Radiation protection for particle accelerators. Lars Hjorth Præstegaard, Ph.D. Technical manager Medical physicist Aarhus University Hospital

Transcription

1 Radiation protection for particle accelerators Lars Hjorth Præstegaard, Ph.D. Technical manager Medical physicist Aarhus University Hospital

2 Health physics

3 Dose quantities Definition: Absorbed dose The 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 factor The radiation weighting factor w R is the biological effectiveness of radiation R relative to photons. Definition: Effective dose The effective dose is given by Equivalent total body dose from photons E where w T is the weighting factor for tissue T and the sum is over all radiation quantities and tissues in the body (unit: Sv). = R, T w T w R D T

4 Dose quantities Neutrons: <10 kev 10 to 100 kev > 0.1 to 2 MeV > 2 to 20 MeV > 20 MeV Type and energy of radiation R Photons, all energies Electrons and muons, all energies Protons, other than recoil protons, > 2 MeV Alpha particles, fission fragments, heavy nuclei Radiation weighting factor w R

5 Dose quantities Tissue or organ Gonads Bone marrow (red) Colon Lung Stomach Bladder Breast Liver Oesophagus Thyroid Skin Bone surface Remainder Whole body total Tissue weighting factor w T

6 Why is radiation dangerous? Ionizing radiation causes damage 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.

7 Why is radiation dangerous? 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 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.

8 Why is radiation dangerous? Effects of acute whole body exposure: Dose (Gy) Effect No observable effect Probable recovery LD 50/30 (half of population death within 30 days) Death within a few weeks Death after hours Curative radiotherapy: Gy within a small region of the body

9 Why is radiation dangerous? Lifetime excess risk of death caused by cancer: Linear fit: 5 % per Sv

10 Dose to the public Contributions to the background radiation: Average background radiation in Denmark: 3 msv/year 3 msv/year ~ 1 cigarette per day Cancer deaths in Denmark due to background radiation: ~750/year

11 Dose limits in the EC Bekendtgørelse nr. 823 af 31. oktober 1997 om dosisgrænser for ioniserende stråling: Dosisgrænserne for dosisovervåget arbejdstagere og enkeltpersoner i befolkningen er henholdsvis 20 msv/år og 1mSv/år.

12 Radiation from a particle accelerator

13 Radiation from an electron accelerator Electron beam + Matter Neutrons: (γ,n) process Neutron Photons: Bremsstrahlung (x-rays) Photon Nucleus Photon β -, β + Induced radioactivity: 1. (γ,n) process 2. Neutron capture neutron nucleus Photon β -, β +

14 Radiation from an electron accelerator Dose equivalent rates per unit beam power: Dose from forward bremsstrahlung dominates at all energies Threshold for neutron production: Most materials: 6-13 MeV However: 9 Be: 1.67 MeV D: 2.23 MeV 16 O: MeV 12 C: MeV Radiation Protection Dosimetry, Vol. 96, No 4, pp (2001)

15 Electron accelerator: Photons Angular distribution of bremsstrahlung (tungsten target):

16 Electron accelerator: Neutron production Neutron yield from a thick target: 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 present in and near the target in medical electron accelerator IAEA Technical report, no. 188, 1979

17 Radiation from a proton accelerator High energy proton beam + Matter Spallation process: Production Neutrons Photons α particles Ions Induced rad.... Neutron production by proton bombardment is the most significant radiation hazard for proton accelerators

18 Radiation from a proton accelerator Thick-target neutron yield for protons/carbon ions: Therapeutic carbon ions on: : Cobber : Carbon Therapeutic protons on: : Iron : Soft tissue Radiation Protection Dosimetry, Vol. 96, No 4, pp (2001)

19 Radiation from a proton accelerator Neutron production in a proton accelerator (forward direction): 1E-14 1E-15 Neutron dose (Sv*m^2 per proton) 1E-16 1E-17 1E-18 1E-19 1E-20 Radiation Protection Dosimetry, Vol. 96, No. 4, pp (2001) Radiation Protection Dosimetry (2005), Vol. 116, No. 1 4, pp Proton energy (MeV) C N Al Fe Cu W

20 Induced radioactivity: Electron accelerators Induced radioactivity is mainly caused by: (γ,n) process Neutron capture Photon Nucleus Neutron Photon β -, β + neutron nucleus Photon β -, β + Largest neutron production at locations with large beam loss Only production of β -, β +, and γ emitters

