Nuclear Medicine. International Atomic Energy Agency Part 2: Radiation units and dose quantities...

Nuclear Medicine

International Atomic Energy Agency

Part 2: Radiation units and dose quantities

Topic 1: Exposure and exposure rate

Nuclear Medicine 5

X = dQ/dm

Exposure: X

Nuclear Medicine 2: Radiation units and dose quantities 6

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



Topic 2: Absorbed dose

Nuclear Medicine 8

Absorbed dose, D

• The absorbed dose D, is the energy absorbed per unit mass.

• D = dE/dm. The SI unit of D is the Gray [Gy].

• 1 Gy = J/kg.• The former unit was the “rad”. 1 Gy = 100

rad.

Nuclear Medicine Part 1. Biological effects of ionizing radiation 9

Adapted from Marco Zaider (2000)

IONIZATION PATTERN



Topic 4: Equivalent dose H


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


Radiation weighting factor, wR

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

• The exceptions are: • alpha particles (wR = 20) • neutrons (wR = 5 - 20).

Nuclear Medicine


Interaction of ionizing radiation with DNA

DIRECT ACTIONDIRECT ACTION INDIRECT ACTIONINDIRECT ACTION


Damage to DNA


Exposure of the cell

radiation hit cell

nucleus!

No change

DNA mutation


Outcomes after cell exposure

DNA Mutation

Cell survives but mutated

Cancer?

Cell death

Mutation repaired

Unviable Cell

Viable Cell


Cell killingRadiosensitivity

• RS = Probability of a cell, tissue or organ of suffering an effect per unit of dose.

• Bergonie and Tribondeau (1906): “RS LAWS”: RS will be greater if the cell:• Is highly mitotic.• Is undifferentiated.


RADIOSENSITIVITY

High RS Medium RS Low RS

Bone MarrowSpleenThymusLymphatic nodesGonadsEye lensLymphocytes (exception to the RS laws)

SkinMesoderm organs (liver, heart, lungs…)

MuscleBonesNervous system


BIOLOGICAL EFFECTSDirect

effectsIndirecteffects

Cell death

Primarydamage

Modifiedcell

Damageto organ

Somaticcells

Germcells

Hereditaryeffects

CancerLeukemia

Death oforganism

Repair

Deterministiceffects

Stochasticeffects


1 0 -6

1 0 -12

1 0 -9

1 0 -15

1 0 -3

1 s e c o n d

1 h o u r

1 d ay

1 y ea r

1 0 0 y e a rs

1 m s

1 0 0

1 0 9

1 0 6

1 0 3

Energy deposition

Excitation/ionization

Initial particle tracks

Radical formation

PHYSICAL INTERACTIO NS

PHYSICO -CHEMICAL INTERACTION S

BIO LOG IC AL RESPO NSE

M EDICAL EFFECTS

Diffusion, chem ical reactions

Initial DNA damage

DNA breaks / base damage

Repair processes

Damage fixation

Cell killing

Promotion/completion

Teratogenesis

Cancer

Hereditary defects

Proliferation of "damaged" cells

Mutations/transformations/aberrations

TIM

E (

sec)

Timing of events leading to radiation effects

Nuclear Medicine

Biological effects of ionizing radiation

• Deterministic• e.g. Lens opacities,

skin injuries, • infertility, epilation, etc

• Stochastic• Cancer, genetic

effects.


Part 1. Biological effects

Module 1.2. Deterministic effects


EFFECTS OF CELL DEATH

Dose (mSv)

Probability of death

D

100%


Threshold Doses for Deterministic Effects

• 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

dose

Severity ofeffect

threshold


Effects in eye

• Eye lens is highly RS.• 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


Eye injuries


Whole body response : adult

Acute irradiation syndrome

Chronic irradiation syndrome

Sur

viva

l tim

e

Dose

Steps:.1Prodromic

.2Latency

.3Manifestation

Lethal dose 50 / 30

BMS(bone

marrow)GIS

(gastrointestinal) CNS

(central nervous system)

1-10 Gy

10-50 Gy

>50 Gy

•Whole body clinic of a partial-body irradiation

•Mechanism: Neurovegetative disorder

•Similar to a sick feeling

•Quite frequent in fractionated radiotherapy


Whole body exposure

Absorbed dose (Gy)

Syndrome or tissue involved

Symptoms

1-10 Bone marrow syndrome

Leucopenia, thrombopenia, hemorrhage, infections

10-50 Gastrointestinal Diarrhoea, fever, electrolytic imbalance

>50 Central nervous syndrome

Cramps, tremor, ataxia, lethargy, impaired vision, coma


Whole body exposure

Absorbed dose (Gy)

Therapy Prognosis

1-10 SymptomaticTransfusions of leucocytes and platelets. Bone marrow transplantationGrowth stimu-lating factors

Excellent touncertain

10-50 Palliative Very poor

>50 Symptomatic Hopeless

Lethality

0-90%

90-100%

100%



Module 1.3. Stochastic effects

Nuclear Medicine

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)


STOCHASTIC EFFECTS OF IONIZING RADIATION


STOCHASTIC EFFECTS OF IONIZING RADIATION

Health consequences of Chernobyl accident

•1800 children diagnosed with thyroid cancer (1998)


Genetic effects

Frequency)%(

10 20 30 40 Absorbed dose (Gy)

10

5

0



Module 1.4. Effects on embryo and fetus


PRE-IMPLANTATION


Fetal Radiation Risk

• There are radiation-related risks throughout pregnancy which are related to the stage of pregnancy and absorbed dose


Incidence of Prenatal &

Neonatal Death and

Abnormalities

Hall, Radiobiology for the Radiologist pg 365


Pre-implant stage (up to 10 days)

Only lethal effect, all or none Embryo contains only few cells which are

not specialized If too many cell are damaged-embryo is

resorbed If only few killed-remaining pluripotent

cells replace the cells loss within few cell divisions

Atomic Bomb survivors - high incidence of both - normal birth and spontaneous abortion




• Radiation risks are most significant during organogenesis and in the early fetal period somewhat less in the 2nd trimester and least in the third trimester

Less LeastMost risk


Radiation-Induced Malformations

• Malformations have a threshold of 100-200 mGy or higher and are typically associated with central nervous system problems

• Fetal doses of 100 mGy are not reached even with 3 pelvic CT scans or 20 conventional diagnostic x-ray examinations

• These levels can be reached with fluoroscopically guided interventional procedures of the pelvis and with radiotherapy


Central Nervous System Effects

• During 8-25 weeks post-conception the CNS is particularly sensitive to radiation

• Fetal doses in excess of 100 mGy can result in some reduction of IQ (intelligence quotient)

• Fetal doses in the range of 1000 mGy can result in severe mental retardation particularly during 8-15 weeks and to a lesser extent at 16-25 weeks

Nuclear Medicine. International Atomic Energy Agency Part 2: Radiation units and dose quantities...

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