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DOSIMETRY FOR MEDICAL APPLICATION OF IONIZING RADIATIONS: Calibration requirements and clinical applications

Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs. Olivera Ciraj-Bjelac , Milojko Kovacevic, Danijela Arandjic, Djordje Lazarevic Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Department

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DOSIMETRY FOR MEDICAL APPLICATION OF IONIZING RADIATIONS: Calibration requirements and clinical applications

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  1. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Olivera Ciraj-Bjelac, Milojko Kovacevic, Danijela Arandjic, Djordje Lazarevic Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Department Laboratory for Radiation Measurements Belgrade, Serbia ociraj@vinca.rs DOSIMETRY FOR MEDICAL APPLICATION OF IONIZING RADIATIONS: Calibration requirements and clinical applications

  2. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Content • Global trends in medical exposures • Dosimetric quantities and units • Dosimetry in diagnostic radiology • Metrology and calibration requirements • Clinical application

  3. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Medical exposure to ionizing radiation • Medical exposure contributes 99% of man-made radiation exposure to humans • The concept of risk is used to quantify possible detrimental effects Total dose from man-made sources of radiation> 0.61 mSv Medical: 0.6 mSv (> 99.97%) Source: United Nations Scientific Committee for Effect of Atomic Radiation (UNSCEAR), 2010

  4. Medical exposure to ionizing radiation Dose? The role of dosimetry is to determine the amount of radiation received by a person from the radiological examination

  5. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Dosimetry in diagnostic radiology • Patient dose assessment • Establishment of Diagnostic Reference Levels (DRL), optimisation of protection • Assessment of x-ray equipment performance • Standards of good practice • Assessment of radiation detriment

  6. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Global trend • 3,6 billion radiological examinations in the period 1997-2007 • Increase of 50% compared to previous decade • Significant increase of CT practice: • Examination frequency • Dose per examination • Interventional procedures

  7. Depends on the examination type Variations for the same type of procedure Dose to patient Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs • 2 mSv • 100CxR 5-20 mSv 400- 1000 CxR 0.02- 0.05 mSv 50 chest radiographies= annual natural background radiation dose

  8. Problems Dose for the same examination type varies up to 2 orders of magnitude Increased utilization of high-dose procedures CT Interventional procedures Increase of probability for stochastic effects, in particular in the case of the repeated examinations Possible radiation injuries in high-dose procedures Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Effects

  9. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs

  10. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Radiation injuries ICRP 85

  11. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Basic metrology elements • International Measurements System (IMS) • Framework for dosimetry in diagnostic radiology • Consistency in radiation dosimetry

  12. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs International Measurements System (IMS) • Bureau International des Poids et Mesures (BIPM) • National Primary Standard Dosimetry Laboratories (PSDL) • Secondary Standards Dosimetry Laboratories (SSDL) • Users performing measurements

  13. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Traceability chain

  14. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Metrology and traceability • Dosimeters used to determine doses received by individuals • Measurements need to be traceable though an unbroken chain of comparisons to national and international standards • Traceability is needed to ensure accuracy and reliability • Legal and economic implications

  15. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Role of the SSDL • The prime function: to provide a service in metrology • Designated by the competent national authorities • SSDL-Secondary standards, calibrated against the primary standards of laboratories participating in the IMS

  16. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Journal of the ICRU Vol 5 No 2 (2005) Report 74

  17. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Dosimetric quantities in units in diagnostic radiology • Basic dosimetric quantity: Air kerma • Easy to measure • Calibration: • Dosimeters calibrated in terms of air kerma • Clinical application: • Quantities derived from air kerma for different imaging modalities

  18. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Dosimetric quantities • Basic dosimetric quantities • Application specific dosimetric quantities • Quantities for risk assessment • Conversion coefficient for tissue and organ dose assessment

  19. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Basic dosimetric quantities • Energy fluence Unit:J/m2 • Kerma Unit:J/kg, Gy • Absorbed dose Unit:J/kg, Gy

  20. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Basic dosimetric quantities • Charged-particle equilibrium • Absence of bremsstrahlung losses

  21. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Application specific dosimetric quantities

  22. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs

  23. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Application specific dosimetric quantities: computed tomography

  24. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Application specific dosimetric quantities: computed tomography

  25. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Quantities describing risk • Organ and tissue dose • Equivalent dose • Effective dose • Dose-conversion coefficients for assessment of organ and tissue doses

  26. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs The Use of Effective Dose (E) • E is a risk-related quantity and should only be used in the low-dose range • Primary use: • to demonstrate compliance with dose limits • in regulation, for prospective planning of radioprotection • Not for: • detailed retrospective dose and risk assessments after exposure of individuals • epidemiological studies, neither in accidents. • In the last cases: organ doses are needed ! ICRP 103, ICRP 105

