Room design for RT Flashcards
What are the sources of radiation that need to be protected against in RT?
Linac treatment beam - photons, electrons, photoneutrons, scatter
Linac head - leakage, gamma photons from activated products
kV imaging - primary beam, tube leakage
Brachytherapy
What are the types of barriers used in RT?
Primary - attenuate primary beam
Secondary - attenuated scatter, leakage, and neutrons
What are the advantages and disadvantages of mazes?
Adv: smaller door, nicer for patients
Disadv: take up a lot of room
When building a maze what consideration needs to be made about the width?
Make it wide enough to pass equipment down
What maze design features can be used to reduce the radiation scattered down the maze?
Lintels - beam on ceiling to reduce cross section of maze
Baffles - extra piece of wall at end of maze
An extended nib
An extra turn in the maze
What features need special consideration for shielding cyberknife?
Beam can be incident on any walls so need primary barrier everywhere
What 5 factors should be considered when deciding on room shielding?
Location - what floor? Adjacent buildings?
Availability of space - new or retro fit?
Type of space to protect - public or restricted?
Type of person to protect - public or worker?
Budget
What issues can nuclear medicine departments have if next to poorly shielded RT departments?
Interference on photon counters and energy window detectors
When planning a room, what considerations need to be made?
Plan for the future - will it still be okay in 20 years?
MV rooms are in the basement
Place bunkers together to use common walls
Make large bunkers
Why are large bunkers desirable?
Distance is effective shielding
Needed for certain treatments
Need space to store accessories
What are the important aspects of the layout surrounding the bunkers?
Patient waiting and changing areas designed so patients can’t wander into treatment room
Clear signs lead to treatment units
Control room positioned so staff have clear view of access corridor, room entrance and in the room via CCTV
What warning signals are needed?
Visible signs in treatment room, at the entrance of the maze, and in the control room
Audible signals in the treatment room and at the entrance to the maze
What are the parts of the room interlock?
Door or light gate
Last man out button
Audible alarms
Why is ventilation important?
High energy beams can cause ozone and induced radioactivity
>6 air changes an hour needed
How should the thickness of a barrier be calculated?
Establish a target dose rate behind the barrier - calculate the barrier needed to achieve this
What are the IRR99 designated areas?
Controlled - persons need to take special precautions to restrict significant exposure to radiation - person could receive greater than 3/10ths of a dose limit
Supervised - keep conditions of area under review - person could receive greater than 1/10th of a dose limit
What are the definitions of IDR, TADR, and TADR2000?
IDR - instantaneous dose rate averaged over 1 min
TADR - time averaged dose rate over 8 hours
TADR2000 - time averaged dose rate over 2000 hours (1 working year)
What are the equations for TADR and TADR2000?
TADR = IDR . duty cycle . use factor TADR2000 = TADR . occupancy
What are the dose rates for controlled areas?
IDR > 2000uSv/hr
TADR > 7.5uSv/hr
TADR2000 > 3uSv/hr
What are the dose rates for supervised areas?
IDR > 7.5uSv/hr
TADR > 2.5uSv/hr
TADR2000 > 0.5uSv/hr
What are the dose rates for non-designated areas?
IDR < 7.5uSv/hr
TADR < 0.5uSv/hr
TADR2000 < 0.15uSv/hr
How is the workload, W, of a linac calculated?
Expressed in Gy/week @ isocentre
Number of patient/day x dose per patient
Typically 1000Gy/wk for <10MV machines, 500Gy/wk for >10MV machines
What is the usage factor?
The factor to account for beam orientation - ie U=0.25 for isocentric techniques for the floor, ceiling, and two walls
What factors can increase the usage factors, U, of barriers?
Dedicated long exposure rooms ie TBI
IMRT - increases secondary barrier fluence due to larger leakage
SRS/SABR - high doses so larger fluences at barriers
What are typical occupancy factors, T, for areas?
Full occupancy - control ares, offices - 1
Partial occupancy - corridors, staff toilets - 1/5
Occasional - public toilets, waiting areas - 1/20
What is the equation for the reduction factor?
B = p/p0 where p = target dose rate, p0 = initial dose rate
What is the tenth value layer of a material?
The thickness of material required to allow 10% transmission
For a primary barrier calculation, what is the equation for B?
B = (P.(d1/d0)^2)/W.U.T
What is the equation for the thickness of material required?
S = TLV1 + (n-1).TLVe
Where TLV1 = first tenth value layer
TLVe = equilibrium TVL
n = number of TLVs = log(1/B)
What is the equation for B from head leakage?
B = (1000.P.dL^2)/WT
Where dL is the distance from target to POI and U=1 as leakage is isotropic
What is the equation for B from patient scatter?
B = (P.d0^2.dP^2.400)/(a.W.T.F) d0 = distance from source to patient dP = distance from patient to barrier F = field size at isocentre a = scatter fraction
What is the equation for the scatter factor, a?
a = dose rate @1m from phantom when field area is 400cm^2 at the phantom surface / dose rate at centre of field 1m from source with no phantom
What is the purpose of doors?
Protects against head leakage and scatter that is transmitted through the nib or reflected off walls
What needs to be taken into account when calculating scatter?
Beam area at first scattering surface
Cross section of maze
Multiple reflection - lowers energy each time
Distance to entrance
What are the differences between high and low energy doors?
Low energy - shielding based on 0.2MeV photons
High energy - need higher energy shielding, use boron or litium with polyethylene to attenuate neutrons
What are the disadvantages of using a linac bunker as a brachy suite?
Not enough space for imaging equipment
Time pressure
Difficult to predict assumptions for shielding calcs
What is the equation for the dose rate of scatter from a brachy procedure?
Kp = (A.Г.alpha.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 = distance to scattering surface d2 = distance from scattering surface to room door
Repeat this at 20 degree intervals for the total
