A<unk>  is a mass of cells (tumor) that lacks the ability either to invade neighboring tissue or metastasize (spread throughout the body). When removed,<unk> s usually do not grow back, whereas malignant tumors are cancerous and sometimes do. Unlike most<unk> s elsewhere in the body,<unk>  brain tumors can be life-threatening.<unk> s generally have a slower growth rate than malignant tumors and the tumor cells are usually more differentiated (cells have more normal features). They are typically surrounded by an outer surface (fibrous sheath of connective tissue) or stay contained within the epithelium. Common examples of<unk> s include moles and uterine fibroids. Although<unk> s will not metastasize or locally invade tissues, some types may be harmful to health. The growth of<unk> s produces a "mass effect" that can compress tissues and may cause nerve damage, reduction of blood flow to an area of the body (ischaemia), tissue death (necrosis) and organ damage. The health effects of the tumor may be more prominent if the tumor is within an enclosed space such as the cranium, respiratory tract, sinus or inside bones. Tumors of endocrine tissues may overproduce certain hormones. Examples include thyroid adenomas and adrenocortical adenomas. Although most<unk> s are not life-threatening, many types of<unk> s have the potential to become cancerous (malignant) through a process known as tumor progression. For this reason and other possible harms, some<unk> s are removed by surgery.
Benign tumor
Vascular tumor
Degenerative disease
Viral disease
Mixed tumor
Cartilage tumor
Precancerous condition
Psychogenic disease