Bunker Design Calculations

Question 1

Label each part of this linac bunker

Answer 1

Grey = linac + couch
Red = isocentre
Orange = primary beam source
Yellow = primary beam
Blue = primary barriers
Green = secondary barriers
Purple = maze

Question 2

Name 6 factors that effect the dose delivered to people behind radiation shielding (e.g. a linac bunker)

Answer 2

• Dose rate at isocentre
• Use factor/duty cycle
• Orientation factor (U)
• Attenuation from barrier
• Distance from source
• Occupancy factor (T)

Question 3

What is meant by the ‘duty cycle’ or use factor of a beam?

Answer 3

The proportion of the time that a radiation beam is on during the working day.

Question 4

What is meant by the orientation factor of a beam?

Answer 4

The proportion of the time that a radiation beam is pointing in a given direction when turned on.

Question 5

What is meant by the occupancy factor of a room?

Answer 5

The proportion of time that people are in a room.

Question 6

What is the dose rate at isocentre of a beam?

Answer 6

The dose rate at the isocentre when the machine is on (typically 600 MU/min = 6 Gy/min)

Question 7

Give the equation for the attenuation of a radiotherapy bunker wall

Answer 7

I = intensity
I_0 = initial intensity
t = barrier thickness
TVL = tenth value layer

Question 8

Give the equation for the barrier thickness of a linac bunker

Answer 8

t = barrier thickness
TVL1 = first tenth value layer
n = number of TVLs
TVLe = equilibrium TVL

Question 9

Give the equation for radiation intensity at a distance, d, from the source

Answer 9

I = intensity
I_0 = intensity at source
d = distance from the source

Question 10

For primary barriers, where is the source ‘located’ for dose calculations?

Answer 10

In the linac head, 1m beyond the isocentre from the barrier (orange = source position)

Question 11

For secondary barriers, where is the cource ‘located’ for dose calculation?

Answer 11

The time averaged source position is at the isocentre

Question 12

Give 4 examples of full (100%) occupancy areas

Answer 12

• Control areas
• Offices
• Nurses stations
• Waiting rooms

Question 13

Give 3 examples of partial (20-50%) occupancy areas

Answer 13

• Clinic rooms
• Corridors
• Washrooms

Question 14

Give 3 examples of occasional (5-12.5%) occupancy areas

Answer 14

• Store rooms
• Plant rooms
• Car parks

Question 15

Define instantaneous dose rate (IDR)

Answer 15

The dose rate when the beam is on (averaged over 1 minute), measured in µSv/h. This is a measureable quantity.

Question 16

Define time averaged dose rate (TADR)

Answer 16

The worst-case dose rate for a working day. Given by multiplying the instantaneous dose rate by the duty cycle and beam orientation.

Question 17

Give the equation for the time-averaged dose rate

Answer 17

TADR = time-averaged dose rate
IDR = instantaneous dose rate

Question 18

Define time averaged dose rate for 2000 hours (TADR2000)

Answer 18

The estimated dose rate for a worker averaged over a year.

Question 19

Give the equation for the time-averaged dose rate for 2000 hours

Answer 19

Question 20

Give the equation for the annual dose rate from a source

Answer 20

Question 21

What is the orientation factor used for secondary barriers?

Answer 21

1

Question 22

Which factors contribute to the dose at the maze entrance?

Answer 22

1) Scatter from primary beam off walls
2) Scatter from patient
3) Leakage from linac head
4) Transmission through the wall

Question 23

Give the equation for the total dose at the entrance of a linac maze

Answer 23