Tumours

Neoplasm
Synonyms	tumour, carcinocytes
Colectomy specimen containing a malignant neoplasm, namely an invasive colorectal cancer (the crater-like, reddish, irregularly shaped tumor)
Classification and external resources
Specialty	Oncology
ICD-10	C00-D48
ICD-9-CM	140-239.99
DiseasesDB	28841
MedlinePlus	001310.
MeSH	D009369
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Neoplasm is an abnormal growth of tissue, and, when it also forms a mass, is commonly referred to as a tumor. This abnormal growth (neoplasia) usually but not always forms a mass.

The World Health Organization (WHO) classifies neoplasms into four main groups: benign neoplasms, in situ neoplasms, malignant neoplasms, and neoplasms of uncertain or unknown behavior. Malignant neoplasms are also simply known as cancers.

Prior to the abnormal growth of tissue, as neoplasia, cells often undergo an abnormal pattern of growth, such as metaplasia or dysplasia. However, metaplasia or dysplasia does not always progress to neoplasia. The word is from Ancient Greek νέος- neo "new" and πλάσμα plasma "formation, creation".

A neoplasm can be benign, potentially malignant, or malignant (cancer).

Neoplastic tumors are often heterogeneous and contain more than one type of cell, but their initiation and continued growth is usually dependent on a single population of neoplastic cells. These cells are presumed to be clonal – that is, they are derived from the same cell, and all carry the same genetic or epigenetic anomaly – evident of clonality. For lymphoid neoplasms, e.g. lymphoma and leukemia, clonality is proven by the amplification of a single rearrangement of their immunoglobulin gene (for B cell lesions) or T cell receptor gene (for T cell lesions). The demonstration of clonality is now considered to be necessary to identify a lymphoid cell proliferation as neoplastic.

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