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000 Radiation Safety > ++++ Room Design for Radiotherapy > Flashcards

++++ Room Design for Radiotherapy Flashcards

1
Q

Other than the primary beam, what other sources of radiation may be produced within a Linac bunker?

A

Head leakage

Activation products

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2
Q

What type of radiation is considered when plannning Brachytherapy bunkers?

A

Gamma

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3
Q

How thick are primary barriers?

A

Equivalent to 1.5-2.5m of concrete.

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4
Q

What is the secondary barrier designed to attenuate?

A

Scatter from patient & walls
Head leakage
Neutrons

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5
Q

How is the thickness of the secondary barrier determined?

A

The largest type of scatter or leakage drives the thickness of the secondary barrier. They are not added together.

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6
Q

What is the purpose of a room maze?

A

Reduce the door thickness by attenuating scatter

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7
Q

What is a disadvantage of a room maze?

A

Takes up space

Has to be large enough to pass equipment and patient stretchers through.

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8
Q

What percentage of the energy of a scattered photon is reflected from a room maze?

A

3-4%

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9
Q

What can be used to reduce scatter down a maze?

A

Lintels
Baffles
An extended nib
An extra turn in the maze

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10
Q

Briefly describe the principle of a lintel in radiation protection.

A

Lintels are used in room mazes. They bring the ceiling down - reducing the radiation ability to scatter down the maze. Fluence that arrives at entrance is influenced by cross sectional area of maze - lintels reduce the cross-sectional area of a maze.

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11
Q

Briefly describe the principle of a baffle in radiation protection.

A

A baffle is an extra piece of attenuating material that is aligned to the extended nib of maze wall. This results in direct line of sight photons attenuating or reflectingbefore entering maze.
The end of the nib is thinner due less attenuation due to presence of baffle.

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12
Q

Briefly describe the principle of an extended nib in radiation protection.

A

An extended nib minimises the angle that photons can get down maze and subsequently reduce scatter down maze. End of nib is thickness of maze wall, but is at an angle coming in towards the Linac.

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13
Q

Briefly describe the principle of an extra turn in the maze in radiation protection.

A

Extra turn in maze makes it impossible for photons to have single reflection, but this requires space.
Compromise between size of room, space available, cost of materials. Want minimal radiation with minimal cost.

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14
Q

What is the driving specification for the thickness of barriers in a Tomotherapy room, and why?

A

Leakage radiation.
There is a lead beam stop at the back of the beam so the primary beam is reduced due to the inbuilt shielding. But as the beam is on for a long period, leakage becomes an issue.

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15
Q

What are the principles behind the room shielding for a CyberKnife?

A

6MV treatment head can point in almost any direction, so need primary shielding in most of the room.
The arm does not rotate over 22 degrees, so it is difficult to point up; do not need primary shielding in all of the roof.

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16
Q

What are the driving principles behind the room shielding design for a Brachytherapy bunker?

A

The gamma rays are emitted isotropically.

Therefore, avoid direct line of sight to the entrance by using a short maze plus door combination.

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17
Q

What should be considered when looking at room shielding for a bunker?

A

Location (basement/3rd floor), availability of space, type of space to protect, type of person to protect, budget, adjacent facilities (may influence nuclear medicine equipment), the future workload/machines.

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18
Q

What are the advantages of a large bunker?

A

Distance is effective shielding
Needed for certain treatments e.g. TBI
Need storage space for accessories and patient immobilisation

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19
Q

Other than things that may influence the bunker shielding, what other considerations should be made when designing a bunker?

A

Signage required in areas leading to treatment units
Waiting areas and patient changing areas should be positioned so that patients are unlikely to accidentally enter treatment areas
Control rooms should be located to give a good view of the treatment room (CCTV), access corridors and entrance to the bunker

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20
Q

What type of warning signals should be used for bunkers?

A

Visible: at the entrance, in the room, and in the control area.
Audible: in the room, and in the control area. Audible signal can be noise produced by equipment itself.

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21
Q

Why is ventilation required for Linacs?

A

It is required especially for Linacs >10MV.
There may be a build up of:
-ozone
-induced radioactivity (Oxygen-15, Nitrogen-13)

Over 6 air changes per hour should be satisfactory.

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22
Q

What is a controlled area?

A

IRR99:
Controlled Area:
It is necessary for any person who enters or works in the area to follow special procedures designed to restrict significant exposure to ionising radiation in that area.
<6mSv constraint (potential to exceed limit of >20mSv).

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23
Q

What is a supervised area?

A

IRR99:
It is necessary to keep the conditions of the area under review to determine whether the area should be designated as a controlled area.
<0.1mSv constraint (potential to exceed >1mSv (public dose limit))

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24
Q

What is the IDR?

A

Instantaneous Dose Rate (averaged over 1 minute)

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25
Q

What is TADR?

A

Time Averaged Dose Rate (Averaged over 8 hours (1 working day))
TADR = IDR x Duty Cycle x Use Factor

Duty cycle is how much of working day the beam is on (if on for 4 mins per hour, then is 4/60).
Use factor is how much of time beam is pointing at particular barrier (VMAT/continuous rotating = 0.25 factor), general/conventional treatments = walls =0.25, floor = 0.3, ceiling = 0.2. TBI = higher factor for that wall.

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26
Q

What is TADR2000?

A

Time Averaged Dose Rate over 2000 hours (1 working year).
TADR2000 = TADR x Occupancy Factor

TADR2000 will change is change to 7 day working weeks.

27
Q

Which drives Linac bunker design: IDR, TADR or TADR2000?

A

TADR2000

28
Q

What is the IDR for a controlled area?

A

> 2000 uSv/hr

29
Q

What is the IDR for a supervised area?

