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Physics · Radiobiology

Biological Effects of Ionising Radiation

Snapshot

Ionising radiation harms cells mainly through DNA double-strand breaks, about two-thirds of them (for X- and γ-rays) caused indirectly by free radicals formed in water. The effects are either tissue reactions (deterministic), which need a threshold dose — skin injury, cataract, sterility, the acute radiation syndrome — or stochastic effects (cancer, heritable), whose probability, not severity, rises with dose. Diagnostic nuclear medicine doses are far below every threshold.

Read the full article →In-depth, fully referenced version

Radiosensitivity is greatest in dividing, undifferentiated cells (law of Bergonié and Tribondeau) and in the M and G2 phases of the cell cycle. Cell survival follows the linear-quadratic model, S = e^(−αD − βD²); LET, dose rate and oxygen all modify the response.

0.5 GyCataract threshold (ICRP 118)
3.3–4.5 GyLD50/60 without treatment
100 mGyFetal dose: termination not justified below
Log-scale dose ladder comparing diagnostic nuclear medicine doses of a few millisieverts with fetal thresholds, tissue reaction thresholds, acute radiation syndromes and the LD50/60.
Figure. Diagnostic nuclear medicine doses (a few mSv) lie two or more orders of magnitude below every tissue-reaction threshold (ICRP 118), the fetal thresholds (ICRP 84) and the doses that cause the acute radiation syndromes, so only a small stochastic risk applies.

How radiation damages cells

  • DNA is the critical target; the double-strand break is the lethal lesion.
  • Direct action dominates for high-LET radiation (α, neutrons); indirect action (OH• radicals) for X- and γ-rays.
  • Oxygen fixes radical damage: OER ≈ 2.5–3 for low-LET radiation, ≈ 1 for α particles.
  • High-LET radiation has a high RBE (peak near 100 keV/µm) and little dose-rate effect.

Deterministic vs stochastic

  • Tissue reactions: threshold dose; severity rises with dose (e.g. temporary male sterility 0.1 Gy, cataract 0.5 Gy, temporary hair loss 4 Gy).
  • Stochastic effects: no threshold assumed (linear no-threshold model); probability rises with dose.
  • BEIR VII: about 1 in 100 people would develop cancer from 0.1 Sv, against about 42 in 100 from other causes.

Acute radiation syndrome

  • Phases: prodromal → latent → manifest illness → recovery or death.
  • Haematopoietic (~0.7–10 Gy), gastrointestinal (symptoms from ~6 Gy, full syndrome >10 Gy) and neurovascular (symptoms from ~20 Gy, full syndrome >50 Gy) syndromes.
  • LD50/60 about 3.3–4.5 Gy without treatment, 6–7 Gy with supportive care.

Embryo and fetus

  • Pre-implantation: all-or-none. Organogenesis: malformations above about 100–200 mGy.
  • 8–15 weeks: intellectual disability from about 300 mGy; IQ falls about 25 points per Gy.
  • Termination of pregnancy is not justified on radiation grounds below 100 mGy (ICRP 84).

In nuclear medicine

  • Diagnostic effective doses are a few mSv — only a small stochastic risk applies.
  • Radionuclide therapy is planned in absorbed dose (Gy); effective dose (Sv) is for radiation protection.
  • α emitters (²²³Ra, ²²⁵Ac): high LET, range of a few cell diameters; β⁻ emitters (¹³¹I, ¹⁷⁷Lu, ⁹⁰Y): low LET, millimetre ranges.
In depth
  • Linear-quadratic model: α/β is about 10 Gy for early-responding tissues and many tumours and about 3 Gy for late-responding tissues; for fractionated treatment BED = nd[1 + d/(α/β)].
  • Radionuclide therapy delivers a continuously falling dose rate. For single-exponential decay (constant λ) and repair rate µ (µ = ln2/T_repair), Dale's formulation gives BED = D[1 + Dλ/((µ + λ)(α/β))], so slow delivery spares late-responding tissue.
  • ICRP 103/118 thresholds for about 1% incidence after acute exposure: temporary male sterility 0.1 Gy, permanent male sterility about 6 Gy, permanent female sterility about 3 Gy, depressed haematopoiesis about 0.5 Gy, cataract 0.5 Gy, circulatory disease 0.5 Gy.
  • ICRP 118 applies the 0.5 Gy cataract and circulatory thresholds to acute and protracted exposure alike, which is why the occupational lens limit fell to 20 mSv per year.
  • RBE peaks at an LET of about 100 keV/µm, where the spacing of ionisations matches the 2 nm diameter of the DNA double helix; higher LET wastes energy (overkill) and RBE falls.
  • The fetal thyroid begins to concentrate iodine at about 10–12 weeks of gestation, so ¹³¹I given after that can ablate it; pregnancy must be excluded before radioiodine.
  • ICRP 103 keeps the linear no-threshold model with a dose and dose-rate effectiveness factor of 2; BEIR VII used 1.5. Both are judgements for protection, not measured thresholds.

Sources: ICRP 103 (2007) · ICRP 118 (2012) · ICRP 84 (2000) · BEIR VII (2006) · Dale, Br J Radiol 1985 (PMID 4063711) · Hall & Giaccia, 8th ed. (2018)

Sources

  1. ICRP. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann ICRP. 2007;37(2-4).
  2. Stewart FA, Akleyev AV, Hauer-Jensen M, et al. ICRP Publication 118: ICRP statement on tissue reactions and early and late effects of radiation in normal tissues and organs – threshold doses for tissue reactions in a radiation protection context. Ann ICRP. 2012;41(1-2):1-322.
  3. ICRP. Pregnancy and medical radiation. ICRP Publication 84. Ann ICRP. 2000;30(1).
  4. National Research Council. Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2. Washington, DC: National Academies Press; 2006.
  5. Hall EJ, Giaccia AJ. Radiobiology for the Radiologist. 8th ed. Philadelphia: Wolters Kluwer; 2018.
  6. Dale RG. The application of the linear-quadratic dose-effect equation to fractionated and protracted radiotherapy. Br J Radiol. 1985;58:515-28.
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