First FRCR Examination in Clinical Radiology Diagnostic Radiology & Radionuclide Radiology (4b) Patient Dosimetry John Saunderson Radiation Protection Adviser 21/09/2018

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 . 21/09/2018

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) 21/09/2018

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

But we are interested in dose to tissue not to air?

Dose to air vs dose to tissue? i.e. from 10 keV to 100 MeV ratio of absorbed dose to air to absorbed dose to tissue within around +/- 3%

Examples of current measured by an ionisation chamber 1 coulomb of charge in dry air is released by 33.97 joules of absorbed energy Fluoro patient entrance dose of 10 mGy/min 0.4 nanoamps through a 60 cc ionisation chamber 0.04 nA through a 6 cc ionisation chamber i.e. very small currents (hence very delicate instruments, cable and connectors) The bigger the chamber, the more sensitive Detailed working out (not needed for FRCR exam!!) Fluoro patient entrance dose of 10 mGy/min measured by a 60 cc ion chamber 60 c.c. air = 78 microgrammes of air 10 mGy per minute = 10 mJ/kg per min = 0.78 J per 78 g per min 0.78 J/min ÷ 33.97 J/C = 23 nC per minute = 0.4 nA

Ionisation Chambers 21/09/2018

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) 21/09/2018

Geiger-Muller (GM Tubes) Low pressure gas, high voltage (1000 V) When irradiated, some gas atoms are ionised High voltage accelerates electrons andions 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) 21/09/2018

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) 21/09/2018

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) 21/09/2018

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) 21/09/2018

Methods General radiology Fluoroscopy Computed Tomography Nuclear Medicine . 21/09/2018

General radiology ESD Dose-Area Product (DAP) Effective dose . Thermoluminescent dosemeter (TLD) exposure factors Dose-Area Product (DAP) Effective dose . 21/09/2018

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 . 21/09/2018

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) . 21/09/2018

Dose Area Product Because dose falls with 1/d2 and area increases with d2, DAP is independent of distance. 21/09/2018

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) . 21/09/2018

Effective dose Complicated to calculate from ESD or DAP Can use computer models which make assumptions on field size patient size field position 21/09/2018

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Fluoroscopy Dose-Area Product (DAP) Exposure factors Effective dose . 21/09/2018

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. 21/09/2018

Exposure factors Based on assumed FSDs, etc. 21/09/2018

Effective dose Can be “fudged” using radiograph software. 21/09/2018

GAFCHROMIC film 21/09/2018

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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 21/09/2018

CT Dose Index (CTDI) Applies to a single slice Can be used to compare different slice widths different physical filter different scanners etc. 21/09/2018

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. 21/09/2018

Effective dose NRPB program similar to radiography one 21/09/2018

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CT dose/risk calculation e.g. wrong patient referred for CT scan (laryngectomy) 21/09/2018

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Diagnostic Reference Levels Early 80’s survey DRLs today IRMER 21/09/2018

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. 21/09/2018

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 !!. 21/09/2018

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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. 21/09/2018

DRLs today A DRL is essentially a guide to the rather indistinct border between “good and normal practice” and “bad and abnormal practice”. 21/09/2018

IRMER National DRLs Local DRLs set as 3rd quartile average from survey of “standard patients” should be below DRL Local DRLs Usually use average of room averages for Trust 21/09/2018

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National DRLs https://www.gov.uk/government/publications/diagnostic-radiology-national-diagnostic-reference-levels-ndrls/national-diagnostic-reference-levels-ndrls 21/09/2018

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Staff and Environmental Monitoring Devices TLD OSLD Electronic Body Extremity Environment 21/09/2018

Relevant measurement techniques 21/09/2018

Measuring Dose Luxel dose badges TLD finger rings Can be cold sterilized Heat sensitive 21/09/2018

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. 21/09/2018

Electronic Dosemeters Used by Radiation Physics staff to test x-ray unit and measure environmental doses 21/09/2018

f i n 21/09/2018