Radiopharmaceuticals and Radiochemistry
Generator and product quality
Tc-99m is eluted from a Mo-99/Tc-99m generator; Mo-99 breakthrough and aluminum breakthrough are tested before use. Radiopharmaceutical quality has several distinct dimensions: radionuclidic purity (fraction of activity in the desired radionuclide), radiochemical purity (fraction in the desired chemical form), chemical purity (absence of interfering chemicals), and specific activity (activity per mass). Traps: confusing these terms; forgetting that eluate can contain free pertechnetate, reduced-hydrolyzed technetium, or oxidant; and ignoring that particle-based agents such as MAA require careful sizing and counting.
Labeling and biodistribution
Understand the chemistry that determines biodistribution: MDP binds bone mineral; sulfur colloid is phagocytosed by reticuloendothelial cells; MAA is trapped in pulmonary capillaries; MAG3 and DTPA are cleared by glomerular filtration or tubular secretion; HIDA is hepatocyte-transported; iodine and pertechnetate are trapped by thyroid and salivary tissue. For PET agents, FDG follows glucose transport and phosphorylation; Ga-68 dotatate binds somatostatin receptors; PSMA agents bind prostate-specific membrane antigen. Radionuclide therapy agents include I-131, Lu-177 dotatate, Lu-177 PSMA, Y-90 microspheres, and Ra-223.
Worked example
A kit has 95% radiochemical purity by chromatography and 2% free pertechnetate. If the product is MAA, free pertechnetate raises thyroid and stomach uptake and lowers lung uptake; it is not merely a cosmetic impurity. Rule of thumb: always match the impurity to the expected artifact-free pertechnetate causes thyroid, salivary, and stomach uptake; reduced-hydrolyzed Tc causes liver or spleen uptake; aluminum can alter bone or colloid behavior.