iaea international atomic energy agency iaea regional training course radiation protection of...

IAEAInternational Atomic Energy Agency

IAEA Regional Training Course

RADIATION PROTECTION OF PATIENTS FOR RADIOGRAPHERS

Accra, Ghana, July 2011

Radiation units, dose quantities and biological effects

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Greetings from Sydney, Australia!

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A few questions for you, to help us

• Who are using automatic wet film processing?

• Who have manual film processing?

• Who have CR or DR?

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Topics

• Exposure and exposure rate

• Absorbed dose and KERMA

• Mean Absorbed Dose in a tissue

• Equivalent dose H

• Effective Dose

• Related dosimetry quantities (surface and depth dose, backscatter factor…..)

• Specific dosimetry quantities (Mammography, CT,…)

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Exposure: X

• Exposure is a dosimetric quantity for ionizing electromagnetic radiation, based on the ability of the radiation to produce ionization in air.

• This quantity is only defined for electromagnetic radiation producing interactions in air.

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Exposure: X

• Before interacting with the patient

(direct beam) or with the staff (scattered radiation), X Rays interact with air

• The quantity “exposure” gives an indication of the capacity of X Rays to produce a certain effect in air

• The effect in tissue will be, in general, proportional to this effect in air

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Exposure: X

• The SI unit of exposure is Coulomb per kilogram [C kg-1]

• The former special unit of exposure was Roentgen [R]

• 1 R = 2.58 x 10-4 C kg-1

• 1 C kg-1 = 3876 R

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Exposure rate: X/t

• Exposure rate (and later, dose rate) is the exposure produced per unit of time.

• The SI unit of exposure rate is the [C/kg] per second or (in old units) [R/s].

• In radiation protection it is common to indicate these rate values “per hour” (e.g. R/h).

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Patient dosimetry quantities

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Absorbed dose, D

• The absorbed dose D, is the energy absorbed per unit mass. This quantity is defined for all ionizing radiation (not only for electromagnetic radiation, as in the case of the “exposure”), and for any material.

• The SI unit of D is the Gray [Gy]. • 1 Gy = J/kg.• The former unit was the “rad”. 1 Gy = 100

rad.

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Absorbed dose, D and KERMA

• The KERMA (kinetic energy released in a material)

The SI unit of kerma is the joule per kilogram (J/kg), termed Gray (Gy).

• In diagnostic radiology, Kerma and D are equal.

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Relation between absorbed dose and exposure

• It is possible to calculate the absorbed dose in a material if the exposure is known

• D [Gy]. = f . X [C kg-1]• f = conversion coefficient depending on medium

• The absorbed energy in a quantity of air exposed to 1 [C kg-1] of X Rays is 0.869 [Gy]• f(air) = 0.869

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Exposure and absorbed dose or KERMA

• Exposure can be linked to air dose or kerma by suitable conversion coefficients.

• For example, 100 kV X Rays that produce an exposure of 1 R at a point will also give an air kerma of about 8.7 mGy (0.87 rad) and a tissue kerma of about 9.5 mGy (0.95 rad) at that point.

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Equivalent dose: H

• The equivalent dose H is the absorbed dose multiplied by a dimensionless radiation weighting factor, wR which expresses the biological effectiveness of a given type of radiation

• To avoid confusion with the absorbed dose, the SI unit of equivalent dose is called the sievert (Sv). The old unit was the “rem”

• 1 Sv = 100 rem

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Radiation weighting factor, wR

• For most radiation used in medicine (X Rays, , e-) wR is = 1, so the absorbed dose and the equivalent dose are numerically equal

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Detriment

• Radiation exposure of the different organs and tissues in the body results in different probabilities of harm and different severity

• The combination of probability and severity of harm is called “detriment”.

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Tissue weighting factor

• To reflect the combined detriment from stochastic effects due to the equivalent doses in all the organs and tissues of the body, the equivalent dose in each organ and tissue is multiplied by a tissue weighting factor, wT, and the results are summed over the whole body to give the effective dose E

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Tissue weighting factors, wT (ICRP103)

Organ/Tissue WT Organ/Tissue WT

Bone marrow 0.12 Lung 0.12

Brain 0.01 Oesophagus 0.12

Bone surface 0.01 Skin 0.01

Breast 0.12 Stomach 0.12

Colon 0.12 Thyroid 0.04

Gonads 0.08 Remainder 0.12

Liver 0.04

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Effective dose, E

• E = T wT.HT

• E: effective dose

• wT: weighting factor for organ or tissue T

• HT: equivalent dose in organ or tissue T

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Entrance surface dose (ESD)

• Absorbed dose is a property of the absorbing medium as well as the radiation field, and the exact composition of the medium should be clearly stated.

