Room design for linacs and brachytherapy facilities

Question 1

What sources of radiation need to be considered for radiotherapy room design shielding?

Answer 1

• Linac electron or photon primary beam (photoneutrons also produced above 8 MV).
• Linac head leakage and activated product emissions.
• On-board imaging primary beam and leakage.
• Sealed source radiation for brachytherapy.

Question 2

What do primary and secondary barriers protect against? What else needs to be considered for > 8-10 MV beams?

Answer 2

• Primary radiation.
• Secondary radiation (e.g. scatter, leakage and activation).
• Photoneutrons.

Question 3

What options are available to afford radiation protection at the linac bunker entrance? What are the advantages/disadvantages?

Answer 3

• Shielded bunker door or maze which increases distance and creates a sufficient number of scattering events.
• A mazed entrance could be doubled for use as a primary barrier. It would reduce cost.
• However, it would incur an increased spatial footprint. It could also impact workload (due to time taken to enter/vacate bunker) and additional access restriction processes would need to be considered.
• A short maze/door combination can be a good trade-off in some cases.

Question 4

What are some aspects of maze design that help afford radiation protection?

Answer 4

• Lintels reduce ceiling height and, therefore, cross-sectional aperture of the maze entrance.
• Extended nibs/baffles at the maze entrance to reduce scatter.
• Additional distance/extra turns in the maze to help reduce scatter.

Question 5

What differences are apparent with Tomotherapy shielding when compared to a typical linac?

Answer 5

Tomotherapy units have a narrower beam and integral beam stops. This means the length of primary barrier required is less.

Question 6

What would a brachytherapy suite room design typically look like?

Answer 6

Combination of a short maze and heavy bunker door.

Question 7

What are some other general considerations for shielding design?

Answer 7

• Location (basement can often be good for radiotherapy facilities due to the natural shielding provided).
• Availability of space (new build or retro-fit).
• Spaces to protect (e.g. public access and restricted access areas).
• People to protect (e.g. workers and members of the public).
• Budget.
• Adjacent facilities and occupancy.
• Potential future changes in workload.
• Removal and installation of new equipment.

Question 8

What are the advantages of a large bunker?

Answer 8

• Additional distance corresponds to additional protection.
• Storage space for equipment.
• Easier patient access and manoeuvrability.
  Required for some treatments (e.g. total body irradiation).

Question 9

What are some general layout considerations for a radiotherapy treatment unit design (e.g. waiting areas, patient change, control room)?

Answer 9

• Waiting areas and patient changing rooms should be located to reduce the chance of accidental access into the treatment area.
• Control room should be located to have good view of the treatment room, access corridors and entrance to the treatment room.

Question 10

What kind of warning signals are apparent with radiotherapy units?

Answer 10

• Visible signals in the treatment room, at the entrance of the maze and in the control area.
• Audible signals in the treatment room and in the control area.

Question 11

Apart from shielding and warning signage/signals, what is one of the other main engineering controls for a radiotherapy unit?

Answer 11

Interlocks.

Question 12

What constitutes a controlled area, as per IRR17?

Answer 12

• Any person working in the area is likely to receive an annual whole body dose of > 6 mSv, > 15 mSv to the lens of the eye or < 150 mSv to the skin or extremities.
• It is necessary for any person entering or working in the area to follow special procedures designed to restrict significant exposure.

Question 13

What constitutes a supervised area, as per IRR17?

Answer 13

• Any person working in the area is likely to receive a whole body dose of > 1 mSv, > 5 mSv to the lens of the eye or < 50 mSv to the skin or extremities.
• Conditions of the area to be kept under review to determine whether the area should be designated as controlled.

Question 14

What are the guideline dose rates for controlled and supervised areas?

Answer 14

• Controlled: IDR > 100 mircoSv/hr, TADR >7.5 mircoSv/hr and TADR2000 > 3 mircoSv/hr.
• Supervised: IDR > 7.5 mircoSv/hr, TADR >2.5 mircoSv/hr and TADR2000 > 0.5 mircoSv/hr.

Question 15

What are the shielding calculation parameters for radiotherapy units?

Answer 15

• Shielding design goal (P): Dose equivalent beyond the barrier (Sv/week).
• Workload (W): How much the machine is used (Gy/week).
• Use factor (U): Fraction of workload directed at a particular barrier.
• Occupancy (T): Fraction of working week that an individual is in a particular location.

Question 16

How is workload for a radiotherapy linac typically expressed? What might a typical workload be?

Answer 16

• Expressed in Gy/week at isocentre (1 m from output).
• NCRP recommend typical values of 1000 Gy/week for low energy machines and 500 Gy/week for high energy machines.

Question 17

What is the usage factor for a conventional isocentric linac? When might it be different?

