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Nuclear medicine
Essential idea
Nuclear radiation, whilst
dangerous owing to its ability to
damage cells and cause mutations,
can also be used to both diagnose
and cure diseases.
Radiotherapy
• Use of ionizing radiation as emitted by radionuclides
to treat diseases (radiotherapy) such as cancer by
destroying the cancer cells.
• Also used to diagnose a disease by providing
detailed information in the form of images (nuclear
imaging) about internal organs affected by the
disease.
• Radioisotopes have an unstable nucleus that decays
spontaneously into a more stable form by emitting
radiation either in the form of subatomic particles or
energy.
Radiotherapy can be …
• Internal: radionuclides are placed in the human
body to target a particular cancerous tissue or
group of receptors. Internal radionuclides are taken
in orally as a solid or as a liquid, as an implant or
injected in the case of some liquid radionuclides.
• External: radiation source remains outside the
human body and the beams of radiation (beta or
gamma rays, photons or neutrons) target specific
cancerous tissues in the body. A commonly used
radionuclide for this approach is Co-60.
Effect of nuclear radiation on cells
Removes electrons from atoms in biological
molecules converting them into ions (ionizing
radiation) that form reactive radicals (H and OH
that interfere with physiological processes. This
causes genetic damage which can lead to cell
death, mutations and cancer:
o Break structure of DNA.
o Changes to the structure of the DNA within the genes
of cells (genetic damage) (mutations).
o A reduced ability of cells to repair DNA damage.
o Limited growth and regeneration of the cells/tissue.
Effect on cancerous cells
Same effect as described in previous
slide but cancerous cells are affected
more by radiation than normal cells so
targeted radiotherapy (internal or
external) is on obvious choice for the
treatment of cancer.
Side effects
Ionizing radiation also affects normal cells but some
cells more than others in particular cells that divide
rapidly such as hair follicle cells, sex cells and cells in
the skin causing more damage to the DNA and
reducing growth.
Some side effects: hair loss, damage to skin and
nails, nausea, fatigue and sterility.
Issue: ionizing radiation can be used for diagnosis
and treatment of cancer but can also cause it!!
Common types of radiation
Alpha radiation:
• involves the emission of particle equivalent to a helium
nucleus as it consists of 2 protons and 2 neutrons and
therefore has a charge of +2 and a relative mass of 4. As a
result of this decay the radioisotope becomes an element
with a decrease of 2 atomic number as shown by the
nuclear equation below:
• Greatest ionizing ability because of its large charge
and mass but penetrates less.
Common types of radiation
Beta emissions
• occur when a neutron in the nucleus splits
into a proton and an electron.
• The electron is emitted and the proton
remains in the nucleus increasing the atomic
number by one unit changing the radioisotope
into a nuclide of a different element.
Other types of radiation
• Gamma is the emission of energy or photons of
high frequency as part of the electromagnetic
spectrum.
• Emission of protons: 1+1 p
• Emissions of neutrons. 10n
• Positron (positively charged electrons) radiation
occurs when a proton changes into a neutron and
a positron is emitted: 0+1 β. Used in a positron
emission tomography or PET.
Exercises on nuclear equations
Write nuclear equations for the following:
• Alpha decay of Pb-212
• Beta decay of Y-90
• Beta decay of Lu-177
• Alpha decay of Ac-255
• Beta decay of Co-60
• Beta decay of Ca-40
Targeted alpha therapy (TAT)
• Used for treating leukemia and other dispersed cancers;
these are cancers in which the cells have spread throughout
the body from the original tumour. As alpha particles have
the greatest charge and therefore the greatest ionizing
ability, they are the most destructive type of emission. Alpha
particles can only penetrate tissue, and therefore cause
cellular damage, over a very short range of 0.05 mm-0.1
mm. This also means that not too many healthy cells should
be effected.
• The radionuclides emitting the alpha radiation are directed
to antibodies and bind themselves onto them. The antibody
then carries the alpha-emitter to the cancer tissue where
the alpha radiation will destroy the cells without too much
damage to the surrounding healthy cells.
