Radiation protection for particle accelerators - AU · Radiation protection for particle...

Radiation protectionfor particle accelerators

Lars Hjorth Præstegaard, Ph.D.Technical managerMedical physicist

Aarhus University Hospital

Health physics

Aarhus University Hospital, Århus Sygehus

Dose quantities

Definition: Effective doseThe effective dose is given by

where wT is the weighting factor for tissue T and the sum is over all radiation quantities and tissues in the body (unit: Sv).

Definition: Absorbed doseThe absorbed dose D is the ratio of dε/dm where dε is the mean energy imparted by ionizing radiation to mass dm (unit: J/kg).

Definition: Radiation weighting factorThe radiation weighting factor wR is the biological effectiveness of radiation R relative to photons.

∑=TR

TRT DwwE,

Equivalent total body dose from photons


Dose quantities

20Alpha particles, fission fragments, heavy nuclei

5Protons, other than recoil protons, > 2 MeV

5> 20 MeV10> 2 to 20 MeV20> 0.1 to 2 MeV1010 to 100 keV5<10 keV

Neutrons:1Electrons and muons, all energies1Photons, all energies

Radiation weighting factor wR

Type and energyof radiation R


Dose quantities

1.00Whole body total

0.05Remainder

0.01Bone surface

0.01Skin0.05Thyroid

0.05Oesophagus

0.05Liver

0.05Breast

0.05Bladder

0.12Stomach0.12Lung

0.12Colon

0.12Bone marrow (red)

0.20Gonads

Tissue weighting factor wT

Tissue or organ


Why is radiation dangerous?

Ionizing radiation causesdamage to DNA:• Direct damage to the DNA.• Indirect damage: Creation of free

radicals that may damage the DNA.• If DNA is damaged, the cell is likely to

die when the cell divides.

Repair of damage:• Most of the damaged DNA is repaired

before the cell divides.• Double-strand breaks are more difficult

to repair than single strand breaks.• Incorrect repair (mutation) may cause

cancer.



Stochastic damage:• Incorrect repair or damage to the

DNA (mutation) which do not cause the cell to die.

• The stochastic damage may cause the cell to transform into a cancer cell after several years.

Deterministic damage:• Damage to tissue due to direct cell

death (e.g. radiotherapy)• Only observable for doses above

0.5 Gy. • The effect of radiation is observed

within a few weeks (acute).42 Gy / 12 fx


Why is radiation dangerous?Effects of acute whole body exposure:

Death after 24-48 hours100Death within a few weeks10

LD 50/30 (half of population death within 30 days)

4.5Probable recovery1No observable effect0.5

EffectDose (Gy)

Curative radiotherapy:35-80 Gy within a small region of the body



Linear fit: 5 % per Sv

Lifetime excess risk of death caused by cancer:


Dose to the publicContributions to the background radiation:

Average background radiation in Denmark: 3 mSv/year

Cancer deaths in Denmark due to background radiation: ~750/year3 mSv/year ~ 1 cigarette per day


Dose limits in the EC

Bekendtgørelse nr. 823 af 31. oktober 1997 om dosis-grænser for ioniserende stråling:

Dosisgrænserne for dosisovervåget arbejdstagere og enkeltpersoner i befolkningen er henholdsvis 20 mSv/år og 1 mSv/år.

Radiation from aparticle accelerator


Radiation from an electron accelerator

Electron beam + Matter

Photons:

Neutrons:

Neutron

NucleusPhoton

(γ,n) process

Induced radioactivity:1. (γ,n) process

neutron nucleus

Photon

2. Neutron capture

Bremsstrahlung (x-rays)

β-, β+

Photon

β-, β+


Radiation from an electron accelerator

Radiation Protection Dosimetry, Vol. 96, No 4, pp. 333–343 (2001)

Threshold for neutron production:Most materials: 6-13 MeV

However:9Be: 1.67 MeVD: 2.23 MeV16O: 15.67 MeV12C: 18.72 MeV

Dose equivalent rates per unit beam power:

Dose from forward bremsstrahlung dominates at all energies


Electron accelerator: PhotonsAngular distribution of bremsstrahlung (tungsten target):


Electron accelerator: Neutron production

Rule of thumb:Neutron yield largest in high-Z materials

Averrage neutron energy:1-2 MeV for medical electron accelerator

Angular distribution:Isotropically for high-Z materials

Neutron yield from a thick target:

IAEA Technical report, no. 188, 1979

present in and near the target in medical

electron accelerator


Radiation from a proton acceleratorHigh energy proton beam + Matter

Spallation process:

ProductionNeutronsPhotonsα particlesIonsInduced rad....