  27. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Effective Dose in Medical Exposure • The relevant quantity for planning the exposure of patients and risk-benefit assessments is the equivalent dose or the absorbed dose to irradiated tissues. • The assessment and interpretation of E is very problematic when organs and tissues receive only partial exposure or a very heterogeneous exposure (x-ray diagnostics) • E can be of value for comparing doses from • different diagnostic procedures • similar procedures in different hospitals and countries • different technologies for the same medical examination. ICRP 103, ICRP 105

  28. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Dosimeters in diagnostic radiology PSDL/SSDL user

  29. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Dosimetry standards in diagnostic radiology • IEC 61674: Dosimeters with ionization chambers and/or semi-conductor detectors as used in X-ray diagnostic imaging • Diagnostic dosimeter: detector and measuring assembly • IEC 60580: Dose area product meters

  30. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Requirements for dosemeters User SSDL Ionization chamber of reference class • IEC 61674 • Ionization chambers • Semiconductor detectors

  31. Calibrations in diagnostic radiology Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs • Air kerma: • Radiography and mammography • Kerma-length product • Dosimeters in CT • Kerma-area product • Radiography and fluoroscopy • PPV: kVp meters • Frequency: according to national regulations

  32. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Calibration in diagnostic radiology • SSDL with relevant measurement capabilities • General requirements: beam qualities, tube voltage and filtration measurements • Dosimeter of reference class (with electrometer) • Calibrated • Quality control • Traceability for all beam qualities • Auxiliary equipment: electrometers, thermometers, barometers… • Environmental conditions

  33. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Equipment Dosimetry Radiation source X-ray generator, 50-150 kVp, 20-40 kVp Ripple less than 10% for radiography and less than 4% for mammography Beam qualities according IEC 61267 “Shutter” mechanism Filters and attenuators Tube voltage meter (ppv, ±1.5%) • Ionization chambers • Position system • HV supply for monitor and reference class ionization chamber • Electrometer

  34. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Reference class dosemeter

  35. Spectrum X-ray beam quality: First half-value layer (HVL1) Second half-value layer (HVL2) Homogeneity coefficient: Tube voltage Total filtration Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Specification of the x-ray beam

  36. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Radiation beam qualities (IEC 61267)

  37. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Apertures X-ray tube window Focal spot Test point Additional filtration Shutter Monitor chamber Typical calibration set up

  38. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Calibration procedures • Procedures before calibration (acclimatization, positioning, stabilization…) • Calibration procedures (methods, number of measurements, interval between measurements…corrections… • Procedures following calibration (uncertainty budget, certificate…)

  39. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Dosimetry formalism • Air kerma: • Reference conditions: set of influencing quantities • Influencing condition: quantities that are not subject of mesusremst but have an impact on the result • Air density correction: • Beam quality correction:

  40. Calibrations of dosemeters for CT Traditionally, irradiation of the whole volume Contras: Information on chamber response only Size on active volume only assumed Far from real situation Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs

  41. Calibration for CT: air kerma length product Aperture Ionization chamber Monitor chamber Focal spot Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs w da dr Cylindrical chamber, 100 mm Non-uniform irradiation Uniform response RQT 9 (120 kVp, HVL: 8.5 mm Al)

  42. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Ref. chamber Film 10 cm 10 cm Calibration for fluoroscopy: air kerma area product • In laboratory (SSDL) • Fieldcalibration Cekerevac at al, Poster B3

  43. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Calibration in terms of practical peak voltage • X-ray tube voltage measurements • PracticalPeakVoltage (ppv): • Property of the whole exposure cycle • Related to image contrast • Invasive or non-invasive measurements • Voltage divider

  44. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Uncertainty budget • Uncertainty of the reference standard • Uncertainty of user’s instrument • Uncertainty due to calibration set up • Uncertainty of the evaluation procedure

  45. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Uncertainty budget • Air kerma: ± 2.7 % • Air kerma length product: ± 3.0 % • Air kerma area product: ± 15 % • Non-invasive tube voltage measuring devices: 2.5 %

  46. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Quality Management System • Goal: minimal uncertainty • Assurance and control of traceability • Quality manual: technical details, methods, traceability, uncertainty budget, QC, safety…. • Continuous improvements and reviews • External peer review/audit

  47. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Patient dose assessment

  48. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Clinical dosimetry • Direct measurement on patients or phantoms • Indirect measurements on patients or phantoms • Output of the X-ray tube, scaled for exposure and geometry

  49. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs BSF Dosimetric quantites KAP Ke Quantities incident air kerma, entrance surface air kerma and kerma-area product (radiography); kerma-area product and entrance surface air kerma rate (fluoroscopy); incident and entrance surface air kerma (mammography); and kerma-length product (computed tomography)

  50. Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs Patients and phantoms Patient Phantoms Objects that simulate real patients in terms of interaction of radiation with matter Easy to perform Standardized • Real situation

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