Radiation protection
Radiation biology and protection; burns, chemical, heat and contrast injury
Radiation, burns and chemical injury for FCPS Part 1: DNA damage, radiosensitivity, dose units, protection, burns, heat loss and poisons. Each is explained by its mechanism.
How radiation damages cells
- Ionising radiation breaks DNA directly, or indirectly by splitting water into hydroxyl radicals (most X-ray damage). Double-strand breaks kill cells, mostly at the next mitosis. Like ischaemia and inflammation, radiation injures through free radicals.
- Oxygen fixes the damage, so hypoxic tumour cells resist. Cells are most sensitive in G2/M, most resistant in late S phase.
- Law of Bergonié and Tribondeau: sensitivity rises with frequent division and poor differentiation. Small lymphocytes are an exception (they die by apoptosis without dividing).
| Most sensitive | Intermediate | Least sensitive |
|---|---|---|
| Lymphocytes, bone marrow, intestinal crypts, spermatogonia, oocytes, lens | Skin, endothelium, growing cartilage | Neurons, muscle, mature bone |
- Whole-body irradiation is the classic cause of bone marrow suppression.
- Most radiosensitive tumours: seminoma (radiotherapy for a painless testicular seminoma), dysgerminoma, lymphoma.
- Fractionation lets normal tissue repair between doses while tumours reoxygenate.
- In utero: first 2 weeks all-or-none; weeks 3–8 malformations; weeks 8–15 microcephaly and intellectual disability.
- Ultraviolet light is non-ionising; UVB forms pyrimidine (thymine) dimers, repaired by nucleotide excision repair (defective in xeroderma pigmentosum). Risks: basal cell carcinoma, squamous cell carcinoma, melanoma, actinic keratosis, cataract.
Deterministic and stochastic effects
| Feature | Deterministic (tissue reaction) | Stochastic |
|---|---|---|
| Threshold | Yes | No |
| Dose raises | Severity | Probability |
| Examples | Skin erythema after long fluoroscopy, posterior subcapsular cataract, sterility, acute radiation syndrome | Cancer, heritable effects |
Units, doses and limits
| Quantity | Unit | Meaning |
|---|---|---|
| Activity | Becquerel | Decays per second |
| Absorbed dose | Gray (J/kg) | Energy deposited |
| Effective dose | Sievert | Weighted for radiation type and organ sensitivity |
- Chest radiograph about 0.02 mSv; CT abdomen and pelvis about 10 mSv; background 2–3 mSv per year.
- Limits: occupational 20 mSv per year averaged over 5 years; public 1 mSv per year.
- FDG-18 (fluorodeoxyglucose) is the tracer for PET imaging of glucose metabolism.
Acute radiation syndrome and late effects
| Syndrome | Approximate whole-body dose | Mechanism |
|---|---|---|
| Haematopoietic | Over 1–2 Gy | Marrow failure over weeks |
| Gastrointestinal | Over about 6–10 Gy | Crypt stem cell loss |
| Neurovascular | Over about 20 Gy | Collapse within hours |
- The fall in the absolute lymphocyte count over 48 hours best estimates the dose.
- Late effects come from obliterative endarteritis and fibrosis. Radiation pneumonitis: straight edge matching the field.
- Radiation-induced tumours: leukaemia (commonest overall), thyroid cancer, breast cancer, sarcoma; meningioma is the commonest radiation-induced brain tumour.
- After a nuclear release, potassium iodide blocks thyroid uptake of iodine-131.
Radiation protection
- Justification, optimisation (ALARA), dose limitation.
- Time, distance, shielding. Dose falls with the inverse square of distance: moving from 1 m to 2 m cuts it to one quarter (to 3 m, one ninth). This applies to a point source; close to a large source the fall is less predictable.
- Most staff dose in fluoroscopy is scatter from the patient.
Burns
- Zones: central coagulation (dead), stasis (salvageable with resuscitation), outer hyperaemia.
| Depth | Look | Healing |
|---|---|---|
| Superficial (epidermal) | Red, no blisters | Days |
| Superficial partial thickness | Blisters, pink, blanches, painful | About 2 weeks from hair follicles and sweat glands |
| Deep partial thickness | Blotchy, slow refill | Weeks, scars |
| Full thickness (third degree) | White or leathery, painless (nerve endings destroyed); whole epidermis and dermis lost | Needs grafting |
- Burn oedema is from increased capillary permeability (histamine, prostaglandins, kinins, free radicals), plus low plasma protein later.
- Rule of nines: head 9%, each arm 9%, front of trunk 18%, back of trunk 18%, each leg 18%, perineum 1%. Front of trunk plus one whole arm = 27%. Patient's palm with fingers is about 1%.
- Parkland: 4 mL × kg × %TBSA in 24 hours, half in the first 8 hours.
