[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Environmental pathology](https://durgical.com/en/general-pathology/#a-environment) › Ionizing radiation injury

[General pathology](https://durgical.com/en/general-pathology/) · [Environmental pathology](https://durgical.com/en/general-pathology/#a-environment)

# Ionizing radiation injury

- Mechanism

Ionizing radiation breaks DNA directly and through free radicals; dividing tissues fail first, and cancer may appear years later.

Updated 2026-11-01

## Key points

- Most radiosensitive: lymphocytes, bone marrow, gut crypts, gonads; neurons and muscle are resistant.
- Oxygen increases radiation damage, so hypoxic tumour areas resist radiotherapy.
- A falling lymphocyte count in the first days is an early marker of exposure severity.
- Leukaemia is the earliest radiation cancer; thyroid cancer in exposed children (Chernobyl).
- Late effects: fibrosis, vascular narrowing, cataract, infertility, second cancers such as sarcoma.

## What it is

X-rays, gamma rays and particles carry enough energy to ionise molecules. They damage DNA, membranes and vessels, which kills dividing cells, causes slow fibrosis and leaves mutations that can cause cancer. Radiation is used on purpose in radiotherapy, so its effects on normal tissue are seen daily in oncology.

## Structure

- Radiation atypia: large, bizarre nuclei in fibroblasts and endothelial cells.
- Vessels with thick, hyalinised walls and fibrin; dense, poorly cellular fibrosis.
- Typical late lesions: radiation pneumonitis and lung fibrosis, proctitis, skin atrophy with telangiectasias.

## Step by step

- Radiation ionises water in the cell, producing hydroxyl radicals and other free radicals.
- Radicals and direct hits break DNA, including double-strand breaks.
- Dividing cells try to repair at checkpoints; if they fail they die by apoptosis or mitotic catastrophe.
- Rapidly renewing tissues fail first: bone marrow, gut lining, skin and gonads.
- Small vessels are injured: endothelial damage and later thick, hyalinised walls with narrow lumens.
- Chronic ischaemia and fibroblast activation lead to fibrosis months to years later.
- Surviving cells with unrepaired mutations can grow into cancer after a latent period.

## Acute radiation syndrome by increasing exposure

| Form | Target | Main features |
| --- | --- | --- |
| Haematopoietic | Bone marrow stem cells | Falling lymphocytes first, then neutropenia and thrombocytopenia: infection and bleeding |
| Gastrointestinal | Crypt cells of the intestine | Vomiting, bloody diarrhoea, fluid loss and sepsis from gut bacteria |
| Neurovascular | Brain and blood vessels | Confusion, seizures, shock; death within days |

## Memory aid

Radiation hits fast what divides fast, and slowly scars what remains.

## Test yourself

Think of the answer, then tap the question to see it.

## Why are hypoxic tumour cells more resistant to radiotherapy?

Oxygen fixes the DNA damage made by free radicals; without it more of the damage is repaired.

## Which blood cell falls first after a large radiation exposure?

The lymphocyte, which is extremely radiosensitive; its early fall predicts severity.

## Which cancer rose in children exposed to radioactive iodine after Chernobyl?

Papillary thyroid carcinoma.

## What causes the late fibrosis of irradiated tissue?

Damage to small vessels causes chronic ischaemia, and activated fibroblasts lay down collagen.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/general-pathology/ionizing-radiation-injury/)

## Related

- [Radiation and cancer: ultraviolet and ionizing](https://durgical.com/en/general-pathology/radiation-and-cancer-ultraviolet-and-ionizing/)
- [Oxidative stress and free-radical injury](https://durgical.com/en/general-pathology/oxidative-stress-and-free-radical-injury/)
- [Apoptosis: the intrinsic and extrinsic pathways](https://durgical.com/en/general-pathology/apoptosis-the-intrinsic-and-extrinsic-pathways/)
- [Genomic instability: mismatch repair, BRCA and xeroderma pigmentosum](https://durgical.com/en/general-pathology/genomic-instability-mismatch-repair-brca-and-xeroderma-pigmentosum/)
- [Fibrosis in organs: TGF-β and the myofibroblast](https://durgical.com/en/general-pathology/fibrosis-in-organs-tgf-and-the-myofibroblast/)

## Related conditions

- [Aplastic anaemia](https://durgical.com/en/conditions/aplastic-anaemia/)
- [Thyroid cancer (papillary)](https://durgical.com/en/conditions/thyroid-cancer/)
- [Acute leukaemia (AML and ALL)](https://durgical.com/en/conditions/acute-leukaemia/)
- [Myelodysplastic syndromes](https://durgical.com/en/conditions/myelodysplastic-syndromes/)
- [Breast cancer](https://durgical.com/en/conditions/breast-cancer/)
- [Soft tissue sarcomas (liposarcoma, leiomyosarcoma, rhabdomyosarcoma)](https://durgical.com/en/conditions/soft-tissue-sarcomas/)
- [Cataract](https://durgical.com/en/conditions/cataract/)
- [Interstitial lung disease](https://durgical.com/en/conditions/interstitial-lung-disease/)
- [Basal cell carcinoma](https://durgical.com/en/conditions/basal-cell-carcinoma/)
- [Melanoma](https://durgical.com/en/conditions/melanoma/)
- [Xeroderma pigmentosum](https://durgical.com/en/conditions/xeroderma-pigmentosum/)

## Elsewhere in DurgiPedia

- [DNA repair](https://durgical.com/en/biochemistry/topics/dna-repair/)
- [Reactive oxygen species and antioxidants](https://durgical.com/en/biochemistry/topics/reactive-oxygen-species-and-antioxidants/)
- [Ionizing radiation, tobacco and cocaine](https://durgical.com/en/embryology/ionizing-radiation-tobacco-and-cocaine/)
- [Cell cycle, mitosis and meiosis](https://durgical.com/en/histology/cell-cycle-mitosis-and-meiosis/)
- [Cell injury, apoptosis and necrosis](https://durgical.com/en/histology/cell-injury-apoptosis-and-necrosis/)

[← All of general pathology](https://durgical.com/en/general-pathology/)

Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Ionizing radiation and health effects](https://www.who.int/news-room/fact-sheets/detail/ionizing-radiation-and-health-effects) (World Health Organization, 2026); [Risk Factors: Radiation - NCI](https://www.cancer.gov/about-cancer/causes-prevention/risk/radiation) (National Cancer Institute, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/ionizing-radiation-injury/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/ionizing-radiation-injury/