21 Induced radioactivity: Electron accelerators Induced radioactivity for a medical electron accelerator: Only shortlived nuclides Low level of induced radioactivity + short lifetime Radiation protection: Time and avoid areas of large beam loss

22 Induced radioactivity: Proton accelerators Many processes contribute to induced radioactivity: Spallation process: 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 Highest neutron production at locations with large beam loss

23 Induced radioactivity: Proton accelerators Irradiation of samples 1.2 m below 12 GeV proton beamline for 3 months (specific activity in Bq/g): Radiation Protection Dosimetry (2007), Vol. 123, No. 4, pp

24 Induced radioactivity: Proton accelerators Half life of most important nuclides: Radiation Protection Dosimetry (2007), Vol. 123, No. 4, pp

25 Induced radioactivity: Proton accelerators CERN activation limits for waste (nuclide specific):

26 Induced radioactivity: Proton accelerators Example: Proton therapy The gamma dose 1 m from the patient copper collimator after a single 2 min treatment session: 3.24 msv/h (halflife of 23.4 min)

27 Shielding materials

28 Shielding materials: Photons Definition of TVL (tenth-value layer): Relative photon dose Buildup region Exponential falloff of photon dose after the first few cm in concrete The TVL formalism can be used TVL Concrete depth (cm) Attenuation length: (1/e reduction of dose) = TVL/ln(10)

29 Shielding materials: Photons Primary photons from a medical electron accelerator: Density TVL, 6 MV TVL, 15 MV (g/cm 3 ) (cm) (cm) Boron-loaded polyethylene Earth (dry, packed) 1.5 Normal concrete Ledite XN Heavy concrete Ledite XN Steel Lead Large density provides good shielding

30 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

31 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 MeV photon for Boron). Few resonances in kev region. Efficient neutron shielding: Boron-loaded polyethylene (hydrogen + boron)

32 Shielding materials: Neutrons Neutrons from a medical electron accelerator (15 MV): Density TVL, slow TVL, fast (g/cm 3 ) (cm) (cm) Boron-loaded polyethylene Normal concrete Ledite XN Ledite XN Steel Lead Hydrogen and boron provides a good shielding 2. Concrete: TVL,neutron <TVL,photon

33 Shielding materials: Neutrons Neutron transmission for 70 MeV protons in concrete (nitrogen target): Neutron transmission is exponential after a depth of 40 cm in concrete. The TVL formalism can be used Radiation Protection Dosimetry (2005), Vol. 116, No. 1 4, pp

34 Shielding materials: Neutrons Attenuation length as a function of neutron emission angle: 70 MeV protons 100 MeV protons target materials iron target Concrete TVL 56 cm Concrete TVL 41 cm Neutron spectra are harder in forward directions for Proton energies > 30 MeV Radiation Protection Dosimetry (2005), Vol. 116, No. 1 4, pp

35 Shielding materials: Neutrons Neutron production and attenuation length in concrete for 250 MeV protons: Attenuation length of 110 g/cm 2 : Concrete TVL 108 cm!!! Neutron spectra are harder in forward directions for Proton energies > 30 MeV Radiation Protection Dosimetry, Vol. 96, No. 4, pp (2001)

36 Shielding materials: Neutrons Relative dose of neutrons in the forward direction for protons on a thick iron target: Neutrons dominate the shielding design for proton energies above ~30 MeV (Neutron TVL>photon TVL) Depth in concrete (cm) Radiation Protection Dosimetry (2005), Vol. 116, No. 1 4, pp

37 Shielding of a particle accelerator

38 Radiation from a medical electron acc. Photon sources from a medical accelerator: 1. Primary photons from target (treatment felt) 2. Scattered photons from patient and walls 3. Leakage radiation (leakage photons from the accelerator) secondary photons

39 Shielding: Primary/secondary barriers DK legislation: Dose limit for shielding design: 1mSv/year

40 Shielding: Workload (W) 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) 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

41 Shielding: Workload (W) Workload for a large accelerator: Many sources of radiation which depend on the beam loss pattern Example: Proton therapy 27 sources of radiation.

42 Shielding: Area occupancy factor (T) NCRP 151 recommendation for radiotherapy facilities: 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.

43 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 regardless of the beam direction.

