Flashcards in Room Design for Radiotherapy Deck (64)
Other than the primary beam, what other sources of radiation may be produced within a Linac bunker?
What type of radiation is considered when plannning Brachytherapy bunkers?
How thick are primary barriers?
Equivalent to 1.5-2.5m of concrete.
What is the secondary barrier designed to attenuate?
Scatter from patient & walls
How is the thickness of the secondary barrier determined?
The largest type of scatter or leakage drives the thickness of the secondary barrier. They are not added together.
What is the purpose of a room maze?
Reduce the door thickness by attenuating scatter
What is a disadvantage of a room maze?
Takes up space
Has to be large enough to pass equipment and patient stretchers through.
What percentage of the energy of a scattered photon is reflected from a room maze?
What can be used to reduce scatter down a maze?
An extended nib
An extra turn in the maze
Briefly describe the principle of a lintel in radiation protection.
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.
Briefly describe the principle of a baffle in radiation protection.
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.
Briefly describe the principle of an extended nib in radiation protection.
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.
Briefly describe the principle of an extra turn in the maze in radiation protection.
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.
What is the driving specification for the thickness of barriers in a Tomotherapy room, and why?
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.
What are the principles behind the room shielding for a CyberKnife?
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.
What are the driving principles behind the room shielding design for a Brachytherapy bunker?
The gamma rays are emitted isotropically.
Therefore, avoid direct line of sight to the entrance by using a short maze plus door combination.
What should be considered when looking at room shielding for a bunker?
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.
What are the advantages of a large bunker?
Distance is effective shielding
Needed for certain treatments e.g. TBI
Need storage space for accessories and patient immobilisation
Other than things that may influence the bunker shielding, what other considerations should be made when designing a bunker?
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
What type of warning signals should be used for bunkers?
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.
Why is ventilation required for Linacs?
It is required especially for Linacs >10MV.
There may be a build up of:
-induced radioactivity (Oxygen-15, Nitrogen-13)
Over 6 air changes per hour should be satisfactory.
What is a 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).
What is a supervised area?
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))
What is the IDR?
Instantaneous Dose Rate (averaged over 1 minute)
What is TADR?
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.
What is TADR2000?
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.
Which drives Linac bunker design: IDR, TADR or TADR2000?
What is the IDR for a controlled area?
What is the IDR for a supervised area?