Radiation Safety. Bethany Gillett 14th Feb After this lecture, you should be able to:

Transcription

1 Radiation Safety Bethany Gillett 14th Feb 2018 Learning Outcomes After this lecture, you should be able to: Understand different radiation protection quantities Explain the difference between radiation dose and radiation risk Describe important factors in radiation protection 1

2 Charged particle interactions Quantifying radiation dose Radiation risks Revision I At diagnostic energies, nuclear interactions are rare Heavy charged particles (e.g. protons) lose only a small fraction of their energy in each collision Light charged particles (e.g. electrons) lose most of their energy in a single collision All forms of ionising radiation eventually result in a distribution of low energy electrons hence these are of central importance in radiation biology Heavy charged particles undergo multiple Coulomb scattering events with negligible deflection 2

3 Revision II Stopping power is the average rate of energy loss in a medium the energy deposition of charged particles differs significantly from that of photons: -de/dx Photons Protons Light charged particles can lose energy via both collisions and radiative mechanisms x Charged particle interactions Quantifying radiation dose Radiation risks 3

4 Kinetic energy released in media (KERMA) measures the overall energy lost by ionising radiation For a monoenergetic beam: For a polyenergetic beam: constant k = 1.6x10-13 Gy kev -1! K = kne µ $ # & " ρ % cm -2 kev med cm 2 g -1 K = k E max E=0 ( ) ρ # µ E & Φ( E) % ( $ ' med EdE photons cm -2 Absorbed dose is the energy gained from ionising radiation per unit mass of material Energy imparted ε = R in R out + Q Sum of all rest mass energies In any nuclear transformations that occur within the volume Radiant energy incident on the volume Sum of all charged and uncharged particle energies, excluding rest mass energies Radiant energy leaving the volume D = dε Absorbed Dose: D T = 1 Mean Absorbed Dose: dm m T Ddm SI Unit of Dose: 1 Gy = 1 J 1 kg 4

5 In radiation protection, we are interested in the damage done by radiation so use equivalent dose 1 Gy of protons or neutrons causes more damage to tissue than 1 Gy of photons or electrons Define equivalent dose: H = Dw R or in a given organ or tissue: H T = D T,R w R 1 Sievert = 1 Gray w R ICRP 2007 Radiation type Energy w R factor Photons All 1 Electrons All 1 Protons > 2MeV 5 Neutrons < 10 kev, > 20 MeV kev, 2 20 MeV kev 2 MeV 20 Atomic nuclei All 20 Effective dose accounts for the different susceptibility of tissue types to radiation (unit still Sv) E = N t=0 w t H t Tissue type w factor (each) Bone marrow, colon, lung, stomach, breast, remainder 0.12 Gonads 0.08 Bladder, liver, thyroid, oesophagus 0.04 Bone surface, brain, salivary glands, skin

6 How much is a microsievert (μsv)? Average dose received from living in the UK 6 μsv per day Annual dose from radioactive fallout in the UK 10 μsv Return flight to Spain 20 μsv Chest X-ray 20 μsv Annual dose to medical physicist 100 μsv CT scan 5,000 μsv The measurement of radiation dose is dosimetry Incident particle Anode Ion Current Air volume ~ 6 cm 3 Cathode DC Voltage Source + - 6

7 Fluence of photons is not proportional to absorbed dose unless electron equilibrium exists Charged particle equilibrium (CPE) requires that the number of charged particles entering the measurement volume is equal to those leaving it Conditions: Separation of boundaries of volume must be at least the range of any secondary charged particle Atomic composition of medium is homogeneous Density of medium homogeneous Uniform field of x-rays passing through the medium (negligible attenuation) No inhomogeneous electric or magnetic fields are present Usually met in modern exposure chambers for x-ray beams in diagnostic radiology The traditional unit of exposure is the Roentgen The Roentgen is defined only in air, under conditions of electron equilibrium: 1 R = 2.58 x 10-4 C kg -1 Exposure: 1 Roentgen 2.08 x 10 9 ionizations 2.58 x 10-4 C kg -1 1cm 3 air = g (at STP) STP: the mass of air in an ionisation chamber is ~ 7.8 mg A 1 R exposure will therefore liberate 2.0 x 10-9 C inside the chamber, corresponding to 1.2x10 10 ions 7

