Radiopharmacy and Radiopharmaceuticals
Core concepts
Radiopharmacy questions focus on producing a safe, pure, correctly assayed dose. The Mo-99/Tc-99m generator is eluted with saline to yield pertechnetate; know that Mo-99 breakthrough and aluminum breakthrough must be checked, and that eluate quality affects labeling. A cold kit contains nonradioactive ingredients; after adding pertechnetate, radiochemical purity is verified because unbound pertechnetate or hydrolyzed reduced technetium changes biodistribution. Distinguish radionuclidic purity, radiochemical purity, chemical purity, and specific activity. Dose calibrators measure activity; decay correction uses A = A0e^(−0.693t/T½). Always account for calibration time, assay time, and administration time.
Traps
Common errors include using the wrong half-life, forgetting that Tc-99m decays to half its activity every 6 hours, and confusing generator eluate with a final labeled product. Do not assume a kit is usable just because it is within its shelf life; follow labeling, storage, and purity limits. FDG and other PET agents require attention to expiry, assay, and patient preparation.
Worked example
A Tc-99m dose is 800 MBq at 08:00. At 20:00, 12 hours later, two half-lives have passed, so activity is 800 ÷ 2² = 200 MBq. If the prescribed dose is 200 MBq, the 08:00 assay is appropriate for a 20:00 administration only if the entire vial were used; in practice, draw and assay the patient dose at the correct time. Rule of thumb: for Tc-99m, each 6 hours halves the activity.