Neutron production by proton bombardment is the most significant radiation hazard for proton accelerators


Radiation from a proton accelerator

Radiation Protection Dosimetry, Vol. 96, No 4, pp. 393–406 (2001)

Thick-target neutron yield for protons/carbon ions:

Therapeutic carbon ions on:

: Cobber

: Carbon

Therapeutic protons on:

: Iron

: Soft tissue


Radiation from a proton acceleratorNeutron production in a proton accelerator (forward direction):

Radiation Protection Dosimetry (2005),Vol. 116, No. 1–4, pp. 245–251

1E-20

1E-19

1E-18

1E-17

1E-16

1E-15

1E-14

0 50 100 150 200 250

Proton energy (MeV)

Neu

tron

dos

e(S

v*m

^2 p

er p

roto

n)

CNAlFeCuW

Radiation Protection Dosimetry,Vol. 96, No. 4, pp. 381–392 (2001)


Induced radioactivity: Electron accelerators

Neutron

NucleusPhoton

(γ,n) process

Photon

β-, β+

neutron nucleus

Photon

Neutron capture

β-, β+

Induced radioactivity is mainly caused by:

Largest neutron production at locations with large beam loss

Only production of β-, β+, and γ emitters


Induced radioactivity: Electron accelerators

Radiation protection: Time and avoid areas of large beam loss

Low level of induced radioactivity + short lifetime

Induced radioactivity for a medical electron accelerator:

Only short-lived

nuclides


Induced radioactivity: Proton accelerators

Spallation process:

Many processes contribute to induced radioactivity:

Highest neutron production at locations with large beam loss

The problem of induced radioactivity is far more serious in proton accelerators than for electron accelerators(approximately a factor of 100!)

Only production of β-, β+, and γ emitters



Radiation Protection Dosimetry (2007), Vol. 123, No. 4, pp. 417–425

Irradiation of samples 1.2 m below 12 GeV proton beam-line for 3 months (specific activity in Bq/g):



Radiation Protection Dosimetry (2007), Vol. 123, No. 4, pp. 417–425

Half life of most important nuclides:


Induced radioactivity: Proton acceleratorsCERN activation limits for waste (nuclide specific):



The gamma dose 1 m from the patient copper collimator after a single 2 min treatment session: 3.24 mSv/h (half-life of 23.4 min)

Example: Proton therapy

Shielding materials


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 5 10 15 20 25 30

Concrete depth (cm)

Rel

ativ

e ph

oton

dos

e

Shielding materials: PhotonsDefinition of TVL (tenth-value layer):

TVL

Attenuation length: (1/e reduction of dose) = TVL/ln(10)

Buildup region Exponential falloff of photon dose after the first few cm in concrete⇒The TVL formalism can be used


Shielding materials: Photons

27.221.03.84Ledite XN-240

22.217.14.80Ledite XN-288

1.5Earth (dry, packed)

131880.98Boron-loaded polyethylene

3.7-4.8Heavy concrete

5.65.711.35Lead10.89.97.80Steel

43.035.02.35Normal concrete

TVL, 15 MV(cm)

TVL, 6 MV(cm)

Density(g/cm3)

Large density provides good shielding

Primary photons from a medical electron accelerator:


Shielding materials: Neutron interactions

Fast neutrons (>0.5 eV):• Emitted in all direction from the source.• Neutron interactions energy loss):

– Elastic collisions:• Dominating interaction.• Billiard-like collisions.• Elastic collision with atomic nucleus ⇒ energy loss ⇒

conversion of fast neutrons to slow neutrons.– Hydrogen: Large energy loss per collision.– Lead: Low energy loss per collision.