8 Measured X-ray exposure in air can be directly related to absorbed dose in a medium under CPE Empirically, ev is needed to produce an ion pair in air: D air = C/kg 33.97J/C X = J/kg X = 8.76mGy X If x-rays are incident upon another medium: µ en µ ρ D med = D air ( en )med ρ = µ en µ en ρ ρ ( )air ( )med ( )air X F factor = fx Sprawls, Radiation Quantities and Units How are radiation protection quantities related? Operational Quantities Equivalent dose (ambient, directional, personal) Physical Quantities Fluence Kerma Absorbed dose Stopping power / LET Compared by measurement and calculation Calculated using weighting factors and anthropomorphic phantoms Protection Quantities Organ absorbed dose Organ equivalent dose Effective dose 8

9 Two main mechanisms of DNA damage can arise from radiation exposure Ionising radiation creates ions which breaks the sugar phosphate backbone or hydrogen bonds of the base pairs of the DNA, releasing electrons. Ionising radiation interacts with water in the body to produce free radicals which subsequently interact with DNA. These effects are more common than direct effects. Cell survival curve demonstrates relative biological effectiveness (RBE) Petri dishes containing clonogenic cells exposed to successively higher doses of ionising radiation the surviving fraction can then be calculated by comparing to a control plate. Note the shoulder of the curve this demonstrates ability of cells to repair at low doses The curve demonstrates relative biological effectiveness ratio of doses giving identical biological effect (remember the radiation weighting factor ) 9

10 Charged particle interactions Quantifying radiation dose Radiation risks Step by step process Exposure Ionisation direct action Chemical changes (free radicals) Molecular changes (DNA) Subcellular damage Cell death (tissue reaction) Cellular level Cell transformation (stochastic effect) 10

11 Radiation effects may be classed as stochastic or deterministic Stochastic effects can occur at any dose (random) Effects are governed by chance No threshold dose, to minimise risks, keep doses as low as reasonably practicable (ALARP) Usually low probability Assumption probability increases linearly with dose Cancer and heritable effects This is the model we accept but there is debate (hormesis!) Genetic effects and cancer are stochastic effects Descendants of survivors of atom bomb and radiotherapy patients haven t shown genetic effects However, this is not proof the risks aren t there: Uncertainty Diverse nature of severe hereditary disease High natural prevalence of severe physical and mental genetically related handicap Risk of hereditary ill health in subsequent children and future generations estimated to be 1 in 500,000 for 1 mgy exposure to gonads. 11

12 Risk of developing cancer Overall risk of developing cancer is 4.1% per Sv for adult workers E.g. for an 8 msv CT scan, the increased risk of cancer induction is 1 in 3000 Natural incidence of cancer 1 in 3 5.5% per Sv for whole population Difficult to quote risk of developing fatal cancer treatments are improving all the time Radiation effects may be classed as stochastic or deterministic Tissue reactions (also known as deterministic effects) Well defined threshold at which the effect will occur As dose increases, severity of effect increases Non-linear relationship Most tissue reactions have repair mechanisms and the rate the dose is delivered influences the threshold dose Effects not seen below 100 msv E.g. skin reaction 12

13 Deterministic effects are considered to occur above certain acute dose thresholds Tissue Acute dose (Gy) Effect Latency Skin 2 6 Lens of eye Reddening Hair loss Detectable lesions Cataracts 1 day 10 days Years Months Ovary 2.5 Reduced fertility Few days Testis 0.15 Temporary sterility Months Bone marrow 0.5 Reduced white cells Few days Observed changes in cells depends on cell turnover time: Rapidly dividing cells, damage can be seen within a few hours Slowly dividing cells, effects observed in months or even years. Deterministic effects from diagnostic procedures 13

14 Radiation risk is assessed from long term studies Evaluated from Hiroshima, Nagasaki, Chernobyl survivors (human epidemiology), occupational exposures (nuclear industry), radiation therapy patients, mouse models Approximate risk 5% per Sv There are many problems with assessing risk, including: High natural incidence of cancer External influences (lifestyle, diet) Japanese data higher doserates/doses than encountered occupationally There are numerous sources of radiation exposure Fallout from atomic weapons 0.2% (6 μsv) Air travel, luminous watches, etc 1% (30 μsv) Occupational exposure 0.2% (6 μsv) Releases from nuclear industry 0.1% (0.1 μsv) Medical irradiation 15% (410 μsv) Natural Background 83% (2230 μsv) Total annual dose = 2700 μsv 14

