Nuclear Chemistry & Radiation in Medicine

Nuclear Chemistry: Beyond the Electron Shell

Nuclear chemistry deals with changes in the nucleus — protons and neutrons — rather than electron rearrangements. These processes release energy orders of magnitude greater than chemical reactions and produce ionising radiation with direct clinical applications in diagnosis and therapy.

Types of Radiation

Alpha (α) particles: helium-4 nuclei (2 protons + 2 neutrons). High mass, +2 charge. Stopped by a sheet of paper or 5 cm of air. Cannot penetrate skin. However, if an alpha emitter is ingested or inhaled (e.g., radon gas, polonium-210), it causes intense local DNA damage. Low external hazard; high internal hazard.

Beta (β) particles: high-energy electrons (β⁻) or positrons (β⁺) emitted from the nucleus. Penetrate a few metres of air; stopped by a few mm of aluminium. β⁺ emission is the basis of PET scanning. Therapeutic beta emitters (¹³¹I, ⁹⁰Y-DOTATATE) deliver local radiation to tumours.

Gamma (γ) rays: high-energy electromagnetic radiation (photons). No mass, no charge. Penetrate deeply — require lead or thick concrete shielding. Used in external beam radiotherapy and emitted by technetium-99m (the most widely used diagnostic radioisotope in NZ).

Radioactive Decay and Half-Life

Radioactive decay follows first-order kinetics:

N(t) = N₀ × e^(−λt)

The half-life (t½) is the time for half the atoms to decay: t½ = ln 2 / λ = 0.693 / λ

Clinical relevance: ⁹⁹ᵐTc has t½ = 6 hours — short enough to minimise patient dose, long enough for the imaging procedure. ¹³¹I has t½ = 8 days — useful for thyroid ablation and differentiated thyroid cancer treatment; patients require radiation precautions.

PET and SPECT Imaging

PET (Positron Emission Tomography): uses β⁺ emitters (¹⁸F-FDG, t½ 110 min). The positron annihilates with an electron, producing two 511 keV gamma rays at 180° — detected in coincidence to produce 3D images of metabolic activity. Widely used in oncology staging, neurology (Alzheimer's disease), and cardiology.

SPECT (Single Photon Emission Computed Tomography): uses gamma emitters (⁹⁹ᵐTc). A rotating gamma camera detects emitted photons directly. Lower resolution than PET but cheaper and more widely available. Used for bone scans (metastases), myocardial perfusion, renal function (MAG3 scan).

Radiation Therapy

External beam radiotherapy uses linear accelerators (LINAC) producing high-energy X-rays or electron beams. Radiation causes DNA double-strand breaks, leading to cell death preferentially in rapidly dividing tumour cells. Multiple fractions allow normal tissue repair between sessions.

Brachytherapy: radioactive seeds (e.g., ¹²⁵I for prostate cancer) implanted directly in or near the tumour.

Targeted radionuclide therapy: e.g., Lutetium-177 DOTATATE (Lu-PRRT) for neuroendocrine tumours available in NZ — combines a somatostatin analogue targeting vector with a β⁻ emitter.