- end product - Radioactive material in a form suitable for administration to a human for the purpose of therapy or diagnostic investigation - may be an injection, gas or capsule

Radiopharmacy Flashcards by Em A

Radionuclide

radioactive atom

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Radiochemical

radionuclide bound to a chemical ligand (radiolabelling)

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Radiopharmaceutical

end product
Radioactive material in a form suitable for administration to a human for the purpose of therapy or diagnostic investigation
may be an injection, gas or capsule

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Definition of units

• Each radionuclide emits radio activity with a characteristic energy
– Expressed in electron volts (eV)
• Radio activity measured as a function of disintegrations per second (dps)
– SI unit = becquerel (Bq)
– Non‐SI unit – curie (Ci)
– 1Ci = 37MBq
• Absorbed dose measured in grays (Gy)
– 1 Gy = 1 joule of energy absorbed in 1kg of tissue

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Radioactive decay types

Internal conversion (e-)
gamma ray
a-particle
b+/b-
Electron capture (xray)

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Radioactive decay

• a particles (alpha decay)

smallest distance travelled - easiest to stop
– 2 protons, 2 neutrons (He nucleus)
large in size so easy to capture - blocked using lead shielding
– Little penetration
– Very damaging
not ideal

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Radioactive decay

• B‐particles (positive & negative beta decay)

ideal
– Positively charged electron (positron)
– Negatively charged electron
– Range 100 feet in air, completely absorbed in a thickness of several mm to several cm
– High kinetic energy (1000s to 1000000s keV) depends on age of source
-can be captured using lead shielding - thick lead shielding can completely absorb b particles

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Radioactive decay

• gamma‐transmission (gamma rays)

– Result from isometric transition
– Electromagnetic vibrations (gamma photons)
– High energy (100s keV)
– Very penetrating (used to sterilise equipment)
- cannot be stopped - go on to affinity, lead shielding cannot be used
- lower energy than b particles so less damaging

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Radioactive decay

• Internal conversion (e‐)

– Electron drops from higher orbital to lower
– X‐ray emitted
– X‐ray hits higher orbital electron
– Electron expelled out of orbital (Auger electron – eA)

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Radioactive decay • Electron capture

– Emits characteristic x‐ray

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Radioactive decay

• Spontaneous fission

– Neutrons
– Gamma photon
-for energy generation processes

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which ones are most interesting for radio pharmacy

alpha, beta and gamma

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Calculation of radioactivity

• Each radionuclide has a characteristic decay constant
• Governed by equation
At = Aoe‐t

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Radioactive half‐life

• Time taken for a given quantity to decay to half its activity
• Half‐life (t1/2) related to decay
t1/2 = ln2/
ln2 ≈ 0.693

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Commonly used radionuclides and half life

-Technetium-99m : 6hr (99mTc)
-Indium-113m : 100m
(113mIn)
-Krypton-81m : 13s (81mKr)
-Gallium-68 : 68m (68Ga)

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Commonly used Parent and half life

-Molybdenum-99 : 66h (99Mo)
-Tin-113 : 115d (113Sn)
- Rubidium-81 : 4.5h
(81Rb)
-Germanium-68 : 270d
(68Ge)

Properties of an ideal radionuclide

• Diagnostic–only emit gamma‐rays and have activity sufficient for purpose but not greater (should not damage)
• Therapeutic–emit B particles to deposit in target organ (designed to cause damage - used to shrink tissues for surgery)
• T1/2
– Sufficient for preparation, transportation, administration, imaging
– Sufficient to prevent unnecessary radiation remaining in the body

Properties of an ideal radionuclide 2

• Readily available (& cost)
• Easy to formulate into different preparations
- Technetium‐99m (99mTc, Tc‐99m) best meets these properties

Technetium‐99m

99Mo –> (T1/2 = 66h, B- and gamma) 99mTc - unstable

99mTc –> (T1/2 = 6h, gamma) 99Tc - stable

T1/2 = 6.01 hours
Emits gamma‐rays only (great for diagnostics)
Principal photon energy= 140keV (relatively low energy so less damaging)
Abundance= 89% (high yield)

Tc‐99m Generation

Produced using Mo‐99 generator
Mo‐99 produced from fission of U‐235
Passed onto Al203 column
Eluted using 0.9% NaCl
Produces sodium pertechnetate (NaTcO4)

What is elution

passing a liquid through a column to wash radioactivity out of the column

Difference between wet and dry generator

Dry generator is a two port system and doesn’t have a permently conncected source of NaCl

Radio pharmacy Isolator

Type II,

Grade D environment

Shielding

Must be sufficient to absorb a and B particles and attenuate (slow them down) gamma‐rays
Tungsten most commonly used for syringes
Lead glass also available
Lead used to provide barriers

Technique (1)

- creating radiopharmaceutical containing ligand (inert carrier of radioactivity) - calculate volume of elute required - may be a dry powder of liquid - ready for dispensing

Technique (2)

- patient dose withdrawn from vial | - inserted into a dose calibrator: measures amount of radio-activity

Tc‐99m common ligands

• DMSA: used to look at outside area of kidney – Dimercaptosuccinic acid (99mTc Succimer injection BP) – Renal cortex scintigraphy • DTPA: used to look at kidney and lungs – Diethylenetramine‐pentaacetic acid (99mTc Pentetate injection BP) – Renal scintigraphy – Lung ventilation (as aerosol) • MAG3: used to look at how kidneys are working – Mercaptoacetyltriglycine (99mTc Mertiatide injection BP) – Renal dynamic scintigraphy • MDP: used to look at bones – Methylene diphosphonate (99mTc Medronate injection BP) – Skeletal scintgraphy • MAA: looks at issues of getting blood to lungs or liver, MAA can be used – Macroaggregated albumin – Pulmonary perfusion scintigraphy – Liver intra‐arterial scintigraphy ``` • HMPAO: used for brain or white blood cell labelling – Hexamethylpropylene amine oxime (99mTc Exametazine injection BP) – Cerebral perfusion scintigraphy – White blood cell labelling ```

fever of unknown origin

take 50 ml blood sample spin it down to get white blood cells and bind them to HMPAO - re inject into patient to get an image of where infection is

Tc‐99m common doses

* DMSA 75 MBq * DTPA 200 MBq * MAG3 100 MBq * MAA 100 MBq

Radioiodine

• 131I first important clinical radio-pharmaceutical – T1/2 = 8 days – B and gamma emissions (364 keV) – Treatment of hyperthyroidism and differentiated thyroid cancer -131I not used for diagnostic due to beta decay • 123I now preferred for diagnostic purposes – T1/2 = 13.2 hours – gamma emissions only (159keV) – 83% abundance

Tc‐99m Ligands

• Pertechnetate (TcO4‐) has highest possible oxidation state for Tc – Valency of +7 • Will not bind to ligand so need reducing agent • Stannous salts most commonly used (e.g. SnCl2)

What is the decay constant for Tc‐99m? T1/2 = 6 hours ln2 ≈ 0.693

``` t1/2 = ln2/ v v = In2/ t1/2 ``` 0.116

What is the decay factor for 1 hour? What is the decay factor for 2 hours?

e-vt e-0.116 x 1 = 0.891 e-0.116 x 2 = 0.794

Volume calculations It is 8:30am. You have eluted your Tc‐99m generator and have obtained 10mL of eluate with activity of 300 MBq/mL. How much should you draw up to ensure a dose of 75 MBq at 10:30am?

75/ 0.794 = 94.4 MBq 94.4 / 300 = 0.315 dose / decay factor = answer answer / activity =