15 Everyday risks are high compared to radiation risk Flu 1 in 5000 Road accident 1 in 10,000 Accident at home 1 in 25,000 Hit by lightning 1 in 10 7 Radiation risks in pregnancy Embryo consists of rapidly dividing cells, we know these are most sensitive to radiation. Tissue reactions: Principal tissue reactions in a foetus exposed to ionising radiation are death, malformation, growth retardation and abnormal brain development. Effects are unlikely to occur below 100 mgy. No risks for occupational/diagnostic exposures. Stochastic effects Thought to be independent of stage of pregnancy after the first three to four weeks. A foetal dose of 25 mgy was found to double the natural incidence rate of childhood cancer (~1 in 500). Lack of evidence on lifetime cancer risks. 15

16 Doses in diagnostic radiology are relatively low; some procedures require higher dose to improve SNR Exam Effective Dose (msv) Additional cancer risk Conventional X-ray Chest in 10 6 Mammogram 0.7 Dental CT Head 2.0 Interventional 0.05 (ave) Chest in 4000 Abdomen 10.0 Angioplasty (heart) in 600 Occupational doses annual limits in msv Whole body Skin Extremities Lens of eye Employees Trainees Any other person

17 High doses lead to death due to organ failure Whole body dose (Gy) Organ or tissue failure Time at which death occurs after exposure (days) < 10 Bone marrow Intestine and lungs >15 Nervous system 1-5 >100 Nervous system Within a few hours The LD 50 for humans is about 3 Gy A real life example - Chernobyl 203 people in whom radiation sickness confirmed Radiation Dose (Gy) No. patients Deaths within 100 days

18 A real life example - Litvinenko Poisoned with polonium-210 Deterministic effect Alpha emitter couldn t be detected outside the body Deposits mainly in soft tissue Particularly liver, spleen and bone marrow Also to kidneys and skin, particularly hair follicles Radiation protection aims to limit both deterministic and stochastic effects Justification Optimization: As Low As Reasonably Achievable (ALARA) Limits (E<20mSv / year) 18

19 All doses to ionising radiation have to be justified Because of the risks of ionising radiation any exposures must be justified. Benefits must outweigh the risks. This includes diagnostic, therapeutic and occupationally. Research is complicated! All doses should also be optimised Quality assurance X-ray equipment. Ensure patient gets the smallest dose for the intended clinical outcome Staff Time (dose directly proportional to time) Distance (1/r 2 ) Shielding (lead PPE) 19

20 Dose limitation Dose limits for staff and members of the public (not for patients) Personal monitoring Limitation means that doses must be kept below specified legal levels : Dose Limits (staff and public not patients) Limits represent a final restriction to keep doses to a reasonable level - not sufficient in itself Should always aim to keep doses As Low As Reasonably Practicable (the ALARP principle) Thermoluminescent dosemeters are used for personal monitoring Electronic band structure of TL materials allow radiation energy to be trapped in trapping centres provided by impurities Controlled heating allows trapped electrons to release stored energy as light as temperature increases deeper traps are depopulated Light output is measured by PM tube and a plot with time give a glow curve TL materials can have several glow curve peaks due to the various depths of trapping centre LiF:Mg:Ti is a popular TL material for dosimetry because: It is nearly tissue equivalent the light emission is 400nm matching the peak response of common PM tubes Main glow peak is at 200 C, high enough to minimize fading but low enough to stop infrared emissions Glow curve is shaped to enable easy separation of low temperature peaks Not adversely affected by ambient conditions except UV 20

21 1/17/18 Personal Monitoring TLDs report result in msv Hp07 skin equivalent dose Hp3 eye equivalent dose Hp10 whole body effective dose Staff wear monitors for whole body (waist), eye, finger rings, leg monitors, collar monitors... Patient dose calculations Measurement of effective dose can never be made directly, instead measure entrance surface dose and estimate using Monte-Carlo simulations Dose Area Product (general X-ray) Dose Length Product (CT) 21

22 Room shielding To ensure doses to members of the public remain within limits we ensure X-ray rooms adequately shielded. Public dose limit 1 msv per year we work to dose constraint of 0.3 msv per year Lead shielding for X-ray rooms, concrete for linac bunkers. Radioactive Materials Not only external exposure risk, also internal exposure Need to avoid contamination because this would increase likelihood of ingestion If spill onto skin need to wash immediately can get significant skin doses Contamination monitoring important 22

23 Good working practice Reduce exposure: Time (directly proportional) Distance (1/r 2 ) Shielding (exponential attenuation typically with X-rays we shield ourselves, with radioactive materials we shield the source) Reduce the risk of contamination work over drip trays with absorbent material) Wear PPE Always keep below dose limits We work well below these limits in the health sector, annual effective doses <1 msv. Staff group with highest occupational exposures airline workers. Charged particle interactions Quantifying radiation dose Radiation risks 23