S H I E D I NL G

UnitsH = D • Q

H: Dose equivalent (Sv)

D: Dose (Gy)

Q: Quality Factor

1Sv = 1 J/Kg

1Gy = 1 J/Kg

if dose is expressed in units of cGy (rad) then dose equivalent is expressed in units of rem.

Other common unit for H is mSv. When solving shielding problems be consistent in using units.

RADIATION PROTECTION AND SAFETY IN RADIOTHERAPY

601

(a)

(b)

North

North

Maze

t2t1

Isocentretpri

w

tpri

w

tsec

High density concrete

Isocentre

t2 t1Maze

w

FIG. 16.2. Typical floor plan for an isocentric high energy linac bunker. (a) The machine gantry rotation axis is parallel to the maze entry corridor; the primary barriers are parts of the floor and ceiling, as well as parts of the east and west walls. (b) The machine gantry rotation axis is perpendicular to the maze entry corridor; the primary barriers are parts of the floor and ceiling and parts of the north and south walls. Normal density concrete (2.35 g/cm3) is used in all walls except for the south wall, which is made of high density concrete (5 g/cm3). The door to the treatment room maze is a neutron shielded door.

Linac Orientation

RADIATION PROTECTION AND SAFETY IN RADIOTHERAPY

601

(a)

(b)

North

North

Maze

t2t1

Isocentretpri

w

tpri

w

tsec

High density concrete

Isocentre

t2 t1Maze

w

FIG. 16.2. Typical floor plan for an isocentric high energy linac bunker. (a) The machine gantry rotation axis is parallel to the maze entry corridor; the primary barriers are parts of the floor and ceiling, as well as parts of the east and west walls. (b) The machine gantry rotation axis is perpendicular to the maze entry corridor; the primary barriers are parts of the floor and ceiling and parts of the north and south walls. Normal density concrete (2.35 g/cm3) is used in all walls except for the south wall, which is made of high density concrete (5 g/cm3). The door to the treatment room maze is a neutron shielded door.

Linac Orientation

1.8 PROTECTIVE BARRIERS / 11

shall be determined by the ratio of the average time the maximallyexposed individual will be present to the total average time thatthe equipment is used during the week. The period over which theaverage shall be estimated is 1 y.

1.8 Protective Barriers

In radiotherapeutic applications, the radiation consists of pri-mary and secondary radiations (Figure 1.2). Primary radiation,also called the useful beam, is radiation emitted directly from theequipment that is used for patient therapy. A primary barrier is awall, ceiling, floor or other structure that will intercept radiationemitted directly from the source. It needs to attenuate the usefulbeam and also any secondary radiation that impinges on it to theappropriate shielding design goal. Secondary radiation consists ofradiation scattered from or produced by interactions with thepatient and other objects as well as the leakage radiation fromthe protective housing of the source. A secondary barrier is a wall,ceiling floor or other structure that will intercept the secondaryradiation. It needs to attenuate the secondary radiation to theappropriate shielding design goal. A full discussion of primary andsecondary barriers is given in Section 2.

Fig. 1.2. Schematic of radiation sources (primary, leakage andpatient-scattered) and the primary and secondary barriers.

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Barriers

Shielding Parameters

Workload, W (cGy m2 week-1)

Use factor, U Occupancy factor, T Leakage Radiation

Workload WDefinition

Output produced by therapy unit per

week at 1 m in cGy

Example: If a unit treats 25 patients per day with an average dose of 200 cGy per fraction, then W = 25000 cGy m2 week-1

Use Factor UDefinition

Fraction of the operating time

during which the radiation is

directed toward a particular barrier

Typical Use Factors are:Floor: 1Walls: 1/4

Ceiling: 1/4 - 1/2

Occupancy Factor TDefinition

Fraction of the operating time

during which the area of interest is occupied by the

individual

Typical values of T are:Full occupancy: 1Partial occupancy: 1/4Occasional occupancy: 1/8 - 1/16