– In elastic collisions: (n,2n), (n,p) and others• Dominates neutron energy loss for heavy nuclei.• (n,2n) process ⇒ increases the neutron fluence.

Concrete: at least ~5 % water (hydrogen) per weight


Shielding materials: Neutron interactions

Slow neutrons (<0.5 eV):• Most thermal neutrons (~0.025 eV).

• Neutron interactions:

– Elastic collisions with atomic nucleus.

– Neutron absorption:

• Capture of thermal neutrons in atomic nucleus: (n,γ) reaction.– Large cross section for capture in boron and cadmium.

– Emission of photons at capture (only 0.478 MeV photon for Boron).

• Few resonances in keV region.

Efficient neutron shielding:Boron-loaded polyethylene (hydrogen + boron)


Shielding materials: Neutrons

153.84Ledite XN-240

164.80Ledite XN-288

9.60.880.98Boron-loaded polyethylene

8841011.35Lead

3810.77.80Steel

21342.35Normal concrete

TVL, fast(cm)

TVL, slow(cm)

Density(g/cm3)

1. Hydrogen and boron provides a good shielding2. Concrete: TVL,neutron <TVL,photon

Neutrons from a medical electron accelerator (15 MV):



Neutron transmission is exponential after a depth of 40 cm in concrete.⇒The TVL formalism can be used

Neutron transmission for 70 MeV protons in concrete (nitrogen target):




Neutron spectra are harder in forward directions for Proton energies > 30 MeV Radiation Protection Dosimetry (2005),

Vol. 116, No. 1–4, pp. 245–251

Attenuation length as a function of neutron emission angle:

targetmaterials

70 MeV protons 100 MeV protons

iron target

Concrete TVL ≈ 41 cm

Concrete TVL≈ 56 cm



Radiation Protection Dosimetry,Vol. 96, No. 4, pp. 381–392 (2001)

Neutron production and attenuation length in concrete for 250 MeV protons:

Attenuation length of 110 g/cm2: Concrete TVL≈ 108 cm!!!

Neutron spectra are harder in forward directions for Proton energies > 30 MeV




Relative dose of neutrons in the forward direction for protons on a thick iron target:

Depth in concrete (cm)

Neutrons dominate the shielding design for proton energies above ~30 MeV(Neutron TVL>photon TVL)

Shielding of aparticle accelerator


Radiation from a medical electron acc.

1. Primary photons from target (treatment felt)

2. Scattered photons from patient and walls

3. Leakage radiation (leakage photons from the accelerator)

Photon sources from a medical accelerator:

secondaryphotons


Shielding: Primary/secondary barriers

DK legislation:Dose limit for shielding design: 1 mSv/year


Shielding: Workload (W)

Typical workloads for values for Medical electron accelerator:• Primary radiation:

– 6 MV: 500 Sv/week– 15 MV: 250 Sv/week

• Secondary radiation:– 6 MV: 3.6 Sv/week– 15 MV: 1.6 Sv/week

Definition:Dose 1 m from a source of radiation per working week (37 hours)

A workload is defined both for primary radiation and secondary radiation (scattered radiation)


Shielding: Workload (W)Workload for a largeaccelerator:

Many sources of radiation which depend on the beam loss pattern

Example: Proton therapy27 sources of radiation.


Shielding: Area occupancy factor (T)

1 (=full occupancy): Offices, treatment planning, control rooms, laboratories etc.

1/2: Adjacent treatment room.

1/5: Corridors, employee lounges, staff rest rooms etc.

1/8: Mace doors.

1/20: Public toilets, unattended waiting rooms, storage areas etc.

1/40: Outdoor area with only transient pedestrian or vehicular traffic, stairways, unattended elevators etc.

NCRP 151 recommendation for radiotherapy facilities:


Shielding: Beam orientation factor (U)Definition:Fraction of accelerator workload the radiation is directed towards a given barrier

Fixed beam accelerators:Single primary barrier for which U=1.

