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Chronic exapnding hematomas occur in many locations, often simulating neoplasms. All have the same structure with a central mass of blood, a wall of granulation tissue, and dense, fibrous tissue at the periphery. The self-perpetuating exapnding nature of the lesion appears to be due to the irritant effects of blood and its breakdown products, causing repeated exudation or bleeding from capillaries in the granulation tissue.
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John D. Reid (1980) studied this question.
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