Radiopharmaceuticals

Question 1

What is the ideal decay mode for radionuclide therapy and why?

Answer 1

Any decay via charged particle emission: alpha or beta.

Need high radiation dose to biologically targeted areas e.g. tumours

Question 2

What are the ideal characteristics of a diagnostic radiopharmaceutical and why?

Answer 2

• Low radiation dose (minimise patient dose while preserving diagnostic quality of image; too low photon energy = attenuated by the body, contribute to dose; too high photon energy = escape the body but poor detection efficiency due to septal penetration)
• High target/nontarget activity (detecting/evaluating lesions depends on concentration of radpharm. in the organ/tissue/lesion of interest; max concentration in target tissue with min update in nontargets improves contrast and therefore ability to detect abnormalities)
• Safety, convenience, cost-effectiveness (low chemical toxicity; use high-specific-activity and carrier-free radionuclides; minimises amount of isotope required; chemical form and pH that facilitate rapid complexing with the pharmaceutical; radpharm should be stable and have clinically compatible shelf life)

Question 3

State the properties that make a radionuclide suitable for using in a radiotracer

Answer 3

• high enough photon energy to exit the body…
• …but low enough to be detected (100-500keV)
• half-life of a few hours
• ‘clean’ photon-emission decay (no alpha or beta particles which add radiation dose)

Question 4

What is a radiotracer?

Answer 4

Ligand + radionuclide

Question 5

What properties must a radiotracer have?

Answer 5

• Suitable biodistribution
• suitable clearance
• be considered safe in ‘trace’ amounts

e.g. 99m-Tc-labelled sestamibi for myocardial blood perfusion imaging

Question 6

Name some common gamma emitters

Answer 6

• Tc-99m
• Indium-111, 113m
• Iodine-123, 125, 131
• Gallium-67
• Thallium-201
• Selenium-75
• Xenon-133
• Strontium-87m
• Chromium-51
• Mercury-197

Question 7

Name some common** positron emitters**

Answer 7

• Carbon-11
• Nitrogen-13
• Oxygen-15
• Fluorine-18
• Copper-64
• Gallium-68
• Arsenic-72
• Bromine-76
• Rubidium-82
• Iodine-122

Question 8

How do charged particles deposit energy in matter?

Answer 8

by scattering, i.e. electromagnetic interactions with atomic electrons in the medium through which they are travelling (many atoms along the particle track are ionised)

alpha and beta

Question 9

What does a charged particle’s “range” in matter depend on?

Answer 9

• Their energy
• The material’s characteristics (e.g. Z, density)