Radiation Biology

Rad T 290

Objectives – Radiation Biology Radiosensitivity

Somatic Effects

Embryonic and Fetal Risks

Genetic Effects

Patient Interactions

**Photoelectric** Classic Coherent

Scatter **Compton

Scattering**

Pair Production Photodisintegration

Interaction in

the body begin at the atomic level

Atoms

Molecules

Cells

Tissues

Organ structures

X-ray photons can change cells

Some radiations are energetic enough to rearrange atoms in materials through which they pass, and can therefore he hazardous to living tissue.

1913

Interactions of X-rays with matter• No interaction; X-ray passes

completely through tissue and into the image recording device.

• Complete absorption; X-ray energy is completely absorbed by the tissue. No imaging information results.

• Partial absorption with scatter; Scattering involves a partial transfer of energy to tissue, with the resulting scattered X-ray having less energy and a different trajectory. Scattered radiation tends to degrade image quality and is the primary source of radiation exposure to operator and staff.

Coherent Scattering

Also called: Classical scattering or Thompson scattering

Occurs with energies below 10 keV

Incident x-ray interacts with an atom of matter, causing it to become excited. Immediately the atom releases this excess energy and the scattered x-ray.

Coherent Scattering

The wavelength is equal to the incident x-ray or equal energy.

The only difference is the direction of travel

Energy in = Energy out - Only changes is direction

Coherent / Classical Scatter

Classical (Coherent) Scattering

Excitation of the total complement of atomic electrons occurs as a result of interaction with the incident photon

No ionization takes place Electrons in shells “vibrate” Small heat is released The photon is scattered in

different directions No loss of E

Thompson scatter

Occurs primarily with low energy x-rays. Classical will occur throughout the diagnostic range.

Coherent contributes slightly to film fog and reduces image contrast.

Compton Effect or Compton Scattering Occurs throughout the diagnostic

imaging range The incident x-ray interacts with the

outer electron shell on an atom of matter, removing it.

It not only causes ionization but scatters the incident x-ray causing a reductions in energy and the change of direction.

COMPTON

SCATTERING –

OUTER SHELL ELECTRON IN BODY –

INTERACTS WITH

X-RAY PHOTON

FROM TUBE

Compton scatter A fairly high energy (high kVp) x-ray photon ejects an

outer shell electron. Though the x-ray photon is deflected with somewhat

reduced energy (modified scatter), it retains most of its original energy and exits the body as an energetic scattered photon.

A Compton e- is also released Since the scattered photon exits the body, it does not

pose a radiation hazard to the patient. It can, however, contribute to film fog and pose a

radiation hazard to personnel (as in fluoroscopic procedures).

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Compton scatter

Both the scattered x-ray and the Compton electron have enough energy to cause more ionization before loosing all their energy

In the end the scattered photon is absorbed photoelectrically

Compton Effect

The Compton electron looses all of its kinetic energy by ionization and excitation and drops into a vacancy in an electron shell previously created by some other ionizing event

The probability of Compton effect increases as photon energy increases, however the atomic number does not affect the chances of the Compton effect

Compton Scatter

Compton is just as likely to occur with soft tissue as bone. Compton can occur with any given photon in any tissue

Compton is very important in Radiography, but not in a good way.

Scattered photons provides no useful diagnostic information

Compton Effect

Scattered radiation produces a uniform optical density on the radiograph that reduces image contrast

Scattered radiation from Compton contributes to the majority of technologists exposure, especially during fluoroscopy

STAY AWAY FROM YOUR PATIENT !

Photoelectric Effect or Absorption

Inner-shell ionization

The photon is not scattered it is totally absorbed

The e- removed from the atom of matter is called a photoelectron, with an energy level equal to the difference between the incident photon and the e- binding energy.

Binding Energy is very important

Photoelectric – Absorption

PHOTOELECTRIC ABSORBTION

IN THE PATIENT

(CASCADE OF ELECTRONS)

Photoelectric effect• A relatively low energy (low kVp) x-ray photon

uses all its energy (true absorption) to eject an inner shell electron,

• leaving an orbital vacancy. • An electron from the shell above drops down

to fill the vacancy and, in doing so, gives up energy in the form of a characteristic ray.