Shielding Equations

P =WUTd 2

⋅ B⇒ B =P ⋅ d 2

WUT

Primary Radiation Barrier

B: Transmission factor to reduce dose to P in the area of interest

P: Permissible dose equivalent.(e.g. 5 rem/year for controlled area & 0.1 rem/year for non-controlled area NCRP91, 0.5 rem/year for controlled area NCRP151)

No. of tenth-value layers

N = log10 B−1( )

Barrier thickness tpri

T1: 1st TVLTe: subsequent TVL

tpri = T1 + N − 1( )Te

RADIATION PROTECTION AND SAFETY IN RADIOTHERAPY

601

(a)

(b)

North

North

Maze

t2t1

Isocentretpri

w

tpri

w

tsec

High density concrete

Isocentre

t2 t1Maze

w

FIG. 16.2. Typical floor plan for an isocentric high energy linac bunker. (a) The machine gantry rotation axis is parallel to the maze entry corridor; the primary barriers are parts of the floor and ceiling, as well as parts of the east and west walls. (b) The machine gantry rotation axis is perpendicular to the maze entry corridor; the primary barriers are parts of the floor and ceiling and parts of the north and south walls. Normal density concrete (2.35 g/cm3) is used in all walls except for the south wall, which is made of high density concrete (5 g/cm3). The door to the treatment room maze is a neutron shielded door.

Example

d

Evaluate the thickness of concrete needed for the primary shield shown here (tpri) at the point (A). This unit treats 40 patients per day with an average dose of 250 cGy per fraction utilizing a 20 MV beam. The distance from source (S) to the point (A) is 4.4 m.

a) Radiation Therapy Supervisor’s officeb) Hospital corridor

S A*

Example

Dose-Equivalent index TVL for X-rays in concrete (NCRP report 51, 1977)

W = 40 pt/day x 250 cGy m2/pt x 5 day/weekW = 50000 cGy m2/week

U = 1/4

a) T = 1, P = (5 rem/year) / 50 (weeks/year) b) T = 1/4, P = (0.1 rem/year) / 50 (weeks/year)

a) B = [0.1x(4.4)2]/[50000x1x1/4] = 1.55x10-4

N = log10(1/B) = 3.81 tpri = 48 + (2.81) * 44 ≈ 172 cm

b) B = [0.002x(4.4)2]/[50000x1/4x1/4] = 1.2x10-5

N = 4.92 tpri = 48 + (3.92) * 44 ≈ 220.5 cm

Note: We used the recommendations of NCRP 91 in this example.

Example

Dose-Equivalent index TVL for X-rays in concrete (NCRP report 51, 1977)

W = 40 pt/day x 250 cGy m2/pt x 5 day/weekW = 50000 cGy m2/week

U = 1/4

a) T = 1, P = (5 rem/year) / 50 (weeks/year) b) T = 1/4, P = (0.1 rem/year) / 50 (weeks/year)

a) B = [0.1x(4.4)2]/[50000x1x1/4] = 1.55x10-4

From the graph tpri ≈ 173 cm

b) B = [0.002x(4.4)2]/[50000x1/4x1/4] = 1.2x10-5

From the graph tpri ≈ 222 cm

Note: We used the recommendations of NCRP 91 in this example.

NCRP 151 Table B.2

Barrier Width28 / 2. CALCULATIONAL METHODS

Fig. 2.4a. Width of primary barrier protruding into the room.

Fig. 2.4b. Arrangement for the primary barrier when the inside wallis continuous.

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Barrier Width

28 / 2. CALCULATIONAL METHODS

Fig. 2.4a. Width of primary barrier protruding into the room.

Fig. 2.4b. Arrangement for the primary barrier when the inside wallis continuous.