Accelerator with gantry:The sum of all use factors for each primary barrier is 1.

Secondary barriers:U=1 as the secondary barrier always protect against stray radiation regardlessof the beam direction.


Shielding: Primary photons

Reduction of dose behind shielding:• Workload (W) ↓ (½ Workload ⇒ ½ dose)• Occupancy (T) ↓ (½ Occupancy ⇒ ½ dose)• Distance (d) ↑ (2 × Distance ⇒ 1/4×dose)• Use factor (U) ↓ (½ Use factor ⇒ ½ dose)• Wall thickness (t) ↑ (1 TVL more ⇒ 1/10×dose)

pTVLtp

dWTU −102

Primary photon dose behind shielding:

Wp: Primary photon workloadd: Distance from primary photon source)

(Radiotherapy: distance from target)


Shielding: Leakage photons

leakTVLtp

dWTL −102

0

Leak photon dose behind shielding (large angle bremsstrahlung):

Leakage photons are assumed to be emitted isotropically with the same TVL in all directions: U=1

WL: Leak photon workloadL0: Fraction of leakage photonsd: Distance from leak photon source.

Application: Leak radiation from a medical electron accelerator:WL=0.001*Wd: distance from the isocenter.

The energy of leakage photons is lower than that of the primary beam (lsmaller TVL).


Shielding: Photons scattered on a surface

2w

p

dWUT ⋅⋅

Primary dose at surface:

Wp: Primary photon workloaddw: Distance from primary photon source to the surface.

Properties of scattered dose (far from surface):

Scattered dose ∝ Primary dose at surface ⋅ A/dr2

dr: Distance from scattering surface to point of interestA: Area of primary field on surface (m2)

Dose of scattered photons:

22rw

p

dA

dUWT α

Constant of proportionality: α = reflection coefficient A dr


Shielding: Photons scattered on a surface

Application: Photons scattered in the patient

2sec

2400

dFa

dWTU

sca

pWp: Primary photon workloaddsca: Distance from primary photon source to the patient.dsec: Distance from the patient to the point of interesta: Scatter fraction (depend on gantry angle)F: Field size at the patient (cm2)d: Distance from secondary photon source.

The energy of scattered photons are lower than those of the primary beam (lower TVL).

Dose of two photon scatterings:

22,

222

1,

112

rrw

p

dA

dA

dUWT αα

A1A2

dr,1

dr,2


Shielding: Neutrons

102d

TW neutronTVLtN

−

Concrete, Ledite:Neutron dose behind shielding can be ignored (neutron TVL< photon TVL)

Neutrons are assumed to be emitted isotropically with the same TVL in all directions: U=1 (not valid for proton energies above 30 MeV)

Neutron dose behind shielding:

d: Distance from neutron source


Shielding: Access to acceleratorDirect access:

• Entrance through secondary barrier.• Short distance from control room to accelerator.• Very thick, heavy door, and complex needed.• Malfunction of door: problem!

Direct access doors:


Maze: Access to accelerator

Maze:• Entrance through long corridor with one or more turns.• Purpose: Reduction of entrance door thickness.• Long distance from control room to accelerator.• Requires more space.

• Malfunction of door: Small problem.

• Widely used.


Maze: Scatter of primary photons in patientDose at the maze door from scatter of primary photons in the patient:

A2

d1

d2

∏=i

ii

sca

pp

id

AFad

WTUD 22 400 α

Wp: Primary photon workloada: Scatter fraction (depend on gantry angle)F: Field size at the patient (cm2)αi: Reflection coefficient for i'th scatteringAi: Area of i'th scattering surfacedi: Distance of i'th scattering leg

dsca


Maze: Scatter of primary photons

A1

A2d1

d2

∏=i i

ii

w

pw d

AdUWT

D 22α

Wp: Primary photon workloaddw: Distance from primary photo source to wallαi: Reflection coefficient for i'th scatteringAi: Area of i'th scattering surfacedi: Distance of i'th scattering leg

Dose at the maze door from scatter of primary photons at the walls:


Maze: Leakage photonsDose at the maze door from leak photons :

