr
Max neutron sky shine = 0.21 mSv / hr
Photon ~= Neutron sky shine
This is 10 ^-6 iso center dose rate
Remember public is 1 mSv / yr
46
Q

HDR Shielding equation ?

A 
B = (P*d^2)/(W * T)
P = dose limit
d= distance from source to point of interest in meters m
W = workload @ 1m ( cGy/wk)
T= occupancy factor
B= required barrier transmission factor
47
Q

Assumptions when working on HDR shielding?

A

Uncontrol area = 2 mrem / week
For shielding only cGy = rad = R = rem
For photon and electron shielding Gy = Sv
For i192. TVL = 15 cm concrete

48
Q

HDR room vs. Linac vault?

A

HDR from needs to be considered as a Surgery suite (sterile)
Eg. Prostate Brachy
There needs to be a last man out button inside the HDR suite by the door

49
Q

Attenuation or TVL in a material for 6X?

A

Measure 1’ away from distal side of barrier
Shielding survey @ 30cm or 1 foot away
TVL for 6X = 35 cm

50
Q

Narrow vs broad beam geometry for shielding ?

A

Narrow beam : No scatter, no secondary
Broad beam : Scatter and secondary particles in detector
Attenuation coef u narrow beam > u broad beam
TVL of narrow beam < TVL of broad beam. Aka. If plot TVLvs FS, TVL increases with Field size!
For shielding calculation broad beam more realistic and conservative

51
Q

Why is TVL 2 < TVL 1 in concrete? For Broad beam only!

A

In MV ranges for linac mass atten coef (u/p) decreases with E. so you will get beam hardening, and should increase TVL2 with depth……BUT in H2O and concrete, main attenuation from Compton, so there is significant increase in scatter from lower energies deeper in the material that requires lower TVL! This Compton component supersedes the beam hardening effect in concrete!!!!!

52
Q

Why is TVL 2 > TVL 1 in concrete for narrow beam????

A

Narrow beam has no scatter component that usually supersedes the beam hardening component, so TVL 2 > TVL 1 with depth!
So the TVL for concrete changes with FS!!!
TVL 1 < TVL 2 for small FS in concrete only!
TVL 1 > TVL 2 for large FS in concrete only!

ABR Oral JSJ (3 decks)

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