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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).
XXXXX
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
49
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
66
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
110
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.
120
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?)
133
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
139
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
156
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