Radiopharmaceutics (3) Flashcards
Why do we need to think about radioactive decay
- Radiopharmaceuticals are constantly decaying away
- By the time we inject them into patients there will be less available than when we made them
- We need to factor this in when we manufacture radiopharmaceuticals
- It needs to be factored in when placing orders
- We need to be able to deal with delays
Radioactive decay graphs
- Must Natural log
Application of the equation
- LnAt = LnA0 +/- Gamma x t
- Where Gamma = 0.693 (Ln2) / t1/2
- t1/2 = half life of isotope
- For Tc99m, t1/2 = 6 hours
- Gamma = 0.693 / 6 = 0.1155
Worked example 1
Activity in vial = 6000 MBq at 8am
What will the activity be in vial at 10am
Worked example 2
It is 8am. You need to draw up a 400 MBq patient dose for administration at 10am. How much is needed in the syringe at 8am
Worked example
Patient is given a bone scan appointment for 10am and arrives 1 hour late at 11am. Injected dose is 500MBq-600MBq. Activity in the syringe calibrated as 550MBq at 10a. Can this dose still be used
Introducing the reference date
- When buying stock isotopes it is not logistically possible for the manufacturer to send out individually tailored amounts of activity on a daily basis
- A batch is therefore made which will be a certain activity in MBq (the reference activity) on a stated date. This date is the reference
- E.g. Cr-51 37MBq in 1mL on Mar 1st. Any product bought before that date will contain more than the reference activit. After 1st march there will be less
Worked example 4
A dose thallium-201 solution for injection is needed for teusaday 9th feb. The injected dose is 80MBq. The vials come in the following sizes
37,74,111,185MBq
What size vial is needed for the patient
How much activity would the 37MBq vial give you on teusday
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How much activity would the 74 MBq vial give you on the teusday
How much activity would the 350MBq capsule give you on the Wednesday
How much would the 400MBq capsule give you on the Wednesday