• The photoelectric effect is more likely to occur in absorbers of high atomic number (eg, bone, positive contrast media)

• and contributes significantly to patient dose, • as all the photon energy is absorbed by the

patient (and for the latter reason, is responsible for the production of short-scale contrast).

Electron transitions

Are accompanied by the emission of more x-rays – secondary radiation

Secondary radiation behaves much like scatter radiation

Secondary contributes nothing to the image The probability that any given photon will

undergo a photoelectric interaction is dependent on the photon energy and the atomic number of the atom

CASCADE

Photodisintegration

Important X-ray Interactions

Of the five interactions only two are important to radiologyPhotoelectric effect or photoelectric

absorptionCompton scatter

Compton scatter

Contributes to no useful information

Is independent of the atomic number of tissue. The probability of Compton is the same for bone atoms and for soft tissue atoms

The probability for Compton is more dependent on kVp or x-ray energy

Compton Scatter

Results in image fog by optical densities not representing diagnostic information

Photon are Photons

IR is does not know

the difference

Photoelectric Absorption

Provides information to the IR because photons do not reach the IR

This represents anatomic structures with high x-ray absorption characteristics; radiopaque structures; tissue with high atomic number; or tissue with high mass density

Attenuation – The total reduction in the # of photons remaining in an x-ray beam after penetration through tissue

Absorption = x-ray disappears (Photoelectric, Pair production & Photodisintegration)

Scattering = partially absorbed, x-ray emerges from the interaction traveling in a different direction (sometimes with less energy)

Absorption + Scattering = Attenuation

Attenuation

3 Types of x-rays are important for IMAGE FORMATION DIFFERENTIAL ABSORPTION = the

difference between those x-rays absorbed and those transmitted to the IR

Compton scatter (no useful information) Photoelectric absorption (produces the light

areas on the image) Transmitted x-rays (produces the grey/dark

areas on the image)

Differential Absorption

Increases as the kVp is reduced

Approximately 1% of photons that interact with the patient (primary beam) reach the IR. Of that 1% approximately 0.5% interact to form the image

Differential Absorption

The difference in x-ray interactions

Fundamental for image formation

Occurs because of Compton Scattering, Photoelectric absorption, and X-ray transmission

Differential Absorption

Compton vs. Photoelectric

Below 80 kVp Photoelectric absorption is predominant above 80 kVp Compton scatter begins to increase.

Dependent on the tissue attenuation properties

Differential absorption factors

High atomic number = larger atoms

Mass Density = how tightly the atoms of tissue are packedZ # for air and soft tissue are about

the same the OD changes are due to mass density difference

Human Biology

X-rays are harmful, low energy photons can cause skin burns, cancer, leukemia

It is not known for certain the degree of effect following diagnostic levels of x-radiation

Technologists Responsibilities

Technologists, Student Technologists, Radiologists & Medical Physicists have ethical & professional responsibilities to produce high-quality x-ray images with minimal radiation exposure

What is the acronym for this?

47

CARDINAL RULESOF RADIATION PROTECTION

•TIME•DISTANCE •SHIELDING

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Natural radiation

• Natural radiation accounts for approximately 300 millirem (mrem)

• 3 sources of environmental radiation: cosmic rays, terrestrial radiation and internally deposited radionuclides. The largest source of natural radiation is radon.

Biological Response toIonizing Radiation X-ray interactions with matter (human

tissue) can cause biological changes.

Technologists must understand cellular biology and how radiation interacts with cells in order to protect oneself and the patient.

