BACKGROUND: Ionizing radiation DNA damage is the main mechanism of radiotherapy (RT) action and the outcome of treatment and healthy tissue toxicity is influenced by a number of external and internal factors, including mutations in DNA damage recognition and repair. Disorders of DNA repair may result in increased sensitivity to cancer treatment. PURPOSE: The mechanism of DNA repair and an overview of genetic syndromes with mutations in genes involved in DNA repair clarify the accelerated carcinogenesis and increased radiosensitivity in RT cancers. Most radiosensitivity syndromes are autosomal recessively inherited; examples are ataxia teleangiectasia, Nijmegen breakage syndrome, xeroderma pigmentosum, Cockayne syndrome, Bloom syndrome and Werner syndrome. CONCLUSION: Radiotherapy is contraindicated in most homozygous patients with recessive radiosensitivity syndromes. Asymptomatic heterozygotes may have an increased risk of tumor incidence and a small part of them slightly increased risk of RT intolerance; however, this does not limit RT treatment. The high risk of secondary malignancies after radiotherapy is a contraindication to adjuvant RT in Li-Fraumeni syndrome.

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