- Complications: contracture (commonest late complication of deep burns across joints), hypertrophic scar and keloid (more common in people with dark skin), Marjolin ulcer (squamous carcinoma in an old burn scar), Curling ulcer, infection.
- Infection: Pseudomonas aeruginosa is the classic burn bacterium (with S. aureus); after weeks of broad-spectrum antibiotics, Candida albicans is the commonest fungus.
- Anaesthesia: suxamethonium is contraindicated from about 24–48 hours after a major burn (up-regulated extrajunctional acetylcholine receptors release dangerous potassium). A family member dying under anaesthesia suggests malignant hyperthermia (RYR1): avoid suxamethonium and volatile agents.
- Electrical injury: small skin wounds hide deep muscle necrosis; myoglobinuria.
Heat loss and heat illness
| Route | When it dominates |
|---|---|
| Radiation | Naked patient in a cool room (largest share at rest) |
| Conduction | Lying on a cold surface such as a metal table |
| Convection | Moving air (fans, draughts) |
| Evaporation | Sweating; the only route when air is hotter than skin |
- Heat stroke: core above 40 °C with CNS dysfunction when heat gain overwhelms evaporative loss; hot dry skin in classic heat stroke.
Chemical and contrast injury
| Agent | Mechanism |
|---|---|
| Hydrofluoric acid | Fluoride binds calcium and magnesium: deep pain, hypocalcaemia |
| Alkali | Liquefactive necrosis, deeper than acid (coagulative) |
| Button battery | Current makes hydroxide by electrolysis at the mucosa; double-ring halo on the frontal film and a step-off on the lateral; injury continues after removal |
| Paraquat | Redox cycling forms superoxide; lung fibrosis; extra oxygen worsens it |
| Aluminium phosphide ("wheat pill") | Releases phosphine, which blocks mitochondrial cytochrome c oxidase: garlic smell, refractory shock, arrhythmias, pulmonary oedema; vomit releases phosphine, so protect staff |
| Iodinated contrast reaction | Direct, non-IgE mast cell activation |
| Contrast nephropathy | Medullary hypoxia and tubular toxicity |
| Gadolinium | Nephrogenic systemic fibrosis in severe renal failure |
Remember Deterministic effects get worse with dose; stochastic effects get more likely with dose.
Exam trap Doubling the distance from the source cuts the dose to a quarter, not a half.
Exam trap Pseudomonas is the classic burn bacterium; Candida is the classic burn fungus after long antibiotics.
How FCPS asks about this
- Rule of nines, front of trunk and one arm: 27%.
- Nurse moves from 1 m to 2 m: dose rate falls to one quarter.
- Button battery injury: electrolysis producing hydroxide at the mucosa.
- Wheat pill poisoning with garlic smell and shock: phosphine blocking mitochondrial respiration.
- Most radiosensitive tumour: seminoma.
- Commonest complication of third-degree burn: contracture; keloid in dark-skinned patients.
- Heat loss from a naked patient lying on a table: radiation in the room; conduction to the table.
Test yourself on this topic
5 of the 40 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 2-year-old boy swallowed a coin-sized lithium button battery from a television remote, and it has lodged in his upper oesophagus. What is the main mechanism of oesophageal injury from this object?
Answer: Electrolysis making hydroxide at mucosa
Lodged against moist mucosa, a button battery completes an electrical circuit. Electrolysis generates hydroxide ions at the negative pole, producing a strongly alkaline environment and liquefactive necrosis that can begin within 2 hours. Damage can extend into the trachea, aorta or recurrent laryngeal nerve, so urgent endoscopic removal is needed. Leakage of contents is a minor factor compared with the current, and a coin causes only slow pressure injury.
Button battery ingestion
- Battery completes a circuit on mucosa
- Current makes hydroxide ions
- Alkaline liquefactive necrosis
- Remove urgently (within 2 hours)
Remember A swallowed button battery burns through in hours: remove now.
Exam trap Injury can continue after removal, so watch for fistula or vascular erosion over the next weeks.
Why the other options are wrong
- Pressure necrosis, as with a swallowed coin Coins cause slow pressure injury; batteries injure within hours.
- Thermal burn from heat stored in the cell Batteries do not cause heat burns of this kind.
- Toxicity from leaked mercury contents Modern button batteries are lithium cells, and leakage is not the main injury.
- Infection from the contaminated plastic Infection is not the mechanism of this rapid injury.
Three weeks after a 70-minute coronary intervention under fluoroscopy, a 64-year-old man has a sharply outlined red, peeling patch on his upper back at the site where the X-ray beam entered. Which type of radiation effect is this?
Answer: Deterministic effect above a threshold dose
Deterministic effects (tissue reactions) occur only above a threshold dose, because many cells must be killed before the tissue shows damage, and their severity increases with dose. Skin erythema appears above about 2 Gy, and desquamation at higher doses, at the beam entry site in long fluoroscopic procedures. Stochastic effects, such as cancer and heritable effects, have no threshold, and their probability, not their severity, rises with dose. The square, sharply outlined patch matching the beam is typical.