44 Shielding: Primary photons Primary photon dose behind shielding: TU W d 2 p 10 t TVL p W p : Primary photon workload d: Distance from primary photon source) (Radiotherapy: distance from target) 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)

45 Shielding: Leakage photons Leak photon dose behind shielding (large angle bremsstrahlung): TL d W 0 2 p 10 t TVL leak W L : Leak photon workload L 0 : Fraction of leakage photons d: Distance from leak photon source. The energy of leakage photons is lower than that of the primary beam (lsmaller TVL). Leakage photons are assumed to be emitted isotropically with the same TVL in all directions: U=1 Application: Leak radiation from a medical electron accelerator: W L =0.001*W d: distance from the isocenter.

46 Shielding: Photons scattered on a surface Primary dose at surface: T U W d 2 w p W p : Primary photon workload d w : Distance from primary photon source to the surface. Properties of scattered dose (far from surface): Scattered dose Primary dose at surface A/d r 2 d r : Distance from scattering surface to point of interest A: Area of primary field on surface (m 2 ) Constant of proportionality: α = reflection coefficient Dose of scattered photons: A d r T UW d 2 w p αa d 2 r

47 Shielding: Photons scattered on a surface The energy of scattered photons are lower than those of the primary beam (lower TVL). Dose of two photon scatterings: T UW d 2 w p α A d r,1 α A d r,2 A 1 A 2 d r,1 d r,2 Application: Photons scattered in the patient TU W d 2 sca p a F d sec W p : Primary photon workload d sca : Distance from primary photon source to the patient. d sec : Distance from the patient to the point of interest a: Scatter fraction (depend on gantry angle) F: Field size at the patient (cm 2 ) d: Distance from secondary photon source.

48 Shielding: Neutrons Neutron dose behind shielding: TW N 10 d t 2 TVLneutron d: Distance from neutron source Neutrons are assumed to be emitted isotropically with the same TVL in all directions: U=1 (not valid for proton energies above 30 MeV) Concrete, Ledite: Neutron dose behind shielding can be ignored (neutron TVL< photon TVL)

49 Shielding: Access to accelerator Direct 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:

50 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.

51 Maze: Scatter of primary photons in patient Dose at the maze door from scatter of primary photons in the patient: = TU W α A p i i Dp a F 2 dsca i d i W p : Primary photon workload a: Scatter fraction (depend on gantry angle) F: Field size at the patient (cm 2 ) α i : Reflection coefficient for i'th scattering A i : Area of i'th scattering surface d i : Distance of i'th scattering leg d sca d 1 A 2 d 2

52 Maze: Scatter of primary photons Dose at the maze door from scatter of primary photons at the walls: T UWp α = i Ai Dw 2 2 d d w W p : Primary photon workload d w : Distance from primary photo source to wall α i : Reflection coefficient for i'th scattering A i : Area of i'th scattering surface d i : Distance of i'th scattering leg i i d 1 A 2 A 1 d 2

53 Maze: Leakage photons Dose at the maze door from leak photons : TL W 0 p α = i Ai DL 2 2 d i di W p : Primary photon workload L 0 : Fraction of leakage photons d: Distance from leak photon source α i : Reflection coefficient for i'th scattering A i : Area of i'th scattering surface d i : Distance of i'th scattering leg d A 1 d 1

54 Maze: Transmission through maze wall Dose at the maze door from head leakage through the maze wall: D = T T L W 0 p 2 t d B W p : Primary photon workload L 0 : Fraction of leakage photons B: Transmission through the maze wall d t : Distance from leak photon source to door The transmission of photons scattered on the patient is ignored

55 Maze: Gamma capture photons Dose at maze door Neutron capture gamma radiation: Photon spectrum: Concrete: 0-8 MeV (average: 3.6 MeV) Boron: MeV Boron in the maze decreases the capture gamma dose

56 Maze: Scattered neutrons Dose at maze door from scattered neutrons: Maze without bend: D n = W d N 2 1 A S r 1 10 d 2 5 m W N : Neutron workload A r : Cross-sectional area of inner maze entrance (m 2 ) S 1 : Cross-sectional area of maze (m 2 ) Maze with bend: D n = W d N 2 1 A S r d2 5 m d m Reduction caused by bend + extra maze length

57 Maze: Total dose at maze door Total dose at maze door for electron accelerators below 10 MeV: < 10 MeV D = d Dp fdw DL DT G Sum over all gantry angles G Gantry rotation axis perpendicular to maze f: Patient transmission factor G G G Total dose at maze door for electron accelerators above 10 MeV: > 10 MeV < 10 MeV D = D + D + D d d D c : Capture gamma dose at maze door. D n : Scattered neutron dose at maze door. c N If D d >1 msv/year a maze door with shielding is needed

58 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.

