Radiation Protection for Assistant Practitioners in Mammography Lecture 2

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Radiation Protection for Assistant Practitioners
in MammographyLecture 2

• John Saunderson
• Radiation Protection Adviser
• (TPRH ext. 6690)

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IRMER Syllabus

• Production of X-rays
• Absorption and scatter
• Radiation hazards and dosimetry
• Special attention areas
• Radiation Protection
• Laws Guidelines
• Equipment .

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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 .

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What harm can X-rays do?
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Wilhelm Roentgen

• Discovered X-rays on 8th November 1895 .

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Colles fracture 1896 .
Frau Roentgens hand, 1895
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Dr Rome Wagner and assistant
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First radiograph of the human brain 1896
In reality a pan of cat intestines photographed
by H.A. Falk (1896) .
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First Reports of Injury

• Late 1896
• Elihu Thomson - burn from deliberate exposure of
  finger

Edisons assistant - hair fell out scalp became
inflamed ulcerated .
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Mihran Kassabian (1870-1910)
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Ionising radiation can cause chemical reactions
in the bodys 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.

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Where very large doses kill many cells

• radiation burns
• cataract
• radiation sickness.

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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
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Stochastic Effects

• chance effects
• e.g.
• cancer
• hereditary disease

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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.
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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 .

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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.

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ICRP risk factors
5.0 x 10-5 per mGy ? 1 in 20,000 chance .
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Radiation Quantities and Units

• Dose
• e.g. skin dose
• Dose equivalent
• Effective dose

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Absorbed Dose (D)

• Amount of energy absorbed per kg
• Measured in Grays (Gy)
• 1 Gy 1000 mGy (milligray)
• 1 mGy 1000 ?Gy (microgray)
• gt 2 Gy to skin causes erythema (sun burn)

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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 .

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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

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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 Oe
sphagus 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 ?T wT.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.
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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 .

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Old Units

• 100 rad 1 Gy 100cGy
• 100 rem 1 Sv
• 100 R ? 0.9 Gy

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Typical Mammography doses

• For a typical single mammogram
• Film needs about 7 ?Gy
• Patients skin gets about 10 mGy
• Breast gets about 1.6 mGy
• Effective dose around 50 ?Sv
• Annual staff dose limit is 6 mSv

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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 .

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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 .

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(Ended lecture 2)
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