RBE – Relative Biological Effectiveness

THE EARLY YEARS

Early measurement of Radiation

Skin dryness & erythemia Ulcers formed

Late Effects: Cataracts Cancers

Radiobiology

Radiobiology

The study of the effects of ionizing radiation on biologic tissue

Most radiobiology research is designed to develop dose-response relationships to determine the effect of planned doses or accidents

Comparsion of Units

R - ROENTGENS

RADS –

PATIENT DOSE

REMS

OCCUPATIONAL EXPOSURE

RADS REMS RADS

GRAYS

PATIENT ABSORBED

DOSE

REMS

SIEVERTS

Employee(technologists)

=

Rad VS. Rem 1 RAD X QF = 1 REM

1 GRAY X QF = 1 SIEVERT

QF FOR X-RAYS = 1

So…… Rads = Rems

TYPES OF RADIATON(ALL CAUSE IONIZATION)

PARTICULATE ALPHA BETA FAST

NEUTRONS Unit of mesaure

is the curie (Ci) or becquerel (Bq)

More destructive

ELECTROMAGNETIC

XRAY GAMMA (damaged caused

by indirect action = free radicals – can be repaired)

QUALITY FACTOR

Qualifies what the damage is from

different types of radiation

Example: QF for X-ray is 1 QF for alpha is 20

Alpha is 20 x more damaging to tissue

Measurement

(Rad + QF = Rem)

RBE-

Measures biologic tissue response to radiation

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Patient dose

Is reported in Entrance Skin Exposure (ESE)

REGULATORY AGENCIES

NCRP – National Council on Radiation Protection and Measurement ?

NRC – Nuclear Regulatory Committee ?

Other regulatory agencies?

REGULATORY AGENCIES

NCRP – National Council on Radiation Protection and Measurement

Reviews recommendation for radiation protection & safety

NRC – Nuclear Regulatory Committee Makes LAWS & enforces regulations

California Department of Public Health, Radiologic Health Branch (CDPH) Title 17

Human Radiation Response

The effects of x-rays on human is the result of interactions at the atomic level Ionization or excitation

The result if a deposit if energy in tissue. The excess energy can result in a molecular change that can be measurable if the molecule involved is critical to metabolic function

At each stage cell repair is possible

Atom ionization

Can cause chemical binding property change. If the atom is part of a large molecule the ionization may cause molecule break down or relocation of the atom within the molecule

Abnormal molecules

In time may function improperly or cease to function. This may cause serious impairment or death of the cell

This process is reversible by the ionized atom attracting a free e- and become neutral again

Cell and tissues can regenerate and recover from the radiation injury

Cell bombarded with photonsWhat damage will they cause?

TARGET THEORY

BIOLOGIC RESPONSE TO IONIZING RADIATION DEPENTS ON WHERE THE PHOTON INTERACTS

CELL STRUCTURE NUCLEUS & CYTOPLASM

The most at risk area of the cell…….

CHROMOSOMES, WHICH ARE MADE UP OF GENES.

Cellular AbsorptionDirect vs. Indirect Hit

Direct Hit Theory: When radiation interacts with DNA. Ionization of a DNA molecule. Break in the bases or phosphate bonds Can injure or kill the cell

Indirect Hit Theory: Occurs when water molecules are ionized Produces chemical changes – injury or cell

death Vast majority of cellular damage is from

indirect hit.

Cells

The most radiosensitive part of the cell is the deoxyribonucleic acid (DNA)

Water is the most abundant molecule in the body. The body is 80% water. Humans are basically made of structure water.

Basic Cell Structure

Two parts:1. Nucleus2. Cytoplasm

Nucleus contains chromosomes – genetic info (DNA)

DNA is at risk when a cell is exposed to ionizing radiation

Cytoplasm – 80% water

Tissue response to radiation

A precise knowledge of various organ radiosensitivities in unnecessary. However, it is important to have a general knowledge of effects of radiation exposure

A few important general principals are important to understand

Response of cells to radiation

CELL SENSITIVITY TO RADIATION TYPE OF CELL AGE OF CELL TYPE OF DAMAGE RECEIVED KIND OF RADIATION EXPOSURE

Human cell types

Two general types:

