Download Quality control and patient dosimetry in diagnostic

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 Laboratory for Radiation Measurements Belgrade, Serbia [email protected]

Content

 Metrology and calibration requirements  Clinical application

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 Global trends in medical exposures  Dosimetric quantities and units  Dosimetry in diagnostic radiology

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

0.005 0.002

0.002

0.005

mSv Medical

Nuclear weapons Occupational Chernobyl 0.61

Atmospheric nuclear tests

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

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

Medical exposure to ionizing radiation

The role of dosimetry is to determine the amount of radiation received by a person from the radiological examination

Dose?

 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

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Dosimetry in diagnostic radiology

Global trend  3,6 billion radiological examinations in the period 1997-2007  Significant increase of CT practice:  Examination frequency  Dose per examination

 Interventional procedures

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 Increase of 50% compared to previous decade

Dose to patient

0.02- 0.05 mSv

2 mSv  100CxR

5-20 mSv 400- 1000 CxR

50 chest radiographies= annual natural background radiation dose

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 Depends on the examination type  Variations for the same type of procedure

 Dose for the same examination type varies up to 2 orders of magnitude  Increased utilization of high-dose procedures  CT  Interventional procedures

Effects  Increase of probability for stochastic effects, in particular in the case of the repeated examinations  Possible radiation injuries in high-dose procedures

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Problems

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Radiation injuries

ICRP 85 10

 International Measurements System (IMS)  Framework for dosimetry in diagnostic radiology  Consistency in radiation dosimetry

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Basic metrology elements

 Bureau International des Poids et Mesures (BIPM)  National Primary Standard Dosimetry Laboratories (PSDL)  Secondary Standards Dosimetry Laboratories (SSDL)  Users performing measurements

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International Measurements System (IMS)

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Traceability chain

 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

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Metrology and traceability

 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

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Role of the SSDL

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Journal of the ICRU Vol 5 No 2 (2005) Report 74

Dosimetric quantities in units in diagnostic radiology  Basic dosimetric quantity: Air kerma  Easy to measure

 Calibration:  Clinical application:  Quantities derived from air kerma for different imaging modalities

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 Dosimeters calibrated in terms of air kerma

Dosimetric quantities

 Quantities for risk assessment  Conversion coefficient for tissue and organ dose assessment

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 Basic dosimetric quantities  Application specific dosimetric quantities

 Energy fluence Unit:J/m2

dR  da

 Kerma Unit:J/kg, Gy

 tr dEtr K   dm  

 Absorbed dose Unit:J/kg, Gy

 en d D   dm  

  

  

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Basic dosimetric quantities

Basic dosimetric quantities

 en   tr    K  D       

  

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 Charged-particle equilibrium  Absence of bremsstrahlung losses

Application specific dosimetric quantities Symbol

Unit

Equation

Incident air kerma

Ki

Gy

Entrance -surface air kerma

Ke

Gy

Ke  Ki  B

Air-kerma area product

PKA

Gym2

PKA   K (x , y)dxdy A

Air-kerma length product

PKL

X-ray tube output

Y(d)

Gym

PKL   Kair (z)dz L

Gy/As

Y (d)  Ka (d) / PIt

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Quantity

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Application specific dosimetric quantities: computed tomography

Application specific dosimetric quantities: computed tomography Symbol

Unit

CT air-kerma index Ca,100 (free in air)

Gy

CT air-kerma index (in standard CPMMA, 100 phantom)

Gy

Weighted CT air kerma index Normalized weighted CT air kerma index Air-kerma length product

Cw

Gy

Equation C a ,100

1  T

50

 50

1 CW  C PMMA,100,c  2C PMMA,100,p  3 CVOL  CW

nCw

Gy/As n CVOL 

PKL

Gym

 K (z)dz

NT C  W I p

CVOL PIt

PKL   n CVOLj l j PItj j

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Quantity

Quantities describing risk  Dose-conversion coefficients for assessment of organ and tissue doses c

dosimetric quantity normalisat ion quantity

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 Organ and tissue dose  Equivalent dose  Effective dose

The Use of Effective Dose (E)  E is a risk-related quantity and should only be used in the low-dose range  Primary use:  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

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 to demonstrate compliance with dose limits  in regulation, for prospective planning of radioprotection

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.