59 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.

60 Example: Shielding of radiotherapy electron accelerators: Accelerator 7 and 8

61 Layout accelerator 8 accelerator 7 Accelerator room 7 and 8 at Århus University Hospital (build in 2004) accelerator 3 Challenges: Limited footprint for treatment rooms Thickness of ceiling shielding: 90 cm

62 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/cm 3. Solution: Ledite

63 Ledite Ledite : Commercial product. Concrete with iron and boron. Pre-harded blocks. High density: XN-240: 3.84 g/cm 3. XN-288: 4.80 g/cm 3. Very fast building process (4-6 weeks per room).

64 Primary shielding Accelerator 7 : Direction Concrete thickness (cm) Number of Ledite XN- 288 layers Ledite XN- 288 wall thickness (cm) d (m) U 6 U 18 T Dose (msv/year) Acc Acc Advantage of Ledite: Ledite XN-288: 122 cm Normal concrete: 236 cm

65 Primary shielding: Ceiling Density (g/cm 3 ) Thickness (cm) No shielding of neutrons Steel Lead times larger hydrogen content density than that of concrete (also boron) Boron-loaded polyethylene Ledite XN-288 Normal concrete (floor) Total thickness (cm) Dose behind shielding: 0.27 msv/year

66 Secondary shielding Accelerator 7 : Direction Concrete thickness (cm) Ledite XN- 240 wall thickness (cm) d (cm) T θ ( ) Photon dose (msv/year) Neutron dose (msv/year) Total dose (msv/year) West / Acc Acc East Control room Small contribution

67 Maze Dose without maze door (μsv/week): Accelerator 7 Accelerator 8 6 MV 18 MV 6 MV 18 MV Scattered primary photons Scattered leak photons Scattered primary photons in patient Secondary photons through maze wall Total for secondary photons Gamma radiation Neutrons Maximum year dose reached within one week!

68 Maze The maze has no turn Large dose from scattered x-rays and neutrons. Thick and heavy door needed: Steel Lead Steel Total Material 5 % boron-loaded polyethylene Thickness (cm) Measured dose at maze door: Acc. 7: 1.22 msv/year (measured: 0.48 msv/year) Acc. 8: 0.74 msv/year (measured: 0.29 msv/year)

69 Example: Shielding of radiotherapy electron accelerators: Accelerator 4 and 6

70 Layout of accelerator 4 - Photons can not reach the maze door by a single scattering - Neutron door due to short maze

71 Accelerator 6: Layout Photons can not reach the maze door by a single scattering

72 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, Radiological Safety Aspects of the Operation of Electron Linear Accelerators, Technical Reports Series No. 188, Radiological Safety Aspects of the Operation of Proton Accelerators, Technical Reports Series No. 283, P. H. McGinley, Shielding techniques for radiation oncology facilities, Medical Physics Publishing, RADIATION PROTECTION AT LOW ENERGY PROTON ACCELERATORS, Radiation Protection Dosimetry, Vol. 96, No 4, pp (2001). RADIATION PROTECTION AT HIGH ENERGY ELECTRON ACCELERATORS, Radiation Protection Dosimetry, Vol. 96, No 4, pp (2001)

73 Literature SHIELDING HIGH ENERGY ACCELERATORS, Radiation Protection Dosimetry, Vol. 96, No. 4, pp (2001). SPECIAL RADIATION PROTECTION ASPECTS OF MEDICAL ACCELERATORS, Radiation Protection Dosimetry, Vol. 96, No. 4, pp (2001). RADIATION PROTECTION AT MEDICAL, ACCELERATORS, Radiation Protection Dosimetry, Vol. 96, No 4, pp (2001). CALCULATIONS OF NEUTRON SHIELDING DATA FOR MeV PROTON ACCELERATORS, Radiation Protection Dosimetry (2005), Vol. 116, No. 1 4, pp 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