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Radiation Protection for Assistant Practitioners in Mammography Lecture 2

Radiation Protection for Assistant Practitioners in Mammography Lecture 2. John Saunderson Radiation Protection Adviser (TPRH ext. 6690). “IRMER Syllabus”. Production of X-rays Absorption and scatter Radiation hazards and dosimetry Special attention areas Radiation Protection

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Radiation Protection for Assistant Practitioners in Mammography Lecture 2

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  1. Radiation Protection for Assistant Practitioners in MammographyLecture 2 John Saunderson Radiation Protection Adviser (TPRH ext. 6690)

  2. “IRMER Syllabus” • Production of X-rays • Absorption and scatter • Radiation hazards and dosimetry • Special attention areas • Radiation Protection • Laws & Guidelines • Equipment .

  3. 1.2 Radiation Hazards and Dosimetry • Biological effects of radiation • Risk/benefits of radiation • Dose optimisation • Absorbed dose, dose equivalent, effective dose and their units .

  4. What harm can X-rays do?

  5. Wilhelm Roentgen • Discovered X-rays on 8th November 1895 .

  6. Colles’ fracture 1896 . Frau Roentgen’s hand, 1895

  7. Dr Rome Wagner and assistant

  8. ”First radiograph of the human brain” 1896 In reality a pan of cat intestines photographed by H.A. Falk (1896) .

  9. First Reports of Injury Late 1896 Elihu Thomson - burn from deliberate exposure of finger Edison’s assistant - hair fell out & scalp became inflamed & ulcerated .

  10. Mihran Kassabian (1870-1910)

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  14. Ionising radiation can cause chemical reactions in the body’s cells which may • do no harm • kill the cell • cause the cell to multiply out of control (cancer) • cause the cell to malfunction in some other way.

  15. Where very large doses kill many cells • radiation “burns” • cataract • radiation sickness.

  16. Threshold risks(Deterministic effects)Very large doses onlyThe bigger the dose, the more severe the effect Staff doses never this big Typical skin dose to mammo. patient = 9 mSv

  17. Stochastic Effects • “chance effects” • e.g. • cancer • hereditary disease

  18. Cancer risksIt is assumed that any dose of radiation could potentially cause cancer.The bigger the dose, the more likely the effect will occur, (but it will probably never occur). i.e. a bit like crossing the road - the more times you cross the more likely you are to be run over, but probably never will.

  19. Data Sources for Risk Estimates • North American patients - breast, thyroid, skin • German patients with Ra-224 - bone • Euro. Patients with Thorotrast - liver • Oxford study - in utero induced cancer • Atomic bomb survivors - leukaemia, lung, colon, stomach, remainder .

  20. Stochastic Effects • Caused by cell mutation leading to cancer or hereditary disease • Current theory says, no threshold • The bigger the dose, the more likely effect.

  21. ICRP risk factors 5.0 x 10-5 per mGy  1 in 20,000 chance .

  22. Radiation Quantities and Units • Dose • e.g. skin dose • Dose equivalent • Effective dose

  23. Absorbed Dose (D) • Amount of energy absorbed per kg • Measured in Grays (Gy) • 1 Gy = 1000 mGy (milligray) • 1 mGy = 1000 Gy (microgray) • > 2 Gy to skin causes erythema (“sun burn”)

  24. Typical Values of D • Radiotherapy dose = 40 Gy to tumour (over several weeks) • LD(50/30) = 4 Gy to whole body (single dose) • Annual background dose = 2.5 mGy whole body • Chest PA skin dose = 160 uGy • Mammo skin dose = 9 mGy .

  25. Dose Equivalent (H) • Measured in Sieverts (Sv) • 1 Sv = 1000 mSv (millisievert) • 1 mSv = 1000 Sv (microsievert) • Dose equivalent = absorbed dose x Q • Q depends on type of radiation • For X-rays, Q = 1, so 1 Sv = 1 Gy • Alpha rays are ten times as dangerous as X-rays, so Q = 10, so 10 Sv = 1 Gy

  26. Effective Dose (E) Tissue or organ wT Gonads 0.20 Red bone marrow 0.12 Colon 0.12 Lung 0.12 Stomach 0.12 Bladder 0.05 Breast 0.05 Liver 0.05 Oesphagus 0.05 Thyroid 0.05 Skin 0.01 Bone surfaces 0.01 Remainder 0.05 • Sum of equivalent doses to each tissue/organ x organ weighting factors [E = TwT.HT] • Units are Sieverts (Sv) • Risk of cancer is proportional to effective dose e.g. if breast alone received 2 mGy to tissue, E = 0.05 x 2 = 0.1 mSv.

  27. Typical Values of E • Barium enema = 7 mSv • CT abdomen = 10 mSv • Conventional abdomen = 1 mSv • Chest PA = 20 uSv • Annual dose limit for radiation workers = 20 mSv • Annual background dose = 2.5 mSv.

  28. Old Units • 100 rad = 1 Gy = 100cGy • 100 rem = 1 Sv • 100 R  0.9 Gy

  29. Typical Mammography doses • For a typical single mammogram • Film needs about 7 Gy • Patient’s skin gets about 10 mGy • Breast gets about 1.6 mGy • Effective dose around 50 Sv • Annual staff dose limit is 6 mSv

  30. Cancer risk • For adult worker, average risk of inducing fatal cancer is 4% per Sv • i.e. risk from 0.1 mSv • 0.04 x 0.0001 • 0.000004 • 1 in 250,000 .

  31. ICRP System of Radiological Protection • Justification • no unnecessary exposures • Optimisation • keep doses as low as reasonably achievable (ALARA) • Limitation • dose limits for workers and staff • diagnostic reference levels (DRL) for patients • DRL for mammo. 2 mGy glandular dose.

  32. (Ended lecture 2)

  33. f i n

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