PHYS C3

Ionisation mechanisms

physical process of energy transfer

Direct ionisation

Particle’s positive charge attracts electrons from surrounding atoms, creating ions by pulling them out of shell

Particles’s negative charge exerts repulsive force on atom’s electrons, ejecting it from shell

Indirect ionisation

Electrically neutral particles/rays transfer energy to surrounding atomic electrons inducing photoelectric effect or Compton scattering, creating ions

Compton scattering

X-ray photon collides with an electron, transferring a fraction of its energy. Causes photon to change direction and electron to be ejected

Linear Energy Transfer (LET)

Rate of energy absorption in the medium, keV/μm

High LET

densely ionising

deposits large amount of energy over very small area

produces essentially all direct action in DNA

e.g alpha particles

Medium LET

moderately ionising

energy deposit depends on speed of neutron (fast moving = little energy deposit)

produces predominantly direct action in DNA

e.g protons and neutrons

Low LET

does not give up energy quickly

deposits energy over a large distance

produces predominantly indirect action in DNA

e.g x-ray and γ-ray photons - produce fast moving electrons that deposit their energy sparsely

Radiation Injury

Ionisation energy may be transferred through a chain of chemical reactions and free radicals formation, causing irreversible damage to important cellular molecules

Radiosensitive cell site

DNA molecule within the nucleus

DNA

DNA’s large size makes it a ready radiation target

DNA damage is the main reason for biological and health effects

Secondary charged particles

Produced when a neutral particle/ray interacts with an atom, transferring kinetic energy and causing the emission of a charged particle

Free radical

Uncharged molecule that contains a single unpaired electron in the outer shell

Direct Action

Occurs when absorbed ionisng energy or secondary charged particles deposit energy in the target atom causing direct ionization or excitation and thus breaking chemical bonds in DNA

Indirect Action

Body absorbs radiation (usually low LET) which ionises cellular water, producing ion pairs and free radicals (like hydroxyl radicals, OH*). The free radicals cause potentially repairable ionizing damage to DNA but in the presence of O₂, they react to form DNA peroxyl radicals which creates chemically stable lesions, permanently damaging DNA (oxygen fixation hypothesis)

Exposure

flow of photons per unit time (photon flux) through a point of interest in air at a given distance from a radiation source

Unit: C/kg

KERMA

Kinetic energy released per unit mass

The amount of kinetic energy transferred to the electrons released in photon-matter interactions

Unit: Gy (1 J/kg)

relates energy released in matter to energy absorbed in matter to determine extent of biological effect

Absorbed Dose

Amount of radiation energy absorbed by medium

This can be from scattered radiation in the room as well as directly from source’s beam

Unit: Gy (1 J/kg)

Equivalent Dose (H_T)

Sum of absorbed dose of all radiation types multiplied by radiation weighting factor (w_r) which accounts for differences in biological effectiveness between radiation types

when w_r=1, equivalent dose = absorbed dose

Unit: Sv (also 1J/kg but only used for H_T)

Most important for radiation therapists

Radiation weighting factor (w_r)

Photons, electrons, myons = 1

protons, charged pions = 2

Alpha particles, heavy ions, fission fragments = 20

Effective Dose (E)

the sum over tissues and organs of the equivalent dose H_T multiplied by the tissue weighting factor wT (organs sensitivity to radiation)

If more than one organ is exposed sum the effective doses to all exposed organs

If only part of organ is exposed, dose is averaged over whole organ not just exposed area

Accounts for stochastic cancer risk and heritable effects allowing estimation of whole body risk

Most important for radiographer

Unit: Sv

Functional Sub-Units (FSU)

compartment of an organ that performs part of the organ's function

Total damage to an organ (tissue response) depends on how many FSUs are destroyed

FSU Arrangements

Serial: each FSU in a line, if one FSU fails organ experiences harmful effects, organs susceptible to high point (hot spot) doses - where radiopharmaceutical is concentrated in one area, e.g spinal cord, oesophagus

