David Sutton or Colin Martin But Borrowed from Jerry Williams IAEA/RCA DRL Kampala What is Dose ? David Sutton or Colin Martin But Borrowed from Jerry Williams Kampala

Why Measure Radiation Dose ? There is a detriment associated with ionising radiation. We want to reduce that detriment to a level that is As Low As Reasonably Achievable. Kampala

An amount of radiation ………. What is Dose? An amount of radiation ………. Kampala

What does dose tell us? Quantity related to clinical outcome - effects of radiation Measurable (assessable) quantity Related to equipment performance Kampala

Dose: What can we measure? Energy imparted: temperature rise Ionisation Kampala

Radiation quantities (dose) Physical Biological Kerma Energy transferred per unit mass Unit: J kg-1 or gray (Gy) Absorbed dose Energy absorbed per unit mass Unit: gray (Gy) Equivalent dose Absorbed dose multiplied by radiation weighting factor and summed over all radiation types Unit: sievert (Sv) Effective dose Equivalent dose weighted for tissue sensitivity and summed over all tissues Kampala

KERMA & ABSORBED DOSE Kampala

KERMA Kinetic Energy Released per unit Mass (to Medium; to Matter) Amount of energy transferred from the photon beam to the material (ΔEtr) in a small mass of the material (Δm) Unit joule/ kg (J/kg) gray (Gy) Kampala

Measurement in practice Kampala

Field instrument calibration Traceable to primary standard Energy dependent because not air equivalent Kampala

…..but the body is not composed of air So we measure air kerma …..but the body is not composed of air National Protocol for Patient Dose measurements in Diagnostic Radiology: Entrance surface dose defined as absorbed dose to air ……. Kampala

Dose measurement Ionisation chamber Measures air kerma Uncertainties In presence of chamber and phantom Uncertainties Energy dependence of calibration factor (±4% to +/- 10%) Dependence of temperature & pressure (±3%) Angular dependence (non-isotropic) Influence of measuring device Overall measurement uncertainty: ±5% to ±15% (Not including systematic uncertainty in dosimetry standard) Kampala

Solid state (diode) dosimeter Energy dependence Use of dual detectors with energy compensating filters Lead backed Back scatter rejection New transparent detectors Kampala

Measurement quantities for dose audit Kampala

Measurement quantities for dose audit: Radiography and Fluoroscopy Entrance Surface AK or Dose (ESAK orESD) Air Kerma Includes back scatter Assessment: Calculation from X-ray factors TLD Units: mGy Incident Air Kerma As above,no BSF Air Kerma Area Product (KAP) Calibrated in scatter free geometry Undercouch systems: Calibrated above table top Assessment: KAP meter Calculated from X-ray factors Units: µGy m2 = 10 mGy cm2 =1 cGy cm2 = 0.01 Gy cm2 IAK rate (mGy/min) Cumulative IAK Kampala

ESD, IAK and KAP BSF is about 1.3 BSF data from Aoki and Koyama, 2002, PMB, 47, 1205 Kampala

Measurement quantities for dose audit CT scanning Kampala

CT Dose Index (CTDI) Kampala

CTDI and Dose Length Product (DLP) Kampala

Doses in mammography Kampala

Mean Glandular Dose Average absorbed dose to glandular tissue Assumes uniform distribution of breast tissue K – Entrance air kerma at surface of the breast ( no backscatter) g – conversion factor from entrance air kerma to MGD c – age dependent factor to correct from assumption of 50% glandular/ 50% adipose breast composition s – spectrum correction factor (normalised to Mo/Mo) Kampala

Effective dose Measurement of Risk Kampala

Organ Absorbed Dose Radiation weighting factors wR Equivalent dose Tissue weighting factors wT Effective Dose Kampala

Equivalent dose ICRP 103 factors (2007) Kampala

Effective Dose Remainder: Adrenals; Extrathoracic region; Gall bladder; Heart; Kydneys; Lymphatic nodes; Muscle; Oral mucosa; Pancreas; Prostate; Small intestine; Spleen; Thymus; Uterus/ cervix. ICRP 103 factors (2007) Kampala

Radiation Doses (mSv) Dose limit (worker) Natural bgd per year 10 Natural bgd per year 1 Dose limit (public) 0.1 Min reading TLD dosimeter Return flight New York 0.01 Natural bgd per day One way flight London 0.001 Natural bgd per hour Kampala 0.0001

Weighting factors ICRP 60 – ICRP 103 Annual dose from natural background radiation Average =2.2 mSv Max = 7.4 mSv What if wT for α’s = 10 or 30? Kampala

Effective dose Measure of risk Good for comparison of techniques CT colonoscopy vs Ba Enema Dose from a medical procedure and another source Cannot give risk to individual NOT GOOD FOR DRLS Kampala

Summary Requirements for dosimetric quantities: Related to clinical outcome Measurable Ionisation is easily measured quantity For dose audit 5 to 20% measurement uncertainty acceptable Quantities used for dose audit: ESD, KAP, CTDI, DLP, MGD Effective is not relevant to dose audit Kampala