Boron Neutron Capture Therapy (BNCT)
This therapy uses a beam of neutrons to produce alpha
particles only at the site of the cancer. The target of the
external neutron radiation are B-10 atoms that have
been taken to the site of the cancer. There the B-10
atoms capture (absorb) the neutrons from the beam
and then change into B-11 nucleii; these then
immediately decay emitting alpha particles that destroy
the surrounding cancerous cells. The B-10 nucleii are
administered using intravenous injections in the form of
a compound such as boronophenylalanine. This
compound tends to accumulate in brain tumours.
When the compound has been absorbed by the tumour
cells the site is radiated by a neutron beam.
BNCT
Write a nuclear equation of:
o The neutron capture of B-10
Radiodiagnostics
• Use of a radioactive tracer that is attached to a
biologically active molecule to form a
radiopharmaceutical
• Radiopharmaceutical is ingested or injected
• Traced used detection equipment that uses for instance
gamma rays producing an image on a scan.
• Commonly used tracers:
o Tc-99m (most common)
o I-131. Different traces accumulate in different parts of the
body e.g. I-131 in the thyroid gland which is why it is used in
the diagnosis and treatment (higher dose than for diagnostic
purposes) of thyroid cancer.
Tc-99m diagnostic nuclide: Why?
• Emits gamma radiation:
o Easily traced as it emits gamma radiation; detected using Xrays.
o As source of the emission is inside body can only be detected
if the radiation can escape the body which gamma rays can.
o Gamma = low energy radiation so less hazardous to the
patient: only affected by a low dose.
• Versatile and easily administered:
o Can be used to diagnose and treat cancer in different organs and tissues
as it can bind onto a number of different biological carrier. Each type of
tissue has its own biologically active molecule that accumulates there.
o Can easily be administered to specific areas in the body.
• Patient is only exposed to a minimum amount of radiation:
half life is 6 hours and most of Tc-99m will have decayed
after 2 days.
• Short biological half life; leaves body after 1 day.
• Beta radiation: low energy electrons.
Lu-177
• Emits strong beta radiation
• Also emits gamma radiation that can be
detected outside the body and can therefore
be used for imaging.
Y-90
• Emits strong beta radiation
• Also emits gamma radiation that can be
detected outside the body and can therefore
be used for imaging.
• Short half life of 64 hours
Activity or rate of decay
Rate of decay is the number of
nuclides that decay or emit
radiation per second. The unit is
Bequerel or Bq.
Half life, t1/2
• Time it takes for :
o Half the initial amount or concentration of a radionuclide to
decay.
o Activity of rate of decay to half
• Dependent on nature of nuclide
• Half life remains constant
• Independent of :
o Concentration or starting amount of nuclide
o Temperature
o Pressure
• The longer the half- life, the more slowly a radionuclide decays,
the lower the activity (inverse relationship between half life and
activity), the lower the doses of radiation emitted.
• As radioactive decay only involves 1 reacting species it follows
first order reaction kinetics i.e. rate = k[N].
Decay constant
The expression λ = ln 2/ t1/2 also indicates that:
• Decay constant is independent of:
o concentration or starting amount of the nuclide
o Temperature
o Pressure
o Dependent on nature of substance
o Decay constant remains constant throughout the
decay process.
Calculating decayed/remaining amount
• Nt = amount/concentration/activity after any
given time t
• N0 = initial amount (mol or
particles/concentration/activity)
• t = any given time
• t1/2 = half life
Medical considerations
Long half life means low value for decay
constant and low value for activity. Nuclides
used in diagnosis or therapy should have shorter
half lives so they do not remain radioactive for
too long but not too short either as otherwise
patient will receive high dose of radiation. Long
half life is also not ideal as often nuclides remain
in the body for a while and will make patient be
exposed to radiation for a longer period of time.
MRI
• Magnetic resonance imaging.
• MRI uses the same principles as NMR as the scans use
very powerful magnets to detect not just H-1 but also
C-13, Na-23, He-3 and P-31 nuclei and also use low
frequency radio waves that are not ionizing therefore
no known impact on human tissue – non-invasive.
• Protons in different tissues absorb radiowaves of
different frequencies e.g. cancerous tissue; computers
identify the type of tissue a proton is in.
• The radiowaves absorbed by the nuclei are detected
and used by computers to identify the type of tissue
the proton is in and produce 2D or 3D images of
internal organs or body parts.