While almost any organ can be damaged by radiation, the clinically important tissues are the skin, lungs, heart, kidney, bladder, and intestine. It is difficult to shield these organs. Localized damage to the bone marrow is clearly of little functional consequence because of the immense reserve capacity of the hematopoietic system.

Persistent damage to radiation-exposed tissue can be attributed to two major factors : (i) compromise of the vascular supply and (ii) a fibrotic repair reaction to acute necrosis and chronic ischemia.

Radiation-induced tissue injury predominantly affects small arteries and arterioles.

The endothelial cells are the most sensitive elements in the blood vessels and acutely exhibit swelling and necrosis.

Chronically the walls become thickened by endothelial cell proliferation and subintimal deposition of collagen and other connective tissue elements.

Striking vacuolization of intimal cells, the so-called foam cell, is typical.

Fragmentation of the internal elastic lamina, loss of smooth muscle cells and scarring in the media, and fibrosis of the adventitia are seen in the small arteries.

The fibroblastic repair reaction is nonspecific but may be exaggerated, possibly owing to altered epithelial – mesenchymal interactions consequent on impaired regeneration.

Bizarre fibroblasts with large, hyperchromatic nuclei are common, and, again probably reflect radiation-induced DNA damage.

Acute necrosis is represented by such disorders as radiation pneumonitis, cystitis, dermatitis and diarrhea from enteritis.

Chronic disease is characterized by interstitial fibrosis in the heart and lungs, stricture in the esophagus and small intestine, and constrictive pericarditis.

Chronic radiation nephritis, which stimulates malignant nephrosclerosis, is primarily a vascular disease characterized by severe hypertension and progressive renal insufficiency.

Since radiotherapy inevitably traverses the skin it often leads to radiodermatitis.

The initial damage is evidenced by dilatation of blood vessels, recognized as erythema.

Necrosis of the skin may follow and linger as indolent ulcers that do not heal because the epithelium is unable to regenerate.

A further consequence of this poor regenerative capacity is the difficulty faced by the surgeon, for whom the impairment of wound healing in irradiated areas poses a serious problem.

Poorly healed or dehisced wounds or persistent ulcers often require full-thickness skin grafts.

Chronic radiodermatitis results from the repair and revascularization of the skin, and is characterized by atrophy, hyperkeratosis, telangiectasia, and hyperpigmentation.

Like other tissues that depend on continuous cell cycling, the gonads, both testes and ovaries, are extremely radiosensitive.

The acute inhibition of mitosis in the testis results in necrosis of the germinal stem cells, the spermatogonia.

The combination of radiation-induced vascular injury and direct damage to the germ cells leads to progressive atrophy of the seminiferous tubules, peritubular fibrosis, and loss of reproductive function.

However, since the interstitial and Sertoli cells do not cycle rapidly, they are more resistant than the germ cells and so persist, thereby preserving the normal hormonal status.

Comparable injury is seen in the irradiated ovary ,the follicles become atretic, and the organ eventually becomes fibrous and atrophic.

Should the eye lie in the path of the radiation beam, lenticular opacities (cataracts) - may result.

The spinal cord is unavoidably irradiated during treatment of certain thoracic or abdominal tumours, and the vascular damage in the cord may bring about localized ischemia which can result in a transverse myelitis and paraplegia.

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