 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

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 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)

Dosimeters in diagnostic radiology

Ionization chambers

Semiconductor dosimeters Others

Accurate

Compact

TLD

Good energy dependence

Energy dependant

OSL

Design for different application (cylindrical, parallel-plate, different volumes..)

PSDL/SSDL

Film (radiochromic) Scintillation (kVp meters)

user

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Tube voltage 20-150 keV, various A/F combinations, various modalities

 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

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Dosimetry standards in diagnostic radiology

User  IEC 61674  Ionization chambers  Semiconductor detectors SSDL  Ionization chamber of reference class

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Requirements for dosemeters

 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

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Calibrations in diagnostic radiology

Calibration in diagnostic radiology

 Calibrated  Quality control  Traceability for all beam qualities

 Auxiliary equipment: electrometers, thermometers, barometers…  Environmental conditions

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 SSDL with relevant measurement capabilities  General requirements: beam qualities, tube voltage and filtration measurements  Dosimeter of reference class (with electrometer)

Dosimetry

Radiation source

 Ionization chambers  Position system  HV supply for monitor and reference class ionization chamber  Electrometer

 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%)

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Equipment

Reference class dosemeter Range tube voltage (kV)

Intrinsic uncertainty (k=2)

Maximum variation of response (%)

Unattenuated beam

General radiography

cylindrical or plane parallel

60-150

3.2

±2.6

Fluoroscopy

cylindrical or plane parallel

50-100

3.2

±2.6

Mammography

plane parallel

22-40

3.2

±2.6

10 μGy/s10 mGy/s

CT

cylindrical

100-150

3.2

±2.6

0.1 mGy/s50 mGy/s

Dental radiography

cylindrical or plane parallel

50-90

3.2

±2.6

1 μGy/s10 mGy/s

1 mGy/s500 mGy/s

Attenuated beam

10 μGy/s5 mGy/s

0.1 μGy/s100 μGy/s

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Application

Type of chamber

Range of air kerma rate

 Spectrum  X-ray beam quality:  First half-value layer (HVL1)  Second half-value layer (HVL2)  Homogeneity coefficient: h

 Tube voltage  Total filtration

HVL1 HVL2

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Specification of the x-ray beam

Radiation quality

RQR

Radiation origin Unfiltered beam emerging from x-ray assembly

Phantom material

No phantom

Application General radiography, fluoroscopy, dental radiology Measurements behind the patient (on the image intensifier)

RQA

Radiation beam from Aluminium an added filter

RQT

Radiation beam from Copper an added filter

CT applications (free in air)

RQR-M

Unfiltered beam emerging from x-ray assembly

Mammography (free in air)

RQA-M

Radiation beam from Aluminium an added filter

No phantom

Measurements behind the patient

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Radiation beam qualities (IEC 61267)

Typical calibration set up Apertures

Focal spot

Test point

Shutter

Additional filtration

Monitor chamber

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X-ray tube window

 Procedures before calibration (acclimatization, positioning, stabilization…)  Calibration procedures (methods, number of measurements, interval between measurements…corrections…  Procedures following calibration (uncertainty budget, certificate…)

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Calibration procedures

Dosimetry formalism  Air kerma:

K  M Q  M 0 N K ,Q0

 Reference conditions: set of influencing quantities K  M Q  M 0 N K ,Q0  ki

 Influencing condition: quantities that are not subject of mesusremst but have an impact on the result P 273.15  T  Air density correction: kTP  0 P 273.15  T0  Beam quality correction: K Q  M Q  N K ,Q0  kQ ,Qo

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i

Calibrations of dosemeters for CT

 Information on chamber response only  Size on active volume only assumed  Far from real situation

