Biological effects of radiation

How ionising radiation damages cells

• X-rays and gamma rays act mostly indirectly: they ionise water (radiolysis), producing free radicals (especially hydroxyl) that cause DNA double-strand breaks; cells die when they next try to divide (mitotic death).
• Oxygen fixes radical damage, so hypoxic cells are 2–3 times more resistant. Anaemia is corrected before radiotherapy for cervical cancer for this reason.
• Rule of radiosensitivity: cells are most sensitive when they divide often and are poorly differentiated.

• Radiosensitive tumours follow the same rule: seminoma (and ovarian dysgerminoma) and lymphomas are the most radiosensitive; melanoma and sarcomas are resistant.
• Lymphocytes fall within hours, so the 48-hour lymphocyte count estimates dose. At moderate whole-body doses the bone marrow fails first (the main cause of death), then gut, then CNS at very high doses.
• Pelvic radiotherapy and the ovary: oocytes in primordial follicles are destroyed (under 2 Gy kills half). With no follicles there is no oestradiol or inhibin, so FSH rises: premature ovarian insufficiency with hot flushes and amenorrhoea. The stroma is not the target.
• Pelvic radiotherapy also kills crypt stem cells (acute diarrhoea); late effects come from endarteritis and fibrosis (strictures, fistulae).
• Ultraviolet light is non-ionising but forms thymine dimers, causing basal cell, squamous cell carcinoma and melanoma; defective repair in xeroderma pigmentosum.

Deterministic versus stochastic effects

• Radiation-induced cancers: overall the commonest is leukaemia (latent period about 5–10 years); solid tumours follow after 10 years or more. The commonest radiation-induced brain tumour is meningioma; papillary thyroid cancer follows childhood neck irradiation and breast cancer follows mantle radiotherapy. These are late effects.

Units and protection

• Gray (Gy): absorbed dose, joules per kg. Sievert (Sv): dose weighted for radiation type and tissue sensitivity, used for protection. Becquerel: decays per second.
• Typical doses: chest X-ray about 0.02 mSv; CT abdomen and pelvis about 10 mSv; natural background 2–3 mSv a year, mostly radon.
• Principles: justification, optimisation (ALARA) and dose limits (staff and public, never patients). Time, distance, shielding; dose falls with the inverse square of distance.
• Limits: workers 20 mSv a year; public 1 mSv; once pregnancy is declared, the fetus is limited to 1 mSv for the rest of pregnancy.

Radiation in pregnancy

• Malformations need a fetal dose above about 100–200 mGy; no increase is seen below 50 mGy. A chest X-ray gives under 0.01 mGy; a CT abdomen and pelvis usually under 50 mGy. Never withhold a needed film.
• The classical ten-day rule kept elective pelvic X-rays to the first 10 days of the cycle.

Ultrasound bioeffects

• Effects are thermal and mechanical, never ionising.
• Thermal index estimates temperature rise; it is highest with pulsed Doppler, which concentrates energy on one spot.
• Mechanical index reflects peak negative pressure and the likelihood of cavitation of gas bubbles, greatest where gas is present (lung, bowel, contrast microbubbles); the fetus has no gas.
• Early pregnancy: the embryonic heart rate is recorded with M-mode (motion along one scan line against time), which deposits much less energy than pulsed Doppler. Pulsed Doppler is not banned but is avoided when M-mode suffices.
• Maternal hyperthermia in weeks 3–4 is linked with neural tube defects.

MRI safety

• MRI uses no ionising radiation and is safe in pregnancy. Gadolinium crosses the placenta, is excreted into amniotic fluid and recirculates, so it is avoided unless essential.

Heat and cold

• Heat loss routes: radiation (the largest in a naked patient or baby, to any colder surface, even when the surrounding air is warm), convection (draughts), evaporation (wet skin, sweating, open wounds), conduction (contact with a cold table or scale). A naked preterm baby in a single-walled incubator beside a cold window loses heat by radiation to the cold walls; double-walled incubators and avoiding windows prevent this.
• Hypothermia slows clotting enzymes and platelets; tests run at 37 °C look normal, so warm blood and fluids in massive haemorrhage.
• Malignant hyperthermia: autosomal dominant RYR1 (ryanodine receptor) mutation. Volatile agents (halothane and others) and suxamethonium trigger uncontrolled calcium release from the sarcoplasmic reticulum: rigidity, rising end-tidal CO2, hyperthermia. Treat with dantrolene; susceptible patients receive total intravenous anaesthesia (propofol) or nitrous oxide, which are safe.
• Diathermy: a partly detached return plate concentrates current density and burns the skin.
• Cryotherapy: below about −20 °C, intracellular ice ruptures cells; a freeze–thaw–freeze cycle deepens destruction.
• Lasers: non-ionising. Nd:YAG (1064 nm) reaches the retina; CO2 (10,600 nm) burns the cornea. Laser vaporisation of warts produces a plume carrying viable HPV DNA, so a smoke evacuator and filtration masks are used.

Sound

• Noise-induced hearing loss above about 85 dB(A): outer hair cells are lost first, with a notch at 4 kHz; permanent sensorineural loss.

Remember Dividing, undifferentiated cells are the ones radiation kills: marrow, gut crypts, gonads, lens and fetal brain.

Exam trap Diagnostic imaging rarely gives the fetus more than 50 mGy; malformation needs 100–200 mGy, so a single CT is not a reason for termination.

Exam trap MRI noise comes from the gradient coils, and the danger to metal clips from the static field, which is never switched off.

How FCPS asks about this

• Most radiosensitive tissue: bone marrow (lymphocytes fall first); most radiosensitive tumour: seminoma.
• Mechanism of radiation injury: free radicals from radiolysis of water.
• Commonest radiation-induced cancer: leukaemia; brain tumour: meningioma.
• Amenorrhoea and high FSH after pelvic radiotherapy: loss of oocytes in primordial follicles.
• Mechanical index: likelihood of cavitation.
• Naked patient losing most heat: radiation; lying on a cold table adds conduction.
• Malignant hyperthermia: RYR1 defect triggered by volatile agents and suxamethonium; treat with dantrolene.