Somatic cells

Genetic cells

MOST CONCERNING EFFECTS OF RADIATION EXPOSURE

LATE EFFECTS SOMATIC EFFECTS =

INDIVIDUAL EXPOSED

GENETIC EFFECTS =

FUTURE GENERATIONS

Target Theory = for a cell to die after radiation exposure, the target molecule must be inactivated

TARGET THEORY

Photons hit master molecule DNA = cell dies

Or doesn’t hit nucleus – and just passes through

No essential damage

Hormoresis – repair that can occur when below 5 rads of exposure

DNA is the target molecule of radiation damage

Radiolysispoison water theory The human body is 80% water molecules and

1% DNA molecules

Irradiation of water represents the principal radiation interaction in the body

When water is irradiation, it dissociates into other molecular products – RADIOLYSIS OF WATER

Formation if ions & free radicals

The ion pair may rejoin into a stable water molecule

In this case, no damage is done

HOH+ recombine to H2O

Radiolysispoison water theory

H 2 O molecules Ejection of electron = free

radical H2 02 = hydrogen peroxideOr H O2 = Hydroperoxyl are

formed

Radiosensitivity of Cells Bergonie & Tribondeau (1906) –

method of classifying a cell’s response to radiation according to sensitivity.

Cells are most sensitive during active division (primitive in structure & function).

Cells that are most sensitive to radiation

Young – immature cells Stem Cells Highly dividing (mitotic) cells Highly metabolic

The Law of Bergonie & Tribondeaux

Categorizing Radiation Exposure

Early vs Late effects of Radiation Early Effect = response that occurs within

minutes or days after exposure

Late Effects = response that occurs within months or years

**most human responses have been observed after LARGE doses. To be cautious we assume even small doses are harmful**

Predicting Radiation Dose

Responses

Radiobiology Irradiated tissue response, besides the

cell properties, is determined by the amount of energy deposited per unit mass

Linear Energy Transfer (LET) = the rate at which energy is transferred from ionizing radiation to tissue

LET The ability of ionizing radiation to produce

biologic response increases as the LET of radiation increases

When the LET of radiation increases ionizations increase. When LET is high, ionizations occur frequently, increasing the potential for biologic damage

Relative Biologic Effectiveness As the LET of radiation increases, the

chances of biologic damage also increases

Relative Biologic Effectiveness (RBE) = standardizes biologic effects of radiation exposure

RBE for diagnostic x-rays is 1 radiation with lower LET is less than 1,

radiation with higher LET is greater than 1

QUALITY FACTOR Qualifies what the damage is from different types of radiation

Example: QF for X-ray is 1 QF for alpha is 20

Alpha is 20 x more damaging to tissue

TYPES OF RADIATON(ALL CAUSE IONIZATION) PARTICULATE (HIGH LET) ALPHA BETA FAST NEUTRONS

More destructive

ELECTROMAGNETIC (LOW LET) XRAY GAMMA (damaged caused by

indirect action = free radicals – can be repaired)

Why did the bunny die??BUNNY A

Received 200 rads

BUNNY B

Received 200 rads

Why did the bunny die??BUNNY A

200 rads x 1 for X-RAY = 200 RADS

BUNNY B200 rads x 20 for alpha

= 4000 rads

LET vs RBE

Biologic Factors Affecting Radiosensitivity Oxygen Effect – tissue is more sensitive

when the tissue is oxygenated

Age – Humans are most sensitive before birth, sensitivity decreases until maturity, after maturity humans are mostly resistant to radiation effects

Age Radiosensitivity

LD 50/30

HIGH DOSES RECEIVED

50% OF THE POPULATION WOULD DIE IN 30 DAYS

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Threshold vs ChanceDeterministic (non stochastic) vs Stochastic

Radiation Dose-Response Relationships Every radiation dose-response relationship

has two characteristics

Linear or Nonlinear

Threshold or Stochastic (chance)

Linear Dose-Response Relationships

Linear dose-response – when radiation dose is doubled the response to radiation is likewise doubled

Nonthreshold dose-response – any dose, regardless of it size is expected to produce a response – chance

Threshold dose-response – a radiation doses below a certain level no response is expected

Linear nonthreshold = A & B

Linear threshold = C & D

FIG. 9–7 Graph indicates no-threshold versus threshold response to radiation.

