Radiation and cancer: ultraviolet and ionizing !
- Mechanism
Sunlight damages the DNA of skin cells; X-rays and other high-energy radiation break DNA anywhere in the body. Both can lead to cancer.
Updated 2026-11-01
What it is
Radiation causes cancer by damaging DNA in two different ways. Ultraviolet light from the sun, mainly UVB, forms pyrimidine dimers in skin cells, which are removed by nucleotide excision repair; when repair fails, the result is basal cell carcinoma, squamous cell carcinoma and melanoma. Ionizing radiation (X-rays, gamma rays, particles, radon) breaks DNA strands directly and through free radicals from water, and causes leukaemias, thyroid, breast and lung cancers, often decades later.
Key facts
- Squamous cell carcinoma tracks with cumulative lifetime sun exposure; melanoma and basal cell carcinoma track more with intense, intermittent exposure and sunburns.
- Children's thyroid glands are very sensitive: papillary carcinoma rose after the Chernobyl accident.
- Chronic lymphocytic leukaemia is the leukaemia not linked to radiation.
- Radon gas in homes and mines is a cause of lung cancer, more so in smokers.
- Rapidly dividing tissues are most radiosensitive; the latency of solid tumours is long.
Step by step
- UVB is absorbed by DNA in the epidermis and joins neighbouring pyrimidines into dimers (mostly thymine dimers).
- Nucleotide excision repair cuts out the damaged segment; in xeroderma pigmentosum this repair is defective.
- Unrepaired dimers cause C-to-T mutations (the UV signature) in genes such as TP53 or PTCH1.
- Ionizing radiation splits water into free radicals and breaks both DNA strands directly.
- Badly repaired double-strand breaks give deletions and translocations (for example RET rearrangements in thyroid cancer).
- Surviving mutant clones expand: leukaemias appear first (years), solid tumours later (decades).
Ultraviolet vs ionizing radiation
| Feature | Ultraviolet (UV) | Ionizing |
|---|---|---|
| Source | Sun (mainly UVB; UVA also contributes), tanning beds | Medical imaging and radiotherapy, radon, nuclear accidents, atomic bombs |
| DNA damage | Pyrimidine dimers | Double-strand breaks, free radicals |
| Repair system | Nucleotide excision repair | Homologous recombination and non-homologous end joining |
| Tissues at risk | Skin only | Bone marrow, thyroid, breast, lung, salivary glands, almost any tissue |
| Classic tumours | Basal cell carcinoma, squamous cell carcinoma, melanoma | Acute and chronic myeloid leukaemia, papillary thyroid carcinoma, breast cancer, sarcoma after radiotherapy |
| Inherited defect that exaggerates it | Xeroderma pigmentosum | Ataxia-telangiectasia |
Clinical notes
- Photoprotection (shade, clothing, hats, sunscreen, no tanning beds) is primary prevention of skin cancer; high altitude in the Andes raises UV exposure.
- Children with xeroderma pigmentosum develop skin cancers at a very young age and need strict sun avoidance.
- Survivors who received chest radiotherapy in youth (for example for Hodgkin lymphoma) have a higher breast cancer risk and are screened earlier.
- Imaging in children follows the principle of using the lowest radiation that answers the question.
Test yourself
Think of the answer, then tap the question to see it.
What DNA lesion does ultraviolet B light produce?
Pyrimidine dimers, mostly thymine dimers, between neighbouring bases on the same strand.
Which repair system is defective in xeroderma pigmentosum?
Nucleotide excision repair, so UV damage persists and skin cancers appear in childhood.
Which thyroid cancer is linked to radiation exposure in childhood?
Papillary thyroid carcinoma, often with RET rearrangements.
Which leukaemia is not associated with ionizing radiation?
Chronic lymphocytic leukaemia; myeloid leukaemias are the classic radiation-induced ones.
Related
- Genomic instability: mismatch repair, BRCA and xeroderma pigmentosum
- Chemical carcinogens: initiation, promotion and the table to know
- Ionizing radiation injury
- Oxidative stress and free-radical injury
- Cancer epidemiology: age, environment, predisposing conditions (and Colombia)
- Hereditary cancer syndromes: the table to know
- Tumour suppressor genes: RB, TP53, APC, BRCA and the two-hit hypothesis
Related conditions
- Melanoma
- Basal cell carcinoma
- Cutaneous squamous cell carcinoma
- Actinic keratosis
- Thyroid cancer (papillary)
- Xeroderma pigmentosum
- Ataxia-telangiectasia
- Acute leukaemia (AML and ALL)
- Chronic myeloid leukaemia
- Breast cancer
- Lung cancer
- Osteosarcoma
Elsewhere in DurgiPedia
- DNA repair
- Types of mutations
- Keratinocytes, melanocytes, Langerhans and Merkel cells
- Skin: epidermis layers, dermis, hypodermis
- Ionizing radiation, tobacco and cocaine
- Skin examination: primary and secondary lesions
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: Radiation (National Cancer Institute (NCI), 2026); Sunlight (National Cancer Institute (NCI), 2026); Xeroderma pigmentosum: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Papillary Thyroid Carcinoma (StatPearls (NCBI Bookshelf), 2024).
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Representative model, for educational purposes only.