Editing Neoplasm (section)

== Causes ==
[[Image:Hidradenoma.jpg|thumb|Neoplastic tumor of the cheek skin, here a [[Benign tumor|benign neoplasm]] of the [[sweat glands]] called [[hidradenoma]], which is not solid but is fluid-filled]]
[[Image:Fibroids.jpg|thumb|Diagram illustrating benign neoplasms, namely [[uterine fibroids|fibroids]] of the [[uterus]]]]
Tumors in humans occur as a result of accumulated genetic and epigenetic alterations within single cells, which cause the cell to divide and expand uncontrollably.<ref>{{cite journal|doi=10.1146/annurev-cancerbio-030419-033413|doi-access=free|title=Investigating Tumor Heterogeneity in Mouse Models|year=2020|last1=Tammela|first1=Tuomas|last2=Sage|first2=Julien|journal=Annual Review of Cancer Biology|volume=4|issue=1|pages=99–119|pmid=34164589|pmc=8218894}}</ref> A neoplasm can be caused by an abnormal proliferation of tissues, which can be caused by genetic [[mutation]]s. Not all types of neoplasms cause a tumorous overgrowth of tissue (such as [[leukemia]] or [[carcinoma in situ]]), however similarities between neoplasmic growths and regenerative processes, e.g., dedifferentiation and rapid cell proliferation, have been pointed out.<ref>{{cite journal | vauthors = Asashima M, Oinuma T, Meyer-Rochow VB|title=Tumors in amphibia|journal=Zoological Science|date=1987|volume=4|pages=411–425}}</ref>

Tumor growth has been studied using mathematics and [[continuum mechanics]]. Vascular tumors such as [[hemangioma]]s and [[lymphangioma]]s (formed from blood or lymph vessels) are thus looked at as being amalgams of a solid skeleton formed by sticky cells and an organic liquid filling the spaces in which cells can grow.<ref>{{cite journal |vauthors=Ambrosi D, Mollica F |title=On the mechanics of a growing tumor | year = 2002 | journal = International Journal of Engineering Science | volume = 40 | issue = 12 | pages = 1297–316 | doi = 10.1016/S0020-7225(02)00014-9 }}</ref> Under this type of model, mechanical stresses and strains can be dealt with and their influence on the growth of the tumor and the surrounding tissue and vasculature elucidated. Recent findings from experiments that use this model show that active growth of the tumor is restricted to the outer edges of the tumor and that stiffening of the underlying normal tissue inhibits tumor growth as well.<ref name="pmid16793355">{{cite journal | author = Volokh KY | title = Stresses in growing soft tissues | journal = Acta Biomater | volume = 2 | issue = 5 | pages = 493–504 |date=September 2006 | pmid = 16793355 | doi = 10.1016/j.actbio.2006.04.002 }}</ref>

Benign conditions that are ''not'' associated with an abnormal proliferation of tissue (such as [[sebaceous cyst]]s) can also present as tumors, however, but have no malignant potential. [[Breast cyst]]s (as occur commonly during pregnancy and at other times) are another example, as are other encapsulated glandular swellings (thyroid, adrenal gland, pancreas).{{cn|date=May 2023}}

Encapsulated hematomas, encapsulated necrotic tissue (from an insect bite, foreign body, or other noxious mechanism), [[keloid]]s (discrete overgrowths of scar tissue) and [[granuloma]]s may also present as tumors.{{cn|date=February 2024}}

Discrete localized enlargements of normal structures (ureters, blood vessels, intrahepatic or extrahepatic biliary ducts, pulmonary inclusions, or [[Enteric duplication cyst|gastrointestinal duplications]]) due to outflow obstructions or narrowings, or abnormal connections, may also present as a tumor. Examples are arteriovenous fistulae or aneurysms (with or without thrombosis), biliary fistulae or aneurysms, sclerosing cholangitis, cysticercosis or hydatid cysts, intestinal duplications, and pulmonary inclusions as seen with cystic fibrosis. It can be dangerous to [[biopsy]] a number of types of tumor in which the leakage of their contents would potentially be catastrophic. When such types of tumors are encountered, diagnostic modalities such as ultrasound, CT scans, MRI, angiograms, and nuclear medicine scans are employed prior to (or during) biopsy or surgical exploration/excision in an attempt to avoid such severe complications.{{cn|date=January 2022}}