Radioisotopes – MBBS First Year (Biophysics / Biochemistry)

1. Basic Concepts

An atom consists of a nucleus (protons + neutrons) surrounded by orbiting electrons. The atomic number (Z) = number of protons; the mass number (A) = protons + neutrons. Isotopes are nuclides of the same element (same Z) but different mass numbers (different neutron number). Radioisotopes (radionuclides) are isotopes with an unstable nucleus that spontaneously disintegrates, emitting radiation, to reach a more stable configuration - Tietz Textbook of Laboratory Medicine, 7th Ed.

Radioactive decay is a fixed property of each nuclide - the rate is unaffected by temperature, pressure, or chemical state, unlike ordinary chemical reactions.

2. Types of Radioactive Decay

Type	Mechanism	Particle/Radiation	Penetration/Hazard
Alpha (α) decay	Heavy nuclei (Z > 70) emit a 2-proton, 2-neutron fragment (helium nucleus)	α-particle	Large mass, stopped by paper, but causes heavy local damage - hazardous if internalized
Beta-minus (β⁻) decay	A neutron converts to a proton + electron + antineutrino	β⁻ particle (electron)	Penetrates paper/cardboard, stopped by metal sheet. e.g., ³H, ¹⁴C, ³²P, ¹³¹I
Beta-plus (β⁺)/Positron decay	A proton converts to a neutron + positron	Positron	Annihilates with an electron producing two 0.511 MeV gamma photons (basis of PET)
Electron capture	Nucleus "absorbs" an orbital electron, converting a proton to a neutron	Characteristic x-rays	e.g., ¹²⁵I decays to ¹²⁵Te
Gamma (γ) emission	High-energy electromagnetic radiation released from an excited nucleus, often accompanying other decay modes	γ-photon (like x-ray)	High penetration, hardest to shield, most useful for external imaging (e.g., ¹³¹I, Tc-99m)

(Source: Tietz Textbook of Laboratory Medicine, 7th Ed., "Basic Concepts")

3. Units of Radioactivity

Becquerel (Bq) - SI unit; 1 Bq = 1 disintegration per second.
Curie (Ci) - older unit; 1 Ci = 3.7 × 10¹⁰ disintegrations per second. (Kaplan & Sadock's Comprehensive Textbook of Psychiatry)

4. Half-Life

Physical (radioactive) half-life - time taken for a radioisotope's activity to fall to half its original value; a fixed characteristic of each isotope.
Biological half-life - time for the body to eliminate half the administered dose of a substance (via excretion/metabolism), independent of physical decay.
Effective half-life - the net half-life observed in the body, combining both physical decay and biological elimination; calculated as the reciprocal of the sum of the physical and biological decay constants. This is the practically important parameter for radiation dosimetry in patients.

Examples of commonly used isotope half-lives: Tc-99m ≈ 6 hours, I-131 ≈ 8 days, Mo-99 ≈ 66 hours, Co-60 ≈ 5.3 years, C-14 ≈ 5,730 years, P-32 ≈ 14 days, ³H (tritium) ≈ 12.3 years.

5. Uses of Radioisotopes in Medicine

A. Diagnostic (Tracer technique) The classic principle: a radioisotope behaves chemically like its stable counterpart, so it can be introduced into the body and tracked by the radiation it emits, without disturbing the biological process being studied. Harper's Illustrated Biochemistry notes that the post-WWII availability of radioisotopes (¹⁴C, ³H, ³²P) as "tracers" was pivotal in identifying metabolic intermediates and pathways (e.g., glycolysis, urea cycle, protein synthesis).

Clinical diagnostic applications include:

Thyroid scanning/uptake studies - I-131 or Tc-99m
Bone scans, renal scans (renography), HIDA (hepatobiliary) scans, cardiac perfusion (myocardial) imaging using Tc-99m-labeled compounds
Radioimmunoassay (RIA) - hormone/drug estimation using radiolabeled antigens
PET scanning - positron-emitting isotopes (e.g., F-18) for metabolic/functional imaging
Gastric emptying and intestinal transit studies via scintigraphy (Yamada's Textbook of Gastroenterology)
CSF leak detection - radioisotope cisternography with indium-111 (Bradley and Daroff's Neurology)

B. Therapeutic

I-131 - treatment of hyperthyroidism (Graves' disease) and differentiated thyroid cancer; the destructive β-particles act locally on thyroid follicular cells while γ-radiation allows external monitoring (Goodman & Gilman's Pharmacological Basis of Therapeutics)
P-32 - used in some hematologic conditions and malignant effusions
Cobalt-60 - external beam teletherapy for cancer
Radioimmunotherapy - monoclonal antibodies labeled with isotopes (e.g., Pb-212) for targeted cancer therapy

6. Radiation Hazards and Safety

Because ionizing radiation damages tissue (via ionization of cellular molecules, particularly DNA), radioisotope use requires precautions: minimal effective dose, shielding appropriate to radiation type (paper/lead for α and β, thick lead/concrete for γ), time limitation of exposure, distance maximization, and monitoring of personnel (dosimetry badges). Forensic and lab-medicine texts note that β radiation, though absorbed by superficial tissue, can still injure handlers (e.g., during autopsy/dissection) if isotopes were used diagnostically before death (Parikh's Textbook of Medical Jurisprudence).

Quick Summary Table for Exams

Isotope	Type of Emission	Half-life	Main Use
Tc-99m	γ	6 hours	Most common diagnostic scanning isotope (bone, renal, cardiac, HIDA)
I-131	β, γ	8 days	Thyroid diagnosis and therapy (hyperthyroidism, thyroid cancer)
Co-60	γ	5.3 years	External radiotherapy
P-32	β	14 days	Therapy for polycythemia, malignant effusions
C-14, ³H	β	Long	Biochemical tracer studies (metabolic pathway research)
F-18	β⁺ (positron)	~110 min	PET imaging

This covers the standard MBBS first-year Biophysics/Biochemistry syllabus scope on radioisotopes: definition, decay types, units, half-life concepts, and medical (diagnostic + therapeutic) applications. If your specific curriculum wants more mathematical detail (decay equations, dosimetry calculations) or deeper coverage of a particular isotope's clinical use, let me know and I can expand that section.

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