∏=i i

iipL d

AdWTL

D 220 α

Wp: Primary photon workloadL0: Fraction of leakage photonsd: Distance from leak photon sourceαi: Reflection coefficient for i'th scatteringAi: Area of i'th scattering surfacedi: Distance of i'th scattering leg

d1

d

A1


Maze: Transmission through maze wallDose at the maze door from head leakage through the maze wall:

20

t

pT d

BWLTD =

Wp: Primary photon workloadL0: Fraction of leakage photonsB: Transmission through the maze walldt: Distance from leak photon source to door

The transmission of photons scattered on the patient is ignored


Maze: Gamma capture photons

Photon spectrum:• Concrete: 0-8 MeV (average: 3.6 MeV)• Boron: 0.478 MeV

Boron in the maze decreases the capture gamma dose

Dose at maze door Neutron capture gamma radiation:

⇒


Maze: Scattered neutronsDose at maze door from scattered neutrons:

m 5

12

1

210 drNn S

AdWD −=

Reduction caused by bend + extra maze length

WN: Neutron workload Ar: Cross-sectional area of inner maze entrance (m2)S1: Cross-sectional area of maze (m2)

m 5m 5

12

1

32 103110 ddrN

n SA

dWD −−=

Maze without bend:

Maze with bend:


Maze: Total dose at maze door

Total dose at maze door for electron accelerators below 10 MeV:

MeV 10 ∑∑∑∑ +++=<

GT

GL

Gw

Gpd DDfDDD

Sum over all gantry angles GGantry rotation axis perpendicular to mazef: Patient transmission factor

If Dd>1 mSv/year a maze door with shielding is needed

Total dose at maze door for electron accelerators above 10 MeV:

Ncdd DDDD ++= <> MeV 10MeV 10

Dc: Capture gamma dose at maze door.Dn: Scattered neutron dose at maze door.


Shielding: Maze

Methods to reduce the dose at the maze entrance:• Long maze.• Long distance from accelerator to maze. • Narrow maze.• Many turns of maze:

– neutrons must scatter many times.• Boron in maze walls:

– Efficient capture of neutrons.– Soft gamma radiation from neutron capture.

• Thick maze door.


Shielding: Safety installations

Emergency stop button in the accelerator room:Press the emergency stop button if the accelerator start running while you are in the accelerator room.

”Last man out” button in the accelerator hall:Confirm you are the last person that leaves the accelerator room. If the button is not pressed immediately before closing the entrance door, the accelerator should be interlocked.

Example:Shielding of radiotherapy

electron accelerators:Accelerator 7 and 8


Layout

accelerator 8

accelerator 7

accelerator 3

Challenges: Limited footprint for treatment roomsThickness of ceiling shielding: 90 cm

Accelerator room 7 and 8 at Århus University Hospital (build in 2004)


Choice of shielding material

Limited footprint for treatment rooms:• Photons:

– Choose larger density than that of normal concrete.• Neutrons:

– Choose shielding material with large hydrogen content.– If possible also boron.– Steel and lead provides insufficient neutron shielding.

Problems with heavy concrete:• Slow hardening process. • Medium density only 3-4 g/cm3.

Solution: Ledite®


Ledite®

Ledite®:• Commercial product.• Concrete with iron and boron.• Pre-harded blocks.• High density:

– XN-240: 3.84 g/cm3.– XN-288: 4.80 g/cm3.

• Very fast building process (4-6 weeks per room).


Primary shielding

121.92

60.96

Ledite XN-288 wall

thickness(cm)

5.6

6.4

d(m)

0.10

0.10

U6

0.33

0.33

U18

1

1

T

0.4480Acc. 8

0.524110Acc. 3

Dose (mSv/year)

Number of Ledite XN-288 layers

Concrete thickness

(cm)Direction

Accelerator 7 :

Advantage of Ledite:Ledite XN-288: 122 cmNormal concrete: 236 cm


Primary shielding: Ceiling

92.1Total thickness (cm)82.35Normal concrete (floor)

60.964.80Ledite XN-2886.350.98Boron-loaded polyethylene15.2411.35Lead1.597.80Steel

