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A Tumor, or neoplasm, is an abnormal growth of tissue with which its continuous growth usually forms a mass. The World Health Organization (WHO) classifies tumors into four main groups: benign neoplasms, in situ neoplasms, malignant neoplasms, and neoplasms of uncertain or unknown behavior.

Malignant tumors 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 do not always progress to neoplasia. The terms tumor and cancer are sometimes used interchangeably which can be misleading.

A tumor is not necessarily a cancer; the word tumor simply refers to a mass. A tumor can be cancerous or benign.

Benign tumors include uterine fibroids and melanocytic nevi (skin moles). They are circumscribed and localized and do not transform into cancer. Potentially-malignant tumors include carcinoma in situ. They are localised, do not invade and destroy but in time, may transform into a cancer. Malignant tumors are commonly called cancer. They invade and destroy the surrounding tissue, may form metastases and untreated or unresponsive to treatment, will prove fatal. Secondary tumors refers to any of a class of cancerous tumor that is either a metastatic offshoot of a primary tumor, or an apparently unrelated tumor that increases in frequency following certain cancer treatments such as chemotherapy or radiotherapy. Rarely there can be a metastatic tumor with no known site of the primary cancer and this is classed as a cancer of unknown primary origin.

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