Physics 300: Week 3

Radiation Protection

What is exposure (X)?

• Total electric charge produced per mass of air

• X = Q/m

• SI unit: C/kg

• Only applies to photons in air

What is KERMA?

• Kinetic Energy Released per unit Mass

• Energy transferred from indirectly ionising radiation to charged particles

• Unit: Gy (J/kg)

• In diagnostic radiology, KERMA ≈ absorbed dose

What is absorbed dose?

• Energy deposited by radiation per unit mass

• D = E/m

• Unit: Gy

• 1 Gy = 1 J/kg

• Applies to any radiation and material

What is equivalent dose?

• Accounts for the type of radiation

• H = D × wR

• Unit: Sv

• X-rays/gamma/electrons: wR = 1

• Alpha: wR = 20

What is effective dose?

• Accounts for the sensitivity of different tissues

• E = H × wT

• Unit: Sv

• Mainly used for radiation protection and comparing overall risk

What do Gy and Sv measure?

• Gy: absorbed dose → energy deposited

• Sv: biological/radiation protection effect

What are the occupational dose limits?

• Effective dose: 20 mSv/year averaged over 5 years

• Maximum in one year: 50 mSv

• Eye lens: 20 mSv/year averaged over 5 years

• Skin: 500 mSv/year

• Hands/feet: 500 mSv/year

What is the public effective dose limit?

1 mSv/year

Do dose limits apply to patients?

• No

• Patient exposure is controlled through justification and optimisation

What are the three principles of radiation protection?

• Justification – benefit must outweigh radiation risk

• Optimisation – keep exposure as low as reasonably achievable

• Dose limitation – prevent unacceptable exposure to workers/public

What are the three fundamental methods of protection?

• Time – reduce time near source

• Distance – increase distance

• Shielding – use appropriate shielding material

How does distance affect radiation dose?

• Dose rate ∝ 1/d²

• Increasing distance greatly reduces dose

What shielding is used for X-rays and gamma rays?

• High-density, high-Z materials

• Lead and concrete

What shielding is used for beta radiation?

• Low-Z materials such as Perspex/acrylic

• High-Z materials can produce Bremsstrahlung X-rays

What shielding is used for neutrons?

• Hydrogen-rich materials

• Examples: concrete, water, paraffin wax

What are the main sources of radiation requiring shielding in an X-ray room?

• Primary radiation – from focal spot

• Scatter – from patient

• Leakage radiation – from X-ray tube housing

What is a primary barrier?

Attenuates the primary X-ray beam

What is a secondary barrier?

Attenuates scatter and leakage radiation

What are the shielding design goals?

• Controlled area: 0.1 mGy/week

• Uncontrolled area: 0.02 mGy/week

What is workload (W)?

• Amount an X-ray unit is used over a week

• Unit: mA-min/week

• Higher workload → potentially more shielding required

What is use factor (U)?

• Fraction of time the primary beam is directed towards a particular barrier

• Range: 0–1

• Secondary barrier: U = 1

What is occupancy factor (T)?

• Fraction of time an area is occupied by people

• Work areas/offices: 1

• Corridors: 1/5

• Toilets/unattended waiting rooms: 1/20

• Transient outdoor areas: 1/40

What is an HVL?

• Half-value layer/thickness

• Reduces radiation intensity by approximately 50%

• 1 HVL = 50% transmission

• 2 HVLs = 25%

• 3 HVLs = 12.5%

What is the role of the Radiation Safety Officer (RSO)?

• Advises on radiation protection and regulations

• Oversees monitoring/dosimetry and shielding

• Provides radiation safety training

• Investigates incidents and safety concerns

• Helps maintain the Radiation Management Plan

What are the main Australian radiation regulatory bodies?

• ARPANSA – national radiation protection codes/standards

• EPA SA – regulates radiation practices/licensing in South Australia

• Organisations implement local radiation safety procedures

What are the main radiation protection codes?

• RPS C-1 – planned exposure situations

• RPS C-5 – medical exposure

• RPS 14.1 – diagnostic/interventional radiology

• RPS 14.2 – nuclear medicine

• RPS 14.3 – radiotherapy

What should radiation workers do to maintain safety?

• Follow Radiation Management Plan/procedures

• Use equipment and protective devices correctly

• Wear personal dosimeter correctly

• Report faults, concerns and incidents

• Keep exposure optimised and within limits

What is an OSL dosimeter?

• Optically Stimulated Luminescent dosimeter

• Worn at chest/waist height

• Used to monitor occupational radiation exposure

• Replaced/tested every 3 months

What is Y-90 used for?

• Targeted therapy for selected liver tumours

• Y-90 microspheres delivered into the hepatic artery

• Beta radiation gives a high local dose to the tumour

How is Y-90 shielded?

• Primary shielding: low-Z material such as Perspex/acrylic

• Lead can be added outside to reduce Bremsstrahlung

• Composite Perspex + lead shielding can be used for syringes/vials

What is Y-90 Bremsstrahlung imaging?

• Y-90 beta particles interact with tissue and produce Bremsstrahlung

• This can be detected with a gamma camera

• Shows distribution of Y-90 and gives an indication of dose delivered

What should you do for a radioactive spill?

• Attend to contaminated people first

• Alert others and isolate the area

• Contain the spill

• Notify the RSO

• Wear appropriate PPE

• Clean from outside → inside

• Monitor for contamination

• Document everything