Radiation Protection for X-Ray Technologists Linacs, radioactive substances and all the rest

1. Radiation Protection for X-Ray Technologists Linacs, radioactive substances and all the rest John Saunderson Consultant Physicist / Radiation Protection Adviser 1

2. Beam energy - kV or MV? mid = 77% mid = 9%

3. Kilovoltage X-ray • good for imaging • good for radiotherapy near skin surface • Megavoltage X-ray • deep therapy • imaging poor, but possible for verification

4. kV + - Electron production in the X-ray tube Applied voltage chosen to give correct velocity to the electrons mA Filament (heats up on prep.)‏ Target Voltage to arc 1 m air at STP = 3.4 MV

5. How to get megavoltage energy photons? • Normal X-ray tube? - insulation needed too thick, arcing, etc. • Van de Graaff generator 2-MeV Van de Graaff accelerator

6. How to get megavoltage energy photons? • Normal X-ray tube? - insulation needed too thick, arcing, etc. • Van de Graaff generator • Radioactive sources - Co-60 (1.2 MeV gamma rays)

7. Linear Accelerator (linac) electrons microwaves Klystron or magnetron

9. CL6

10. Typical dose rates • Fluoroscopy • entrance dose rate < 50 mGy/min • > 40 minutes to erythema threshold • > 3 min to annual hand dose limit • Linac • entrance dose rate > 2.4 Gy/min • < 1 minute to erythema threshold • < 4 seconds to annual hand dose limit

11. MV – electrons / X-ray photons

13. MV – electrons / X-ray photons

15. Hull & East Yorks. CL1 & CL2

17. Tenth Value Layers / mm i.e. • @ 80 kVp, 1 mm lead  90 mm concrete • @ 15 MVx, 1 mm lead  8 mm concrete • (lead 5 x density of concrete)

18. Neutron Production • Binding energy (BE) of neutrons 7-20 MeV (mostly)

19. Pb207 B10

20. Neutron Production • Binding energy (BE) of neutrons 7-20 MeV (mostly) • Probability increases with (EX – BE) up to (2 x BE) • Very little neutron production below 10MVx

21. Neutron Shielding • TVL neutrons • ~ 400 cm lead (thermal) • < 31cm concrete for medical linacs • < 10 cm polythene • (TVL for 15MVx is 43.2 cm)

22. Hull & East Yorks. CL1 & CL2

23. Neutron Activation

24. Neutron Activation Products Port 1 m

25. Neutron Activation Products(2)Half lives • Aluminium ≈ 2.5 minutes • Bed end ≈ 9.2 minutes • Linac head ≈ 8.4 minutes • Plasterboard, water, Perspex – no activity detected

27. Why 7.5 Sv/h ? • Originally • 7.5 Sv/h x 8 h/d x 5 d/wk x 50 wk/y = 15mSv • Pre 1/1/2000, 15 mSv was unclassified person effective dose “limit” • Today • Post 2000, 6 mSv is unclassified person effective dose “limit” • 6mS/y / (7.5 Sv/h x 7.5h/d) = 100 d/y

29. Why 75 Sv/h ? • 75 Sv/h x 8 h/d x 5 d/wk x 50 wk/y = 150mSv/y • Eye dose limit = 150 mSv/y • Skin dose limit = 500 mSv/y • Remember • Dose limit not like a speed limit • Optimisation • Keep doses As Low As Reasonably Achievable • Follow local rules.

30. Half-life no more than 81/2 minutes • So in ½ hour, dose rate will be 10 times or more lower

31. Radioactive materials • Iridium-192 • Used for brachytherapy in wire form • 0.1-0.7 MeV beta particles absorbed by platinum coating • 0.2-1.06 MeV gamma rays emitted (effective energy 0.4MeV) • TVL = 12 mm Pb, 185 mm concrete

32. Smoke detector vs Flexitron

33. Smoke detector Americium-241 37 kBq (kilobecquerels) @10 cm • 0.005 mSv/h @ 1 cm • Erythema threshold in 45 years Direct skin contact • 0.2 mSv/h • Erythema threshold in > 1 year • Hand dose limit in 1 month

34. Flexisource 400 GBq (gigabecquerels) • 400 trillion gamma rays per second 44 mGy/hour @ 100 cm • 44 x 1002/12 = 440,000 mGy/h @ 1 cm • 122 mGy/second @ 1 cm • erythema threshold in 16 secs • hand dose limit in 1.2 secs (direct skin contact 1.2 - 72 Gy/s)

35. “Radioactive Patients” Patients may be radioactive if • they’ve been injected with or swallowed radioactive pharmaceuticals • they have solid radioactive sources surgically implanted • they have been involved in an accident with radioactive materials.

36. Radioactive Decay - half life

37. Nuclear Medicine Scan • Patient injected with or swallows a radioactive pharmaceutical • Gamma camera traces where that radiopharmaceutical is concentrated .

39. Thyroid treatments with radio-iodine • Hyperthyroidism / thyrotoxicosis • overactive thyroid - 400 MBq • Thyroid cancer • must destroy all tumour - 3000 MBq • c.f. thyroid scan - 0.2 MBq.

40. Other unsealed source therapies • Phosphorus-32 for polycythemia (too many red blood cells) • Yttrium-90 colloid for arthritic conditions • Strontium-90 for bone metastases.

41. Brachytherapy (radioactive implants) • Intracavity afterloading • Iridium wire afterloading • Iridium pins • Iodine-125 seeds .

42. HDR-microSelectron

43. Iridium Implant

44. Iodine-125 seeds in Prostate

45. New HDR Flexitron • Iridium-192 • Used for brachytherapy in wire form • 0.1-0.7MeV beta particles absorbed by platinum coating • 0.2-1.06MeV gamma rays emitted (effective energy 0.4MeV) • TVL = 12mm Pb, 185mm concrete

46. Non-Ionising Radiations (briefly) e.g. • lasers • ultraviolet • MRI scanners

47. Laser Device Classes & Hazards • Class 1 • Class 1M • Class 2 • Class 2M • Class 3R • Class 3B • Class 4 • Applies to device as a whole.

48. Class 1 • no risk to eyes (including using optical viewing instruments) • no risk to skin • (either low power device or totally encased)

49. Class 1M • no risk to the naked eye • no risk to skin

50. Class 2 • no risk to eyes for short term exposure (including using optical viewing instruments) • no risk to skin • (visible, so blink response protects) • (may cause dazzle or flash blindness)