Nuclear Medicine and Radiation Therapy Fundamentals
This domain tests radiopharmaceutical basics, gamma camera and PET/SPECT principles, decay, and the distinction between diagnostic nuclear medicine and radiation therapy.
Core rules
- Radiopharmaceuticals localize by physiology or metabolism. The gamma camera detects emitted photons; SPECT rotates detectors; PET detects annihilation photons.
- Half-life determines activity over time. Decay calculations use activity remaining = initial activity × (1/2)^(time/half-life).
- Patient preparation, uptake time and blocking agents affect image quality and biodistribution.
- Radiation therapy uses controlled high doses to treat disease. External beam delivers photons or particles; brachytherapy places sources near or inside tissue.
- Fractionation spares normal tissue while allowing tumor control. Therapy doses are much higher than diagnostic doses and require strict shielding and verification.
Worked example
A technetium-99m source has a 6-hour half-life. After 12 hours, two half-lives have passed, so activity is 1/2 × 1/2 = 1/4 of the original. The reasoning is exponential decay, not linear subtraction. If the original activity was 800 MBq, 200 MBq remains.
Traps
Do not apply diagnostic imaging assumptions to therapy doses. Do not subtract half-life linearly. Do not confuse PET with SPECT. Do not ignore pregnancy, breastfeeding or patient preparation instructions. Do not treat a therapy source as an imaging source.