First FRCR Examination in Clinical Radiology Diagnostic Radiology & Radionuclide Radiology (4b) Patient Dosimetry John Saunderson Radiation Protection Adviser 18/04/2019

RCR Syllabus Methods Diagnostic reference levels (including high dose techniques) Magnitude and measurements Radiation detectors and dose meters Measurement of absorbed dose and dose rate in air . 18/04/2019

Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air Ionisation of air detectors Ionisation chambers Geiger-Muller tubes Proportional counters Others Scintillation detectors Solid state (e.g. diodes) Stimulated luminescence (TLD / OSLD) 18/04/2019

Ionisation Chambers Conducting anode (+) and Cathode (-) Typically between 100 and 400 V between When irradiated, some air atoms are ionised Positive ions attracted to cathode, negative to anode A current flows and can be measured Current  absorbed dose to air rate 1 coulomb of charge in dry air is released by 33.97 joules of absorbed energy (no need to learn this!) joules ÷ kg of air in chamber = grays absorbed dose to air

Ionisation Chambers 18/04/2019

Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air Ionisation of air detectors  Ionisation chambers  Geiger-Muller tubes Proportional counters Others Scintillation detectors Solid state (e.g. diodes) Stimulated luminescence (TLD / OSLD) 18/04/2019

Geiger-Muller (GM Tubes) Low pressure gas, high voltage (1000 V) When irradiated, some gas atoms are ionised High voltage accelerates electrons and ions towards electrodes, giving them more energy These ionise more gas atoms causing a cascade A pulse is produced, which is not dependant on the energy of the radiation in

GM Tube Energy response i.e. “grays per click”

GM Tube uses Very sensitive to small amounts of radioactive material, so good contamination monitor Thin window can detect alpha particles If energy compensated, sensitive dose rate meter, for a particular range. Can be very small and still sensitive, so good for pocket dosemeters.

Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air Ionisation of air detectors  Ionisation chambers  Geiger-Muller tubes  Proportional counters Others Scintillation detectors Solid state (e.g. diodes) Stimulated luminescence (TLD / OSLD) 18/04/2019

Proportional counter Half way between an ionisation chamber and a GM-tube The size of the “click” is proportional to the energy of the photon

Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air Ionisation of air detectors  Ionisation chambers  Geiger-Muller tubes  Proportional counters  Others Scintillation detectors Solid state (e.g. diodes) Stimulated luminescence (TLD / OSLD) 18/04/2019

Scintillation Detector Good contamination monitor Pulse height gives energy of X-ray photon Can also be used for dose rate meter

Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air Ionisation of air detectors  Ionisation chambers  Geiger-Muller tubes  Proportional counters  Others Scintillation detectors  Solid state (e.g. diodes) Stimulated luminescence (TLD / OSLD) 18/04/2019

Silicon diode detector Act like ionisation chamber Silicon much more dense than air, so can be a lot smaller than air ionisation chamber Used for testing X-ray sets Need to correct for energy to get tissue dose.

Radiation detectors and dose meters, measurement of absorbed dose and dose rate in air Ionisation of air detectors  Ionisation chambers  Geiger-Muller tubes  Proportional counters  Others Scintillation detectors  Solid state (e.g. diodes)  Stimulated luminescence (TLD / OSLD) 18/04/2019

Methods General radiology Fluoroscopy Computed Tomography Nuclear Medicine . 18/04/2019

General radiology ESD Dose-Area Product (DAP) Effective dose . Thermoluminescent dosemeter (TLD) exposure factors Dose-Area Product (DAP) Effective dose . 18/04/2019

T.L.D. Crystals, e.g. lithium fluoride Radiation causes electrons to be caught in “traps” At lab. TLDs heated to 240oC Electrons released, light emitted Amount of light emitted proportional to dose . 18/04/2019

T.L.D.s +/- Small Tissue equivalent Read 1 Gy Easy for radiographer No direct readout Sensitive to heat, UV, dirt Tricky to calibrate Easy to loose Special ones needed for low dose (e.g. chest) . 18/04/2019

Dose Area Product Because dose falls with 1/d2 and area increases with d2, DAP is independent of distance. 18/04/2019

DAP +/- No fiddly TLD for radiographers! Only one number to record Instant answer Doesn’t take into account backscatter Initial cost (several thousand £) Units sometimes cause confusion (cGy.cm2, or Gy.cm2, Gy.m2) . 18/04/2019

Effective dose Complicated to calculate from ESD or DAP Can use computer models which make assumptions on field size patient size field position 18/04/2019