Thickness(cm)

Density (g/cm3)

No shielding of neutrons

3-10 times larger hydrogen content

density than that of concrete (also boron)

Dose behind shielding: 0.27 mSv/year


Secondary shielding

0.51

0.32

0.22

0.08

0.06

Photon dose

(mSv/year)

0.05

0.01

0.01

0.00

0.00

Neutron dose

(mSv/year)

61.0

76.2

76.2

15.24

76.2

Ledite XN-240 wall thickness

(cm)

8.6

7.6

5.1

6.1

4.3

d(cm)

0.08251110Acc. 3

0.0601/160West

0.232810Acc. 8

1

1

T

30

0

θ(°)

0.56

0.33

Total dose(mSv/year)

0Control room

0East

Concrete thickness

(cm)Direction

Accelerator 7 :

Small contribution


Maze

5454Gamma radiation

9463161110Secondary photons through maze wall

178282336539Total for secondary photons

1498

55

52

77

18 MV

120

68

31

6 MV

Accelerator 8

2595Scattered primary photons

18 MV6 MV

26135Scattered leak photons

786

34

Neutrons

198Scattered primary photons in patient

Accelerator 7

Dose without maze door (μSv/week):

⇒ Maximum year dose reached within one week!


Maze

18.43Total0.64Steel4.45Lead

12.75 % boron-loaded polyethylene

0.64Steel

Thickness(cm)Material

Measured dose atmaze door:

Acc. 7: 1.22 mSv/year (measured: 0.48 mSv/year)Acc. 8: 0.74 mSv/year (measured: 0.29 mSv/year)

The maze has no turn⇒ Large dose from scattered x-rays and neutrons.⇒ Thick and heavy door needed:

Example:Shielding of radiotherapy

electron accelerators:Accelerator 4 and 6


Layout of accelerator 4

- Photons can not reach the maze door by a single scattering- Neutron door due to short maze


Accelerator 6: Layout

Photons can not reach the maze door by a single scattering


Literature• NCRP Report No. 144, Radiation Protection for Particle Accelerator

Facilities, 2003• NCRP Report No. 151, Structural Shielding Design and Evaluation for

Megavoltage X- and Gamma-Ray Radiotherapy Facilities, 2005.• Radiological Safety Aspects of the Operation of Electron Linear

Accelerators, Technical Reports Series No. 188, 1979.• Radiological Safety Aspects of the Operation of Proton Accelerators,

Technical Reports Series No. 283, 1988.• P. H. McGinley, Shielding techniques for radiation oncology facilities,

Medical Physics Publishing, 2002.• RADIATION PROTECTION AT LOW ENERGY PROTON

ACCELERATORS, Radiation Protection Dosimetry, Vol. 96, No 4, pp. 297–309 (2001).

• RADIATION PROTECTION AT HIGH ENERGY ELECTRON ACCELERATORS, Radiation Protection Dosimetry, Vol. 96, No 4, pp. 333–343 (2001)


Literature

• SHIELDING HIGH ENERGY ACCELERATORS, Radiation Protection Dosimetry, Vol. 96, No. 4, pp. 359–371 (2001).

• SPECIAL RADIATION PROTECTION ASPECTS OF MEDICAL ACCELERATORS, Radiation Protection Dosimetry, Vol. 96, No. 4, pp. 381–392 (2001).

• RADIATION PROTECTION AT MEDICAL, ACCELERATORS, Radiation Protection Dosimetry, Vol. 96, No 4, pp. 393–406 (2001).

• CALCULATIONS OF NEUTRON SHIELDING DATA FOR 10–100 MeV PROTON ACCELERATORS, Radiation Protection Dosimetry (2005), Vol.116, No. 1–4, pp. 245–251

• EVALUATION OF THE RADIOACTIVITY OF THE PRE-DOMINANT GAMMA EMITTERS IN COMPONENTS USED AT HIGH-ENERGY PROTON ACCELERATOR FACILITIES, Radiation Protection Dosimetry (2007), Vol. 123, No. 4, pp. 417–425

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