First FRCR Examination in Clinical RadiologyDiagnostic Radiology & Radionuclide Radiology (4b) Patient DosimetryJohn SaundersonRadiation Protection Adviser
RCR Syllabus Methods Diagnostic reference levels (including high dose techniques) Magnitude and measurements .
Methods • General radiology • Fluoroscopy • Computed Tomography • Nuclear Medicine .
General radiology • ESD • Thermoluminescent dosemeter (TLD) • exposure factors • Dose-Area Product (DAP) • Effective dose .
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 .
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) . T.L.D.s +/-
Using exposure factors • kV, mAs, field size, FSD • If D = dose at 100 cm for 80 kV, 100 mAs then • ESD = D • x kV2/802 • x mAs/100 • x 1002/FSD2 • x BSF • BSF = backscatter factor, depends on HVL and field size.
E.g. • What is ESD for 100 kV, 50mAs, 15 x 15 cm field, 75 cm FSD for Room 3? • From annual survey • @ 80 kV, D = 9.1 mGy/100mAs @ 1 m • HVL for 100kV was 3 mmAl • From tables • BSF for 3mmAl, 15 x 15 field = 1.33 • Therefore, entrance surface dose • 9.1mGy x 1002/802 x 50/100 x 1002/752 x 1.33 • 0.17 mGy .
No fiddly TLD for radiographers! No fiddly TLDs for physicist! Radiographers can calculate ESDs No minimum dose No direct readout Exposure parameters must be recorded Assumes tube output not changed from last survey. Using exposure factors +/-
Dose Area Product • Because dose falls with 1/d2 and area increases with d2, DAP is independent of distance.
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). DAP +/-
Effective dose • Complicated to calculate from ESD or DAP • Can use computer models which make assumptions on • field size • patient size • field position
Fluoroscopy • Dose-Area Product (DAP) • Exposure factors • Effective dose .
Instant answer, etc. DAP moves with the tube Gives good indication of relative risks of inducing cancer Not directly linked to erythema risk. DAP +/- for fluoroscopy
Exposure factors • Based on assumed FSDs, etc.
Effective dose • Can be “fudged” using radiograph software.
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-Width Product (DWP) • Effective dose
CT Dose Index (CTDI) • Applies to a single slice • Can be used to compare • different slice widths • different physical filter • different scanners • etc.
Dose-Length Product (DLP) • DWP = CTDI x n x T • Gives an idea of relative dose for a whole scan • Can be used to compare effect of pitch, etc.
Effective dose • NRPB program similar to radiography one
CT dose/risk calculation • e.g. wrong patient referred for CT scan (laryngectomy)
Nuclear Medicine • MIRD • Add up dose to each organ from the dose irradiating from each organ.
Diagnostic Reference Levels • Early 80’s survey • DRLs today • IRMER
Mid-80’s surveyMethod • Survey of twenty UK hospitals • 13 most common views • For each 10-20 patients (60-80kg) at • DAP or ESD by TLD measured.
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 !!.
Mid-80’s surveyRecommendation • 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.
DRLs today • A DRL is essentially a guide to the rather indistinct border between “good and normal practice” and “bad and abnormal practice”.
IRMER • National DRLs • set as 3rd quartile • average from survey of “standard patients” • should be below DRL • Local DRLs • ?Average for Trust? • Most should be below average
Staff and Environmental Monitoring • Devices • TLD • OSLD • Film • Electronic • Body • Extremity • Environment
Measuring Dose • Luxel dose badges • TLD finger rings • Can be cold sterilized • Heat sensitive
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.