Parallel: each FSU functions independently, loss of one FSU leads to a slight decrease in organ function, organs more sensitive to volume effects - toxicity caused by large number of FSUs being irradiated, e.g liver, kidneys

Mixed organs: combination of serial and parallel, e.g lungs, brain

Biological Response

Cells respond to DNA damage and protein and lipid oxidation within minutes by changing the activation of certain genes and modifying proteins

Types of Effects

Stochastic effects

Tissue Reactions

Stochastic effects

random

long-term

probability increases with dose

no threshold dose

severity does not depend on dose

caused by accumulated exposure

dominant concern in diagnostic imaging

e.g hereditary effects, radiation carcinogenesis

Natural Incidence

The background radiation dose (not from medical radiation procedures) at which a disease occurs spontaneously (‘normal risk’)

Tissue Reactions

immediate

whole body doses of >1Gy needed

severe

there is a threshold dose below which no effect is seen and for which repair mechanisms will prevent effects

above the threshold, effects always occur and severity increases with dose

different effects, tissues and people have different threshold doses

Dominant concern in radiation therapy and lengthy high-dose-rate fluoroscopic procedures

e.g hair loss, sterility (inability to conceive), cataracts (blurry vision)

Thresholds

No Observed Effects (<100 mSv)

Increased Cancer Risk (100 mSv+)

Radiation Sickness (0.5 Sv - 1 Sv)

Lethal Doses (>10 Sv)

Acute Radiation Syndrome

acute illness caused by irradiation of entire body by a high dose of penetrating radiation in minutes

Radiation Sickness: 4 Organ Systems

Cutaneous (C)

Haematopoietic (H)

Gastrointestinal (GI)

Neurovascular/Cerebrovascular/Central Nervous System (N/CV/CNS)

Cutaneous (C)

hair follicle damage (epilation) and skin damage

Few hours = itching and erythema (redness)

Days/weeks = increased reddening and ulceration

Possible regeneration

High doses can be permanent

Haematopoietic (H)

0.7–10 Gy

bone marrow and lymphatic organs damage

causes drop in blood cell counts

results in infection and haemorrhage

usually death within weeks

Gastrointestinal (GI)

>8 -10 Gy

destroys intestines lining

causes severe diarrhea, dehydration, electrolyte imbalance and sepsis

death within 2 weeks

Neurovascular/Cerebrovascular/Central Nervous System (N/CV/CNS)

>20 Gy

Brain and cardiovascular system damage

causes extreme nervousness, confusion, nausea, vomiting, loss of consciousness and circulatory collapse

Death within hours to a few days

Treatment Goals

manage symptoms, prevent infection and support the body's recovery from bone marrow damage

Treatment Options

Immediate decontamination via washing skin and clothing

Supportive care via fluids, antibiotics, pain relief, and blood products

Specific Medical Countermeasures like Potassium Iodide to protect thyroid from radioactive iodine, Hematopoietic Stem Cell Transplant (HSCT) for severe bone marrow damage if cytokine therapy fails, and Prussian Blue to help remove internal cesium and thallium

Embryo and Foetus Radiation Effects - Cancer

In-utero exposure of dose >10–20 mGy = increased stochastic risk of childhood cancer, particularly leukaemia (6% per Gy)

Embryo and Foetus Radiation Stages

Pre-implantation: first days of pregnancy, highest risk of death, if fetus survives they will be symptom free

Organogenesis: first weeks, highest risk of malformations

Fetus: until birth, highest risk of growth and mental retardation, lowest death risk

Why is baby’s likelihood of death highest in earliest stage of pregnancy?

Baby has:

Rapid cell division

Less capacity for DNA damage repair

Higher likelihood that a single mutation is propagated.

Relative risk

is compared to a baseline risk (natural rate), it is not a measure of the absolute chance of something occurring

e.g if RR = 1.3 it is 1.3 times the baseline risk