Room design for RT

Question 1

What are the sources of radiation that need to be protected against in RT?

Answer 1

Linac treatment beam - photons, electrons, photoneutrons, scatter
Linac head - leakage, gamma photons from activated products
kV imaging - primary beam, tube leakage
Brachytherapy

Question 2

What are the types of barriers used in RT?

Answer 2

Primary - attenuate primary beam

Secondary - attenuated scatter, leakage, and neutrons

Question 3

What are the advantages and disadvantages of mazes?

Answer 3

Adv: smaller door, nicer for patients
Disadv: take up a lot of room

Question 4

When building a maze what consideration needs to be made about the width?

Answer 4

Make it wide enough to pass equipment down

Question 5

What maze design features can be used to reduce the radiation scattered down the maze?

Answer 5

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

Question 6

What features need special consideration for shielding cyberknife?

Answer 6

Beam can be incident on any walls so need primary barrier everywhere

Question 7

What 5 factors should be considered when deciding on room shielding?

Answer 7

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

Question 8

What issues can nuclear medicine departments have if next to poorly shielded RT departments?

Answer 8

Interference on photon counters and energy window detectors

Question 9

When planning a room, what considerations need to be made?

Answer 9

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

Question 10

Why are large bunkers desirable?

Answer 10

Distance is effective shielding
Needed for certain treatments
Need space to store accessories

Question 11

What are the important aspects of the layout surrounding the bunkers?

Answer 11

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

Question 12

What warning signals are needed?

Answer 12

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

Question 13

What are the parts of the room interlock?

Answer 13

Door or light gate
Last man out button
Audible alarms

Question 14

Why is ventilation important?

Answer 14

High energy beams can cause ozone and induced radioactivity

>6 air changes an hour needed

Question 15

How should the thickness of a barrier be calculated?

Answer 15

Establish a target dose rate behind the barrier - calculate the barrier needed to achieve this

Question 16

What are the IRR99 designated areas?

Answer 16

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

Question 17

What are the definitions of IDR, TADR, and TADR2000?

Answer 17

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)

Question 18

What are the equations for TADR and TADR2000?

Answer 18

TADR = IDR . duty cycle . use factor
TADR2000 = TADR . occupancy

Question 19

What are the dose rates for controlled areas?

Answer 19

IDR > 2000uSv/hr
TADR > 7.5uSv/hr
TADR2000 > 3uSv/hr

Question 20

What are the dose rates for supervised areas?

Answer 20

IDR > 7.5uSv/hr
TADR > 2.5uSv/hr
TADR2000 > 0.5uSv/hr

Question 21

What are the dose rates for non-designated areas?

Answer 21

IDR < 7.5uSv/hr
TADR < 0.5uSv/hr
TADR2000 < 0.15uSv/hr

Question 22

How is the workload, W, of a linac calculated?

Answer 22

Expressed in Gy/week @ isocentre
Number of patient/day x dose per patient
Typically 1000Gy/wk for <10MV machines, 500Gy/wk for >10MV machines

Question 23

What is the usage factor?

Answer 23

The factor to account for beam orientation - ie U=0.25 for isocentric techniques for the floor, ceiling, and two walls

Question 24

What factors can increase the usage factors, U, of barriers?

Answer 24

Dedicated long exposure rooms ie TBI
IMRT - increases secondary barrier fluence due to larger leakage
SRS/SABR - high doses so larger fluences at barriers

Question 25

What are typical occupancy factors, T, for areas?

Answer 25

Full occupancy - control ares, offices - 1
Partial occupancy - corridors, staff toilets - 1/5
Occasional - public toilets, waiting areas - 1/20

Question 26

What is the equation for the reduction factor?

Answer 26

B = p/p0 where p = target dose rate, p0 = initial dose rate

Question 27

What is the tenth value layer of a material?

Answer 27

The thickness of material required to allow 10% transmission

Question 28

For a primary barrier calculation, what is the equation for B?

Answer 28

B = (P.(d1/d0)^2)/W.U.T

Question 29

What is the equation for the thickness of material required?

Answer 29

S = TLV1 + (n-1).TLVe
Where TLV1 = first tenth value layer
TLVe = equilibrium TVL
n = number of TLVs = log(1/B)

Question 30

What is the equation for B from head leakage?

Answer 30

B = (1000.P.dL^2)/WT

Where dL is the distance from target to POI and U=1 as leakage is isotropic

Question 31

What is the equation for B from patient scatter?

Answer 31

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

Question 32

What is the equation for the scatter factor, a?

Answer 32

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

Question 33

What is the purpose of doors?

Answer 33

Protects against head leakage and scatter that is transmitted through the nib or reflected off walls

Question 34

What needs to be taken into account when calculating scatter?

Answer 34

Beam area at first scattering surface
Cross section of maze
Multiple reflection - lowers energy each time
Distance to entrance

Question 35

What are the differences between high and low energy doors?

Answer 35

Low energy - shielding based on 0.2MeV photons

High energy - need higher energy shielding, use boron or litium with polyethylene to attenuate neutrons

Question 36

What are the disadvantages of using a linac bunker as a brachy suite?

Answer 36

Not enough space for imaging equipment
Time pressure
Difficult to predict assumptions for shielding calcs

Question 37

What is the equation for the dose rate of scatter from a brachy procedure?

Answer 37

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