Biological and Medical Uses of Radiation: MDCAT Physics notes
Biological and Medical Uses of Radiation MDCAT notes: rad, gray, rem, sievert, RBE, somatic vs genetic effects, background dose and radioisotope uses.
Ionizing radiation and dose
Radiation is classified as ionizing (alpha, beta, gamma, X-rays, neutrons) or non-ionizing (radio, microwaves, visible light). Ionizing radiation damages living cells by breaking molecules.
| Quantity | Meaning | SI unit | Older unit |
|---|---|---|---|
| Absorbed dose $D$ | Energy absorbed per kg | gray (Gy) = 1 J kg$^{-1}$ | rad = 0.01 J kg$^{-1}$ = 0.01 Gy |
| Equivalent dose $H$ | Absorbed dose $\times$ RBE | sievert (Sv) | rem = rad $\times$ RBE |
RBE (relative biological effectiveness) compares how damaging a radiation is: about 1 for X-rays, gamma and beta; about 10 for slow neutrons; up to about 20 for alpha particles and fast neutrons. So 1 Sv = 1 Gy $\times$ RBE and 1 rem = 0.01 Gy $\times$ RBE = 0.01 Sv.
Worked example
A 60 kg man receives 200 rem from radiation of RBE 10. Absorbed dose = 200/10 = 20 rad = 0.20 J kg$^{-1}$. Energy absorbed = $0.20\times60=12$ J. Notice how small the energy is compared with the biological harm.
Biological effects
- Somatic effects appear in the irradiated person: skin burns, loss of hair, drop in white blood cells, sterility, cataract, induction of cancer.
- Genetic effects are damage to genes or chromosomes of reproductive cells, passed on to offspring: gene mutation.
Natural background radiation (cosmic rays, rocks, radon, food) gives each person roughly 1 mSv per year (textbook value). Protection: keep distance, reduce exposure time, use shielding such as lead.
Medical and other uses of radioisotopes
| Radioisotope | Use |
|---|---|
| Iodine-131 | Diagnosis and treatment of thyroid cancer (collects in the thyroid) |
| Sodium-24 | Tracing blood circulation, locating blockages |
| Cobalt-60 | Gamma-ray radiotherapy of tumours; sterilizing instruments |
| Phosphorus-32 | Tracer in plants (fertilizer uptake); some skin/blood treatments |
| Carbon-14 | Radiocarbon dating |
| Technetium-99m | Medical imaging |
| Americium-241 (alpha) | Smoke detectors |
- Tracers: a small amount of radioisotope is followed with a detector, e.g. to find leaks in underground water pipes.
- Gamma camera: images the gamma rays from radioisotopes concentrated in an organ.
- Smoke detectors: alpha particles ionize air, giving a small current; smoke absorbs the ions and the current drops, triggering the alarm.
Key formulas
- $D=\dfrac{E}{m}$ (Gy = J kg$^{-1}$)
- $H=D\times\mathrm{RBE}$
- 1 rad = 0.01 Gy; 1 rem = 0.01 Sv
Common MDCAT traps
- Gene mutation is genetic, not somatic.
- Gray is for absorbed dose; sievert is for equivalent dose.
- To get energy from rem: divide by RBE, convert rad to J kg$^{-1}$, then multiply by mass.
- Thyroid cancer: iodine-131, not cobalt-60 or strontium-90.
Quick revision
- 1 Gy = 1 J kg$^{-1}$; 1 rad = 0.01 J kg$^{-1}$.
- Sodium-24 traces blood flow.
- Smoke detectors use alpha emitters.
- Background dose about 1 mSv per year.
- Skin burns and hair loss are somatic effects.