Answer 17

• Same usage for floors, ceiling and walls (U = 0.25).
• TBI, IMRT, SRS etc.

Question 18

List the types of areas surrounding a linac bunker that may have full, partial or occasional occupancy.

Answer 18

• Full: Control areas, offices, receptions & attended waiting areas.
• Partial: Corridors, staff toilets, staff rooms & treatment room door area.
• Occasional: Public toilets, unattended waiting areas, stairways, storage areas & cleaning cupboards.

Question 19

What is the tenth value layer (TVL)? What does it depend on? If the first TVL (TVL_1) is different from the equilibrium TVL (TVL_e), how are they combined? How is the TVL related to the reduction factor?

Answer 19

• TVL: Thickness of material required to allow 10% transmission.
• Depends on photon beam energy and barrier material.
• S = TVL_1 + (n-1)TVL_e.
• 10^n = 1/B where n is the number of TVLs required to achieve a given reduction factor (B).

Question 20

How is the amount of shielding required determined from the reduction factor (B)?

Answer 20

• The number of TVLs (n) required to achieve the reduction factor (B) is given by: 10^n = 1/B.
• The first TVL (TVL_1) and the equilibrium TVL (TVL_e) can then be combined to determine shielding thickness using: S = TVL_1 + (n-1)TVL_e.

Question 21

What is the general equation for the reduction factor required for primary shielding design? Explain it.

Answer 21

• B = (P(d_1/d_0)^2)/WUT
  where P is the shielding design goal, (d_1/d_0)^2 represents the ISL correction, W is the workload, U is the usage factor and T is the occupancy factor.
• The workload (i.e. Gy/week at 1 m) is ISL corrected to the location of interest. It is multiplied by the usage factor and occupancy factor. The reduction factor is then determined by dividing the shielding design goal (i.e. the dose constraint of 0.3 mSv/yr or 0.3/52 = 6x10^-3 mSv/week) by this corrected workload.

Question 22

What is the equation for the head leakage reduction factor? When does head leakage dominate over patient scatter?

Answer 22

• B = 1000.P.d_L^2/WT where P is the shielding design goal, d_L^2 represents the ISL correction, W is the workload and T is the occupancy factor (U is assumed to isotropic for leakage). Head shielding is designed to reduce intensity by a factor of 1000 (i.e. should be 0.1% of primary beam).
• Leakage is dominant for energies above 10 MV.

Question 23

What is the general equation for the reduction factor required for patient scatter?

Answer 23

B = P.d_0^2.d_P^2.400/aWTF
where d_0 is the distance from source to patient, d_P is the distance from patient to barrier, F is the field size at isocentre and a is the scatter fraction = dose rate @ 1 m from phantom with field area 400 cm^2/Dose rate @ centre of field 1 m from source with no phantom.

Question 24

What does the scatter fraction depend on?

Answer 24

Scatter angle and energy.

Question 25

What factors need to be considered in determining radiation levels in the maze?

Answer 25

• Photons originating from head leakage and patient scatter can be transmitted through the nib and reflected from walls. Reflected photons can be determined by considering the beam area at the first scattering surface, the cross-sectional area of the maze, multiple reflections (and the reduced energy after each) and the distance to the entrance.
• Neutrons must also be considered above 8-10 MV.

Question 26

How are photoneutrons produced and what hazards do they present in radiotherapy?

Answer 26

• Photoneutrons are produced when a photon has enough energy to interact with the target nucleus causing ejection of a neutron.

Hazards:
- Direct exposure from photoneutrons.
- Indirect exposure from short-lived activation products produced in the treatment head.

Question 27

What are some options for neutron shielding materials?

Answer 27

• Hydrogen-rich materials have a large neutron interaction cross-section (although they do produce gamma radiation during this interaction).
• Boron absorbs well and produces few gamma photons.
• Up to 15 MV, normal concrete is typically adequate due to the number of water molecules present.
• Higher energy installations should lines walls with hydrocarbons (e.g. borated polyethylene).

Question 28

What radiation safety design features are likely to be present in a HDR room that are not in a linac room?

Answer 28

• In-room environmental monitors.
• Shielded afterloader for sources.

Question 29

What would need to be considered when calculating the air kerma rate at the HDR room door K_p (down the maze due to reflection from the wall) from the radiation source? How is this typically performed in practice?

Answer 29

• Activity of the source (A).
• Air kerma rate per unit activity at 1 m (F).
• Reflection coefficient per unit area for scatter (alpha).
• Area of wall exposed (A1).
• Distance from source to wall (d1) and wall to door (d2) for ISL corrections.
• K_p = AF.alpha.A1/(d1.d2)^2 (i.e. ISL correct the air kerma to the wall, multiple by the area exposed then ISL correct to the door).
• In practice, the scatter contribution is calculated at 20 degree intervals and then each contribution is summed.