A

> 7.5 uSv/hr

30
Q

What is the IDR for a public area?

A

<7.5 uSv/hr

31
Q

What is the TADR for a controlled area?

A

> 7.5 uSv/hr

32
Q

What is the TADR for a supervised area?

A

> 2.5 uSv/hr

33
Q

What is the TADR for a public area?

A

<0.5 uSv/hr

34
Q

What is the TADR2000 for a controlled area?

A

> 3 uSv/hr

35
Q

What is the TADR2000 for a supervised area?

A

> 0.5 uSv/hr

36
Q

What is the TADR2000 for a public area?

A

<0.15 uSv/hr

37
Q

What is the shielding design goal in radiation protection?

A

P = dose equivalent beyond the barrier (Sv/week)

38
Q

What is the workload in radiation protection?

A

W = How much is the machine used (Gy/week)
W = Number of patients per week * dose at isocentre
It is good to overestimate this if not too costly as allows for expansion and future work.
Typical values (NCRP 49, 51):
Low X machine (<10 MV) – 1000 Gy/wk
High X machine (>10 MV) – 500 Gy/wk

39
Q

What is the use factor in radiation protection?

A

U = fraction of workload directed at a particular barrier

40
Q

What is the occupancy factor in radiation protection?

A 
T = The fraction of working week that an individual is in a particular location.
T = may not be same person (eg 2 people in a room – one in am, one in pm = factor of 0.5, not 1)
41
Q

What is the occupancy factor of a control area?

A

1

42
Q

What is the occupancy factor of a reception?

A

1

43
Q

What is the occupancy factor of a waiting area?

A

1

44
Q

What is the occupancy of a corridor?

A

1/5

45
Q

What is the occupancy of staff toilets?

A

1/5

46
Q

What is the occupancy of a treatment room door area?

A

1/5

47
Q

What is the occupancy factor of public toilets?

A

1/20

48
Q

What is the occupancy factor of stairways?

A

1/20

49
Q

What is the occupancy factor of storage areas?

A

1/20

50
Q

What is the reduction factor in radiotherapy radiation protection?

A 
B = Factor by which the intensity of radiation (Po) must be reduced to achieve the target dose rate P
B = P / P(0)
51
Q

What is the TVL?

A

Thickness of material required to allow 10% transmission
n = log(1/B)
S = TVL1 + (n-1)TVLe
Where TVL1 is 1st TVL includes bit to get to electronic equilibrium (build up effect) – requires thicker TVL1
and TLVe is TVL Equilibrium – electronic equilibrium is achieved.

52
Q

What is the disadvantage of using steel shielding for high energy Linacs?

A

Photoneutrons are produced if the energy if high enough.

53
Q

What is the Forster sandwich technique?

A

Concrete used externally

Filled gap with mineral (old stuff from ground used as pulverised to fill concrete gap)

54
Q

What is Ledite?

A

High density mineral aggregate (can include scrap steel).
Preshaped to avoid direct line of sight going straight through = can offset. May be able to dismantle as not mortared into place. Very heavy though.

55
Q

What is the equation to calculate the reduction factor from the primary barrier at point P?

A

B = [P * (d1/d0)^2] / [W * U * T]
Where
D0 = distance from focus to isocentre
D1 – distance from focus to point P

56
Q

At what energy does head leakage become the dominant consideration for secondary shielding?

A

> 10 MV

57
Q

What is the equation to calculate the reduction factor from head leakage?

A

B = [1000 * P * dL^ 2] / [W * T]
Head shielding designed to reduce intensity by factor of 1000
dL is distance from target to POI
Leakage assumed to be isotropic: U = 1

58
Q

What is the equation to calculate the reduction factor from patient scatter?

A

B = [P * d0^2 * dP^2 * 400] / [a * W * T * F]
d0 is the distance from source to patient
dP is the distance from patient to barrier
F is the field size at isocentre
a is the scatter fraction : dose rate at 1 m from the phantom when field area is 400 cm2 at the phantom surface / Dose rate at centre of field 1 m from source with no phantom

Rule of thumb: 0.1-0.2%

59
Q

What is the rule of thumb for patient scatter?

A

Between 0.1-0.2% worst case.

60
Q

What consideration should be made when choosing a door for shielding high energy Linacs?

A

The material may produce photoneutrons at high energies.
Use boron or lithium loaded polyethylene to attenuate neutrons by (n,γ) reaction. The 0.5-10MeV gamma rays produced by this may then require subsequent shielding.

61
Q

What is the composition of a door in a bunker?

A

Approximately 5cm thick:

  • lead: reduce neutron energy
  • borated polyethylene: (n,ϒ)
  • lead: attenuation of ϒ rays

Polyethylene attenuates neutron produced by lead, but produces photon, so need lead to attenuate the photons produced.
Door liable to needing motorised system as heavy.

62
Q

What are the disadvantages of using a Linac bunker for HDR use as well?

A

Usually not enough space for diagnostic equipment for source localization
Time pressure - next external beam radiotherapy patient scheduled
Difficult to predict assumptions for shielding calculations

63
Q

What is the equation for air kerma rate at point P?

A

Kp = [A * Γ * α * A1] / [ (d1 * d2)^2]

Kp = air kerma rate at P from area A1
A = activity of source
Г = air kerma rate per unit activity at 1 m from source
α = reflection coefficient per unit area for scatter
A1 = area defined by dotted lines 10° either side of centre line
d1 and d2 are distances shown in the figure

64
Q

How is the shielding determined in a Brachytherapy bunker?

A

Use the equation for air kerma rate at point P, then repeat by stepping around the room in 20 degree intervals & summing to get total air kerma rate at entrance.
The transmission is also calculated through the walls.

000 Radiation Safety (71 decks)

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