• Usually ESD refers to soft tissue (muscle) or water

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Entrance surface dose (ESD)

• On the other hand, the ESD measured on the surface of the patient or phantom includes a contribution from photons scattered back from deeper tissues, which is not present for free air measurements

• For this reason, correction factor (backscatter factor) must be introduced

• If measurements are made at other distances than the true focus-to-skin distance, doses must be corrected by the inverse square law

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Dose area product (I)

• The dose-area product (DAP) quantity is defined as the dose in air in a plane, integrated over the area of interest

• The DAP (cGy·cm2) is constant with distance since the area of the beam increases with the square of the distance, and cancels the inverse square law effect

•

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Inverse square law

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DAP-meter (Diamentor ®)


Specific dosimetry quantities for mammography and CT

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Mammography

• The Average Glandular Dose (AGD) is the dosimetry quantity generally recommended for risk assessment in mammography

• The use of AGD is recommended by the ICRP, the British Institute of Physical Sciences in Medicine, the NCRP, the BSS and the Netherlands Commission on Radiation Dosimetry (NCS)

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The average glandular dose AGD

• The AGD cannot be measured directly but it is derived from measurements with the standard phantom for the actual technique set-up of the mammographic equipment

• The Entrance Surface Air Kerma (ESAK) free-in-air (i.e. without backscatter) has become the most frequent used quantity for patient dosimetry in mammography

• For other purposes (compliance with reference dose level) one may refer to ESD which includes backscatter

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The ESAK (mammography)

• ESAK is usually determined by a radiation dosimeter with a dynamic range covering at least 0.5 to 100 mGy (better than 10% accuracy)

• Tables are then used to convert the ESAK to AGD, knowing the x-ray beam properties (see later)

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Dosimetric quantities for CT

• CTDI (Computed Tomography Dose Index)

• DLP (Dose-Length Product)

• MSAD (Multiple Scan Average Dose)

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Computed Tomography Dose Index (CTDI)

Do

se

Nominal slice width

CTDI

Dose profile

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Computed tomography dose index (CTDI)

• The CTDI is the integral along a line parallel to the axis of rotation (z) of the dose profile (D(z)) for a single slice, divided by the nominal slice thickness T

• In practice, a convenient assessment of CTDI can be made using a pencil ionization chamber with an active length of 100 mm so as to provide a measurement of CTDI100 expressed in terms of absorbed dose to air (mGy).

D(z)dz T

1 =

+

-

CTDI

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Computed tomography dose index (CTDI)

• Various definitions of CTDI are used, but the most relevant is CTDIW

• From this we can obtain an even more relevant figure, the CT dose-length product (DLP)

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• On the assumption that dose in a particular phantom decreases linearly with radial position from the surface to the centre, then the average dose to the slice is approximated by the weighted CTDI, measured as mGy(mAs)-1

• Without going into details, CTDIW is measured in a perspex phantom, with doses being measured with a 100mm long ion chamber near the surface, and at the centre

CTDIW

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Dose-length product

• A very useful dose indicator for a complete CT examination

• Now routinely displayed or available at the CT console

• Can be recorded for future use

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Dose-length product DLP

• DLP Dose-length product for a complete examination: [mGy • cm]

where:

• i represents each serial scan sequence forming part of an examination

• N is the number of slices, each of thickness T (cm) and radiographic exposure C (mAs), in a particular sequence.

N.B.: Any variations in applied potential setting during the examination will require corresponding changes in the value of CTDIW used.

C N T CTDI = DLP wi

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Reference dose quantities

• In the case of helical (spiral) scanning [mGy • cm]:

• where, for each of i helical sequences forming part of an examination: • T is the nominal irradiated slice thickness (cm)• A is the tube current (mA)• t is the total acquisition time (s) for the sequence.

• N.B.: CTDIW is determined for a single slice as in serial scanning.

t A T CTDI = DLP wn

i

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Summary

• Dosimetric quantities are useful to know the potential hazard from radiation and to determine radiation protection measures to be taken.

• The old, non-S.I. quantities and units are mentioned, since these are still used in some countries, notably the United States of America.


Biological effects of radiation

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Radiation health effects

DETERMINISTICSomaticClinically attributable in the exposed individual

CELL DEATH

STOCHASTICsomatic & hereditaryepidemiologically attributable in large populations

ANTENATALsomatic and hereditary expressed in the foetus, in the live born or descendants

BOTH

TYPEOF

EFFECTS

CELL TRANSFORMATION

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Radiation Effects (1)

• Three types of potential effect:

Stochastic : probability of effect related to dose, down to (?) zero dose

Deterministic : threshold for effect - below, no effect; above, certainty, and severity increases with dose

Hereditary (genetic)

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Stochastic Effect

Examples :

carcinogenesis

leukaemogenesis

Involves DNA/chromosome

damage

Probability of Effect

Dose

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The Linear No-Threshold Hypothesis (LNT)

Above the prevalent background dose,

an increment in dose results in

a proportional increment in the probability of

incurring stochastic effects. Risk is also

regarded as additive.

