A benign tumor is a mass of cells (tumor) that does not invade neighboring tissue or metastasize (spread throughout the body). Compared to malignant (cancerous) tumors, benign tumors generally have a slower growth rate. Benign tumors have relatively well differentiated cells. They are often surrounded by an outer surface (fibrous sheath of connective tissue) or stay contained within the epithelium. Common examples of benign tumors include moles and uterine fibroids. Some forms of benign tumors may be harmful to health. Benign tumor growth causes a mass effect that can compress neighboring tissues. This can lead to nerve damage, blood flow reduction (ischemia), tissue death (necrosis), or organ damage. The health effects of benign tumor growth may be more prominent if the tumor is contained within an enclosed space such as the cranium, respiratory tract, sinus, or bones. For example, unlike most benign tumors elsewhere in the body, benign brain tumors can be life-threatening. Tumors may exhibit behaviors characteristic of their cell type of origin; as an example, endocrine tumors such as thyroid adenomas and adrenocortical adenomas may overproduce certain hormones. Many types of benign tumors have the potential to become cancerous (malignant) through a process known as tumor progression. For this reason and other possible harms, some benign tumors are removed by surgery. When removed, benign tumors usually do not return. Exceptions to this rule may indicate malignant transformation. Signs and symptoms
Benign tumors are very diverse; they may be asymptomatic or may cause specific symptoms, depending on their anatomic location and tissue type.
Prurigo pigmentosa is a rare skin condition of unknown cause, characterized by the sudden onset of erythematous papules that leave a reticulated hyperpigmentation when they heal. : 57  The condition has been associated with a strict ketogenic diet in case reports in the medical literature. It was first reported by Masaharu Nagashima in 1978. Research has shown that it may be caused by gut bacteria dysbiosis as a result of ketosis. See also
Pruritus
Skin lesion
Masaharu Nagashima

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