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 Traditionally, irradiation of the whole volume  Contras:

Calibration for CT: air kerma length product Cylindrical chamber, 100 mm Non-uniform irradiation Uniform response RQT 9 (120 kVp, HVL: 8.5 mm Al)

Focal spot

Aperture

Monitor chamber

Ionization chamber

w da dr

N PKL ,Q  K  w / M 

dr da

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   

Calibration for fluoroscopy: air kerma area product N PKA ,Q 

M KAP

Ref. chamber

Film

10 cm

Cekerevac at al, Poster B3

10 cm

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 In laboratory (SSDL)  Field calibration

M ref N Kref,Qo kQ Anom

Calibration in terms of practical peak voltage  X-ray tube voltage measurements  Practical Peak Voltage (ppv):

 Invasive or non-invasive measurements  Voltage divider

Uˆ 

 pU U wW  i 1 n

i

i

i

 pU wU  i 1

i

i

 Property of the whole exposure cycle  Related to image contrast

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n

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

   

   

Air kerma: ± 2.7 % Air kerma length product: ± 3.0 % Air kerma area product: ± 15 % Non-invasive tube voltage measuring devices: 2.5 %

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Uncertainty budget

 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

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Quality Management System

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Patient dose assessment

Clinical dosimetry

 Output of the X-ray tube, scaled for exposure and geometry

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 Direct measurement on patients or phantoms  Indirect measurements on patients or phantoms

Quantities a) incident air kerma, entrance surface air kerma and kermaarea product (radiography); b) kerma-area product and entrance surface air kerma rate (fluoroscopy); c) incident and entrance surface air kerma (mammography); and d) kerma-length product (computed tomography)

KAP

BSF

Ke

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Dosimetric quantites

Patient  Real situation

Phantoms  Objects that simulate real patients in terms of interaction of radiation with matter  Easy to perform  Standardized

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Patients and phantoms

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Radiography

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Fluoroscopy

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Mammography

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Computed tomography

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Patient dose levels

Uncertainty of clinical dose assessment  Use of k=2 for expression of uncertainty of dose assessment  Typically >10% and close to 25%*

*if correction for beam quality and for individual patient is not applied

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 Radiographers taking the x-ray images  Determining tube output  Calculation of individual patient doses  Determining dose to an average patient

Form measurements towards risk assessment  Conversion of measured quantity into organ doses and effective dose  Ratio of the dose to a specified tissue or effective dose divided by the normalization quantity  Measured using phantoms or calculated using computer models  Voxel phantoms based on images of human anatomy

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 Conversion coefficients

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Organ dose assesment

ICRU 74 Vinca Institute of Nuclear Sciences Radiation and Environmental Protection Laboratory www.vinca.rs

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Optimization of protection

Application of DRLs

 DRLs will be intended for use as a convenient test for identifying situations where the levels of patient dose are unusually high. Quantities that are easily measured!

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 Values of measured quantities above which some specified action or decision should be taken  Values must be specified  Action must be specified

Doses to patients from radiographic and fluoroscopic X-ray imaging procedures in the UK—2005 review. HPA RPD-029, HPA; 2007.

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Diagnostic Reference Levels

 Diagnostic radiology is major contribution to total dose from man-made sources of radiation  Dose measurements: population dose assessment, optimization of practice  Application-specific dosimetric quantities (patients, phantoms)  Calibration of dosimeters in the conditions that are similar to the clinical environment, in terms of  air kerma  kerma-area product  kerma-length product

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Re-cap

Olivera Ciraj-Bjelac, PhD, research associate, dosimetry and radiation physics Milojko Kovacevic, MSc, Head of MDL, radiation physicist Danijela Arandjic, MSc, PhD student, dosimetry and radiation physics Djordje Lazarevic, MSc, PhD student, dosimetry and radiation physics Dragana Divnic, technician Milos Jovanovic, technician Nikola Blagojevic , technician

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