Radiopharmaceuticals

Radioactivity is the spontaneous process by which certain types of matter emit energy and subatomic particles. This fundamental attribute of the atomic nucleus drives the field of radiopharmaceuticals, which utilizes these emissions for diagnostic imaging and therapy.

What Makes an Atom Radioactive?

Atoms are composed of stable and unstable nuclei.

A stable nuclide possesses the appropriate energy levels and nuclear composition to remain unchanged.
A radioisotope or radionuclide is an unstable nucleus that undergoes spontaneous nuclear decay to reach a more stable configuration.

This decomposition involves the emission of various forms of radiation, including:

Alpha rays ( $\alpha$ )
Beta rays ( $\beta$ )
Gamma rays ( $\gamma$ )

Radioactive elements naturally occur in three main series: uranium, thorium, and actinium.

Radioactive Decay Rate: Half-Life

The rate at which a radioactive element decays is measured by its half-life ( $t_{1/2}$ ). This is the time required for one-half of any given quantity of the element to decay. Half-lives can range drastically, from fractions of a second to billions of years.

The by-products of radioactive decay are called daughter isotopes. These daughters may themselves be unstable and continue to disintegrate until a stable nuclide is finally produced.

🧪 Radiopharmaceuticals in Medicine

Radiopharmaceuticals are medicinal products containing a radionuclide. They are categorized into:

Radiopharmaceutical precursor
Radiopharmaceutical preparations
Kit for radiopharmaceutical preparation
Radionuclide generator

Storage and Safety

Due to the risks of ionizing radiation, radiopharmaceuticals must be stored in a special area and kept in well-closed containers. Protection from ionizing radiation must strictly follow national regulations, and the dose rate must be kept below the maximum tolerated level. Vials are often made of transparent glass for visual inspection, although prolonged radiation exposure can cause the glass to darken.

📏 Measurement of Radioactivity and Dose

Radioactivity, first discovered by Henri Becquerel in 1896, is measured in:

Becquerels ( $\text{Bq}$ ): The $\text{SI}$ unit of radioactivity, representing one disintegration per second ( $\text{dps}$ ).

When radiation interacts with matter, it transfers energy, which can cause biological effects. The magnitude and severity of injury depend on the radiation dose and type.

Gray (Gy): The unit for absorbed dose.
$1 \text{ Gy} = 1 \text{ joule absorbed per kg of matter}$
Sievert ( $\text{Sv}$ ): The unit for equivalent dose. This concept accounts for the different biological effects caused by different types of ionizing radiation at the same absorbed dose, allowing for a standardized risk assessment. For example, a single chest $\text{X}$ -ray might expose a patient to $0.14 \text{ millisieverts} (\text{mSv})$ .

✨ Properties of Alpha, Beta, and Gamma Radiation

The three main types of radiation emitted during decay have distinct properties that dictate their potential medical and safety applications.

Property	Alpha Rays (α)	Beta Rays (β)	Gamma Rays (γ)
Identity	Helium nucleus ( $^4_2\text{He}$ ), two protons and two neutrons	High-energy electron ( $\text{negatron}$ ) or positron ( $\text{positron}$ )	High-energy electromagnetic wave (similar to $\text{X}$ -rays)
Charge	Positive ( $+2$ )	Negative ( $\text{negatron}$ ) or Positive ( $\text{positron}$ )	Neutral (massless and chargeless)
Mass	Heavy (4 $\text{amu}$ )	Very small	Massless
Penetration Power	Low. Cannot penetrate tissue/skin.	Medium. Greater than $\alpha$ , less than $\gamma$ .	High. Most penetrating.
Ionizing Power	High (due to charge and mass).	Weaker than $\alpha$ .	Low.
Biological Use	Not used externally; confined to heavy metals.	Used in some therapeutic applications.	Widely used in imaging (e.g., SPECT) and therapy.

Alpha particles have a low penetrating power, meaning they do not penetrate tissue and are not useful in biological applications unless introduced internally (e.g., in targeted alpha therapy).

Beta particles have a greater penetration power than alpha rays and are often associated with gamma radiation.

Gamma rays have the greatest penetrating power. They are massless, chargeless, and generated by nuclear disintegration. Their low ionizing power and high penetration make them useful for external medical procedures.