Radiopharmacy Flashcards
Radionuclide
radioactive atom
Radiochemical
radionuclide bound to a chemical ligand (radiolabelling)
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
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
Radioactive decay types
- Internal conversion (e-)
- gamma ray
- a-particle
- b+/b-
- Electron capture (xray)
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
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
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
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)
Radioactive decay • Electron capture
– Emits characteristic x‐ray
Radioactive decay
• Spontaneous fission
– Neutrons
– Gamma photon
-for energy generation processes
which ones are most interesting for radio pharmacy
alpha, beta and gamma
Calculation of radioactivity
• Each radionuclide has a characteristic decay constant
• Governed by equation
At = Aoe‐t
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
Commonly used radionuclides and half life
-Technetium-99m : 6hr (99mTc)
-Indium-113m : 100m
(113mIn)
-Krypton-81m : 13s (81mKr)
-Gallium-68 : 68m (68Ga)
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
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 =