Deterministic effects
- Occur only above a threshold
- Severity rises with dose
- Skin erythema above about 2 Gy
- Also cataract, sterility, marrow failure
Remember Deterministic: threshold, then worse with dose.
Exam trap Radiation skin injury often appears weeks after the procedure, so it is easily missed or misdiagnosed.
Why the other options are wrong
- Stochastic effect with no threshold Stochastic effects are cancer and heritable change, not visible skin injury.
- Allergic reaction to the contrast medium A contrast reaction would be generalised and occur within minutes or hours.
- Thermal burn from the defibrillator pad A pad burn would be on the chest at the pad site, not the beam entry site.
- Delayed effect of a genetic mutation Genetic effects affect offspring and do not cause a skin patch.
A newborn with suspected oesophageal atresia is given a high-osmolar water-soluble contrast swallow. The baby aspirates some contrast and within an hour becomes hypoxic, and the chest radiograph shows bilateral airspace opacification. What is the mechanism?
Answer: Osmotic shift of water into the alveoli
High-osmolar ionic contrast agents such as diatrizoate have an osmolality several times that of plasma. In the alveoli they draw fluid out of the capillaries by osmosis, producing rapid pulmonary oedema and chemical pneumonitis, which can be fatal in infants. For suspected aspiration or tracheo-oesophageal fistula, low-osmolar or iso-osmolar non-ionic agents are used, or a small amount of dilute barium, which is relatively inert in the airways. The radiograph shows bilateral airspace opacities developing soon after aspiration.
Aspiration of hyperosmolar contrast
- Diatrizoate osmolality several times plasma
- Draws fluid into alveoli
- Rapid pulmonary oedema
- Use low or iso-osmolar agents if aspiration risk
Remember Never give high-osmolar contrast to a baby who may aspirate.
Exam trap The same high osmolality that helps adhesive bowel obstruction makes these agents dangerous in the lung.
Why the other options are wrong
- Barium particles blocking small airways This was a water-soluble agent, not barium.
- IgE-mediated anaphylaxis to iodine Contrast reactions are mostly non-IgE, and this picture is local pulmonary oedema.
- Chemical burn from acidic contrast The contrast is not acidic.
- Infection introduced by the feeding tube Infection takes days and would not explain hypoxia within an hour.
A 28-year-old man with a deep flame burn of the forearm has a white, leathery centre, a surrounding ring of pale, poorly perfused but viable tissue, and an outer red area that blanches. Which zone can be saved or lost depending on early fluid resuscitation and wound care?
Answer: The zone of stasis
Jackson described three zones in a burn. The central zone of coagulation has irreversible protein coagulation and cell death. Around it, the zone of stasis has reduced perfusion from microvascular injury and oedema; its cells are viable at first but can die if hypoperfusion, infection or oedema continue, which is why adequate resuscitation and careful wound care can save it. The outer zone of hyperaemia has increased blood flow from inflammation and usually recovers.
Jackson's burn zones
- Centre: coagulation (dead)
- Middle: stasis (salvageable)
- Outer: hyperaemia (recovers)
- Resuscitation decides the stasis zone
Remember The middle ring is the battleground in burns.
Exam trap Under-resuscitation can deepen a burn over 48 hours as the zone of stasis dies.
Why the other options are wrong
- The zone of coagulation Already dead tissue that cannot be saved.
- The zone of hyperaemia Usually recovers fully without special intervention.
- The surrounding normal skin Not injured by the burn.
- All zones equally The zones have different fates; only stasis is variable.
During a fluoroscopic procedure, a nurse standing 1 metre from the patient receives a certain scatter dose rate. If she moves back to stand 2 metres away, with everything else unchanged, what will her dose rate be?
Answer: One quarter of the original
Radiation from a point source spreads over the surface of a sphere, whose area increases with the square of the distance. The intensity therefore falls with the inverse square of the distance: doubling the distance reduces the dose rate to one quarter, and tripling it to one ninth. Distance, together with shorter time and shielding, is one of the three basic methods of reducing dose. Staff should stand as far from the patient, the main source of scatter, as their work allows.
Inverse square law
- Dose rate proportional to 1/d²
- Double distance: one quarter
- Triple distance: one ninth
- Time, distance, shielding
Remember Step back one pace and the dose falls sharply.
Exam trap The law applies to a point source; very close to a large source the fall is less predictable.
Why the other options are wrong
- One half of the original This would be an inverse linear relationship, which does not apply.
- Unchanged Distance always reduces dose from a point source.
- One eighth of the original This would be an inverse cube relationship.
- Twice the original Moving away cannot increase dose.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Biological, chemical and radiation injury, Biological, chemical, radiation and nuclear injury; radiation biology and protection