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Secondary BarrierScatter

Bs =

PαWT

i400F

id 2 i ′d 2

α: fractional scatter @ 1 m for a f.s. 400cm2 incident @ scattererF: area of the beam @ scatterer

d’: distance from source to scatterer

Scattering Angle α (6MV X-ray)

15 9x10-3

30 7x10-3

45 1.8x10-3

60 1.1x10-3

90 0.6x10-3

135 0.4x10-3

NCRP No. 51, 1977

d: distance from scatterer to area of interest

Secondary BarrierLeakage

BL =

Pid 2

0.001WT

Workload (WL): 0.001 Wpri

d: distance from source to area of interest

2.3 SECONDARY BARRIERS / 33

As noted, the scattered-radiation energy is significantlydegraded (beyond 20 degree scattered radiation) from that ofthe primary beam and thus separate data are used to computeits transmission through the barrier. Tables B.5a and B.5b giveTVL values in concrete and lead, respectively, for radiations scat-tered from the patient at different scattering angles and beamenergies. For other materials, the TVL for the patient-scatteredradiation can be estimated by using the mean energy of thescattered radiation from Table B.6 (Appendix B) and the TVLvalues from Figures A.1a and A.1b (Appendix A).

The barrier transmission of leakage radiation alone (BL) is givenby Equation 2.8.

(2.8)

In Equation 2.8, the factor 10–3 arises from the assumption thatleakage radiation from the accelerator head is 0.1 % of the usefulbeam. The use factor again is taken as one, and dL is measuredfrom the isocenter if it can be assumed that the accelerator gantryangles used are, on average, symmetric. If this is not the situation,then the distance to the individual barriers should be taken fromthe closest approach of the accelerator head to each barrier and

Fig. 2.6. Room layout showing distances associated with patient-scattered (dsca, dsec) and leakage radiations (dL).

BL

P dL

2

103–

W T

-----------------------=

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Distanceslook up in NCRP 151

Secondary BarrierScatter

Bs =

PαWT

i400F

id 2 i ′d 2

Leakage

BL =

Pid 2

0.001WT

For Megavoltage installations, the leakage barrier usually far exceeds that required for the scattered radiation, since the leakage radiation is more penetrating than the scattered radiation.

1If the thickness of the two barriers differ by at least 3 HVLs (1 TVL) of primary beam, the thicker of the two would be adequate. If the difference is less than 3 HVLs, then 1 HVL should be added to the larger one.

2

NeutronsNeutron fluence

a: transmission factor (1 for Pb)

Q: Neutron source strength per unit dose of x-ray

d: distance from target to point of interest

S: Surface area of treatment room

H0: neutron dose eq. at d0

d1: distance from isocenter to centerline of maze

d2: length of maze

T/T0 is the ratio of outer maze area to the inner maze entrance

K: ratio of captured gamma to total n (0.77x10-10)

TVD2: tenth value distance (6.2 m)

Φ total = Φdir + Φsc + Φ th

Φdir =aQ4πd1

2 ;Φsc =5.4aQS

;Φ th =1.26QS

Neutron H

H = (H0 )(T / T0 )(d0 / d1 )210−d2 / 5

D = KΦ total10−d2 /TVD2

RADIATION PROTECTION AND SAFETY IN RADIOTHERAPY

601

(a)

(b)

North

North

Maze

t2t1

Isocentretpri

w

tpri

w

tsec

High density concrete

Isocentre

t2 t1Maze

w

FIG. 16.2. Typical floor plan for an isocentric high energy linac bunker. (a) The machine gantry rotation axis is parallel to the maze entry corridor; the primary barriers are parts of the floor and ceiling, as well as parts of the east and west walls. (b) The machine gantry rotation axis is perpendicular to the maze entry corridor; the primary barriers are parts of the floor and ceiling and parts of the north and south walls. Normal density concrete (2.35 g/cm3) is used in all walls except for the south wall, which is made of high density concrete (5 g/cm3). The door to the treatment room maze is a neutron shielded door.

Example

d1

d2