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Backgroundincidence

Backgrounddose

Probability ofstochastic effects, p

Annual dose, D

p

DIn this zone the relationshipis irrelevant

average 2.4 mSvtypical 10 mSvhigh 100 mSv

5% / Sv

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Models of Stochastic Effects

Linear

Supralinear

Linear-quadratic

Dose

Probability of Effect

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Deterministic Effect (or non-stochastic)

Examples :

epilation

radiation sickness

erythema

Involves cell death

ThresholdDose

Severity of Effect

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Deterministic effect thresholds

• Cataracts of the lens of the eye 2-10 Gy

• Permanent sterility

• males 3.5-6 Gy

• females 2.5-6 Gy

• Temporary sterility

• males 0.15 Gy

• females 0.6 Gy

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Radiation injury from an industrial source

Deterministic effects

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Severe Deterministic Effect - 48 y.o. woman following RFA

3 weeks following RFA 5 months 6.5 months

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Effects in eye

• Eye lens is radiosensitive.

• Coagulation of proteins occur with doses greater than 2 Gy.

• There are 2 basic effects:

From “Atlas de Histologia...”. J. Boya

Histologic view of eye:

Eye lens is highly RS, moreover, it is surrounded by highly RS cuboid cells. > 0.155.0

Visual impairment (cataract)

> 0.10.5-2.0Detectable opacities

Sv/year for many years

Sv single brief exposure

Effect Sv single brief exposure

Effect Sv/year for many years


Effect

> 0.1



Effect

0.5-2.0 > 0.1



Effect

Detectable opacities

0.5-2.0 > 0.1



Effect



0.5-2.0 > 0.1



Effect

5.0



0.5-2.0 > 0.1



Effect

> 0.155.0



0.5-2.0 > 0.1



Effect

> 0.155.0



0.5-2.0 > 0.1



Effect

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Delayed effects of radiation

• Classification:

• SOMATIC: they affect the health of the irradiated person. They are mainly different kinds of cancer (leukemia is the most common, with a delay period of 2-5 years, but also colon, lung, stomach cancer…)

• GENETIC: they affect the health of the offspring of the irradiated person. They are mutations that cause malformation of any kind (such as mongolism)

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Latency - The Time for Stochastic Effects to Become Apparent

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Links Between Radiation and Effects

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Lethal dose 50 / 30

• “Dose which would cause death to 50% of the population in 30 days”.

• Its value is about 2-3 Gy for humans for whole body irradiation.

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Radiation and Pregnancy

• Three relevant stages of gestation :• preimplantation (0 - 9 days post conception)

• organogenesis (10 days - 6 weeks)

• major organogenesis is however 3 - 6 weeks

• foetal (6 weeks to term)

• Main radiation effects are failure to implant, organ malformation, carcinogenesis, foetal death

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Preimplantation

• Undetectable embryonic death (failure to implant) is deterministic effect, requiring ~ 100 mGy (low LET radiation)

• Malformations do not occur

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Post-Implantation Death

• Spontaneous abortion, or foetal death

• Difficult to prove unless at high doses

• Deterministic effects, with threshold probably at least 100 mGy and probably much higher

• Highest risk period ~16 days PC

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Foetal Death

Days PC Threshold0- 1 ?

1-8 20 - 100 mGy

9 - 60 250 - 500 mGy

61 - 104 500 mGy

105 - 175 < 500 mGy

> 175 > 1000 mGy

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Organogenesis

• Sensitive period is > 21 days PC when the CNS and heart develop

• A-bomb survivors have shown some problems, mainly related to brain development, and seeming to have a threshold but with a risk factor of ~30 IQ points Sv-1

• These relate to high dose/high dose rate, and probably an overestimate

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Severe Mental Retardation

• Greatest risk 8-15 wks PC

• Risk factor about 0.04% / mGy to foetus

• Equates to 0.03 IQ points / mGy

• Could be threshold of ~100 mGy

• Lower risk 15-25 wks PC (0.01% / mGy), with possible threshold of > 500 mGy

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Carcinogenesis

• Due to risk of cell damage, we assume the risk exists from ~3 weeks to term

• Data is not consistent, and has high uncertainty

• Thus assume that the risk factor is higher than for adults (ICRP 103 says “about 3 times”)

• This is the only radiation risk which can easily be greater than the spontaneous risk (~ 0.1 % / mSv)

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Normal Risks of Pregnancy - Keep these in mind

• Spontaneous abortion 15%

• Major malformation 3%

• Severe mental retardation (SMR) 0.5%

• IUGR 4%

• Genetic disease 8-10%

• Childhood cancer 0.1%

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Proportion of Fatal Cancers Attributable to Different AgentsAgent or Class Percentage of all Cancer Best estimate Range

Smoking 31 29 - 33Alcoholic beverages 5 3 - 7Diet 35 20 - 60Medicines, medical practices 1 0.5 - 2 Electromagnetic radiation 8 5 -10Ionizing (85% natural) 4.5Ultraviolet 2.5Industrial products <1 <1 - 2Pollution 2 <1 - 4

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Does Low Level Radiation Produce Genetic Effects?

• No conclusive human data

• Risk estimates based on animals - difficult to extrapolate to humans

• Hiroshima/Nagasaki survivors have shown no genetic effects as of the third generation

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What is the relevance to radiology?

• Basic philosophy of diagnostic radiology is :

• Appropriately high (NOT best) image quality

• At minimum reasonable (NOT lowest) dose

• Avoid deterministic effects

• Limit stochastic effects (CANNOT eliminate)

• Patient diagnosis and treatment is of primary importance

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Thank you!

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