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Fluoroscopy Dose-Area Product (DAP) Exposure factors Effective dose . 18/04/2019

DAP +/- for fluoroscopy Instant answer, etc. DAP moves with the tube Gives good indication of relative risks of inducing cancer Not directly linked to erythema risk. 18/04/2019

Exposure factors Based on assumed FSDs, etc. 18/04/2019

Effective dose Can be “fudged” using radiograph software. 18/04/2019

GAFCHROMIC film 18/04/2019

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GAFCHROMIC film optical density is proportional to the absorbed dose 0.01 Gy to 50 Gy Energy independent from 30 keV to 30 MeV £20 per 14” x 17” sheet

Computed Tomography CT Dose Index (CTDI) Dose-length Product (DLP) Effective dose 18/04/2019

CT Dose Index (CTDI) Applies to a single slice Can be used to compare different slice widths different physical filter different scanners etc. 18/04/2019

Dose-Length Product (DLP) DLP = CTDI x n x T Gives an idea of relative dose for a whole scan Can be used to compare effect of pitch, etc. 18/04/2019

Effective dose NRPB program similar to radiography one 18/04/2019

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Here on 22/11/2018 18/04/2019

Diagnostic Reference Levels Early 80’s survey DRLs today IRMER 18/04/2019

Mid-80’s survey Method Survey of twenty UK hospitals 13 most common views For each 10-20 patients (60-80kg) at DAP or ESD by TLD measured. 18/04/2019

Mid-80’s survey Results E.g. Chest PA Median ESD = 0.18 mGy Minimum ESD = 0.03 mGy Maximum ESD = 1.43 mGy Max / min = 48 !!. 18/04/2019

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Mid-80’s survey Recommendation Use 75th percentile as reference value i.e. carry out local surveys and take action if average dose is greater than ¾ of national survey doses e.g. for chest PA reference = 0.3mGy ESD Send results to NRPB to review national reference doses every 5 years. 18/04/2019

DRLs today A DRL is essentially a guide to the rather indistinct border between “good and normal practice” and “bad and abnormal practice”. 18/04/2019

IRMER 2017 r.2 “diagnostic reference levels” means dose levels in medical radiodiagnostic or interventional radiology practices, or, in the case of radio-pharmaceuticals, levels of activity, for typical examinations for groups of standard-sized individuals or standard phantoms for broadly defined types of equipment; 18/04/2019

IRMER & DRLs The employer must Operators must Regularly review DRLs Make available to operators Have regard to EU and UK national DRLs Review whenever DRLs are consistently exceeded and take action where appropriate. Medical Physics Expert must contribute Collect dose estimates and provide to PHE on request Operators must adhere to employers DRLs (This does not apply to individual patients)

National DRLs https://www.gov.uk/government/publications/diagnostic-radiology-national-diagnostic-reference-levels-ndrls/ndrl 18/04/2019

https://www.gov.uk/government/publications/diagnostic-radiology-national-diagnostic-reference-levels-ndrls/ndrl 18/04/2019

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Staff and Environmental Monitoring Devices TLD OSLD Electronic Body Extremity Environment 18/04/2019

IRR2017 schedule 3 Dose Limits Type of limit Employee & trainees > 18 Trainee < 18 Others Effective dose 20 mSv/y a 6 mSv/y 1 mSv/y f Equivalent dose Lens of the eye 20 mSv/y b 15 mSv/y Skin c 500 mSv/y 150 mSv/y 50 mSv/y Extremities d a /y = in a calendar year, i.e. 1st January to 31st December b With HSE permission up to 50 mSv/y if total in 5 years < 100 mSv (i.e. average <20mSv/y) c averaged over 1 cm2 d “extremities” means a person’s hands, forearms, feet and ankles e overaged over 5 years, as long as < 50 mSv in every year f if resulting from medical exposure of another, total in 5 years must be < 5mSv This regulation does not apply to persons undergoing medical exposures

Monitoring Effective Dose External dose TLD OSLD EPD - Electronic personal dosimeter For classified workers, must come from an HSE Approved Dosimetry Service (ADS) For fluoroscopy, if worn on trunk under lead apron HSE allow us to assume effective dose  badge dose Internal radiation – e.g. measurement of tritium in urine

Luxel badges Wear underneath lead rubber apron Assume dose to badge = effective dose Can be worn for 2 weeks to 3 months (usually 1 month) Must be returned promptly. 18/04/2019

Monitoring Skin Dose TLD Electronic Wear position depends on task Finger stalls Electronic Wear position depends on task

Eye dose monitoring TLD

f i n 18/04/2019