Elsevier items and derived items © 2007, 2003 by Saunders, an imprint of Elsevier Inc.

LINEAR RESPONSE TO

RADIATION –

ASSUMES NO PHOTON

IS SAFE

A. DIAGNOSTIC X-RAY - No Threshold –

LOW DOSE – OVER LONG EXPOSURE

B. Early Radiology Exposure

Threshold amount needed to see affect

SOMATIC & GENETICSTOCHASTIC VS NON STOCHASTIC

A = STOCHASTIC “CHANCE” EFFECTS NONTHRESHOLD GENETIC, LEUKEMIA,

CANCERDIAGNOSTIC RADIOLOGY

B= NON-STOCHASTICTHRESHOLD EFFECTSDETERMINISTICSOMATIC EFFECTSSKIN ERYTHEMA, CATARACTS,

STERILITYRAD -MALIGNANCIES

Linear vs Non linear• Linear – direct

response to the dose and the effects seen (proportionally)

• Non linear – effects are not proportional to the dose received

• S curve – rad therapy, skin erythema, most somatic, deterministic radiation effects.

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Organ Systems

Are identified by their rate of cell proliferation and their stage of development. Each organ system have different rates

Immature cells are called undifferentiated cells, precursor cells or stem cells.

Stem cells are more sensitive to radiation than mature cells

Tissue types

Radiosensitivity of tissue is also dependent on structural or functional features

Tissue types include: Epithelium, Connective (supporting tissues), Muscle and Nervous

The various organs of the body exhibit a wide range of sensitivity to radiation. This is determined by the function of the organ, the rate at which cells mature in the organ, and the inherent radiosensitivity of the cell type

Example of cell sensitivity

Organ or Tissue Weighting FactorEffective Dose

NCRP: report # 116

Total Body Response to Radiation

Acute Radiation Syndrome – full body exposure given in a few minutes.

3 stages of response:1. Prodromal Stage: NVD stage

(nausea, vomiting, diarrhea)

2. Latent Period: Feels well while undergoing biological

changes3. Manifest Stage: Full effects felt,

leads to recovery or death

3 Acute Radiation SyndromesEarly Effects

• Bone marrow syndrome: results in infection, hemorrhage & anemia

• Gastrointestinal syndrome: results in diarrhea, nausea & vomiting, fever

• Central nervous syndrome: results in convulsions, coma, & eventual death from increased intracranial pressure.

CNS least sensitive in ADULTS – MOST sensitive in the FETUS

Late Effects of Radiation

Somatic Effects: develop in the individual who is exposed

Most common: Cataract formation & Carcinogenesis

Genetic Effects: develop in future generations as a result of damage to germ cells.

SENSITIVITY TO RADIAITION

Which (Male or Female) GONADs are external vs internal

Which gender is born with all their reproductive cells?

Which gender constantly produces new cells?

Which GENDER is more sensitive to radiation at birth? Why?

Response of cells to radiation

CELL SENSITIVITY TO RADIATION TYPE OF CELL AGE OF CELL TYPE OF DAMAGE RECEIVED KIND OF RADIATION EXPOSURE

• What is this called • What type classification (direct or indirect?)

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Pg 619

Permissible Occupational Dose

• Annual dose:• 5 Rem / year 50 mSv /

year (NOT TO EXCEED 1.25 rem/quarter)

• Cumulative Dose• 1rem x age 10mSv X age

OCCUPATIONAL EXPOSURES

• 5 REMS / YEAR

BUT NOT TO EXCEED 1.25 REM/QUARTER

• Technologist essentially receive all exposure during fluoroscopy exams

Occupational DoseANNUAL LIMITS

• WHOLE BODY = 5 rems / 5000mrem

• LENS OF THE EYE = 15 rems

• EXTREMITIES = 50 rems

PUBLIC EXPOSURE• 10 % OF OCCUPATIONAL• (MUST BE MONITORED IF ABOVE 10%)

• NON MEDICAL EXPOSURE• .5 RAD OR 500 MRAD• UNDER AGE 18 AND

STUDENT• 100 mrem 1 mSv

GSD• GENETICALY SIGNIFICANT DOSE• Takes all of the population into account• Annual AVERAGE gonadal dose to

population of childbearing age

• 0. 20 mSv or 20 millirem • *Bushong

• *30 mrem per NRC website

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Fetus Exposure

Radiation exposure is most harmful during the first trimester of pregnancy

Embryo-Fetus Exposure limit (Monthly)0.05 rem or 0.5 mSv

Effects of radiation in utero are time and dose related Effects include: Prenatal death, neonatal death,

congenital abnormalities, malignancy inductions, general impairments of growth, genetic effects, and mental retardation.

Irradiation in Utero

The first trimester is the most radiosensitive period. After the 2 weeks of fertilization

The first 2 weeks of pregnancy may be of least concern because the response is all or nothing

After 200 rads delivered at various times

Declared Pregnant Worker• Must declare pregnancy – 2 badges

provided• 1 worn at collar (Mother’s exposure)• 1 worn inside apron at waist level

Under 5 rad – negligible risk

Risk increases above 15 rad

Recommend abortion (spontaneous) 25 rad

(“Baby exposure” approx 1/1000 of ESE)

Pregnancy & Embryo

Mother –

occupational worker (5 rem)• Baby – (500 mRem)• .5 rem/ year • .05 rem/month• 5 mSv .5 mSv / month

Pregnant patient

• ALWAYS ASK LMP before exposure made• “10-day Rule” No longer used• “Grace period” of implantation

• What is the State Law for gonadal shielding?

Pregnant Patients

Should never knowingly expose a pregnant patient unless a documented decision to so has been made

If you must expose; use precise collimation & protective shields. Use a high kVp technique and only the minimal projections

Unsuspected pregnancy

Always screen female patients for last LMP don’t assume ages (patient privacy)

If unsure obtain a blood test or reschedule exam if possible

PREGNANT PATIENTS

• Ascertain LMP - if fetus is exposed• Medical Physicist will need information:• Which x-ray machine used (mR/mAs)• # Of projections (including repeats)• Technique for each exposure• SID • Patient measurement at C/R• Fluoro time & technique used• Physicist will calculate fetal dose

90 % of cell damage will repair.At each stage cell repair is possible

Protraction & Fractionation cause less biological effect If radiation is delivered over a long period

of time rather than quickly, the effect of that dose is lessened. Allows for intercellular repair and tissue recovery.

Protraction Dose is delivered continuously but at a lower

dose rate Fractionation

Same dose rate in short doses over a longer period (occupational exposure)

Biologic Factors Affecting Radiosensitivity Recovery – human cells can recover from

radiation damage. If the radiation dose is not sufficient to kill the cell before its next division. Then given sufficient time, the cell will recover If a tissue or organ receives a sufficient

radiation dose it responds by shrinking or atrophy. Cells disintegrate and are carried away as waste products

Hormesis Pg. 518

repair that can occur when below 5 rads of exposure

A growing body of radiobiologic evidence suggests that a little bit of radiation is good for you. It stimulates hormonal and immune responses to other toxic environmental agents

We still practice ALARA

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Why cancer risks at low doses are uncertain It has been difficult to estimate cancer

induction risks, because most of the radiation exposures that humans receive are very close to background levels.

At low dose levels of millirems to tens of rems, the risk of radiation-induced cancers is so low,

that if the risk exists, it is not readily distinguishable from normal levels of cancer occurrence.

In addition, leukemia or solid tumors induced by radiation are indistinguishable from those that result from other causes. 

Always remember….

IMAGE GENTLY, LIGHTLY & WISELY !!

Objectives – Radiation Biology Radiosensitivity

Somatic Effects

Questions

Embryonic and Fetal Risks

Genetic Effects