Tumor

A tumor is an abnormal mass of tissue that forms when cells grow and divide more than they should, and it may be benign or malignant.

Benign tumors, such as lipomas and most moles, grow slowly, stay in one place and do not spread, though they can cause problems by pressing on nearby structures. Malignant tumors are cancers: they invade surrounding tissue and can spread through blood or lymph to form secondary tumors elsewhere. Tumors are classified by the tissue they arise from, for example carcinomas from lining cells, sarcomas from bone or connective tissue, and lymphomas from immune cells. Dogs and cats develop tumors frequently, with skin masses, mammary tumors and lymphoma among the most common.

A new lump is assessed by physical examination, imaging such as ultrasound, CT or MRI, and biopsy, in which a sample is examined under a microscope to determine whether it is benign or malignant and, if malignant, its type and grade. Staging then describes how far a cancer has spread. Cannabinoids have been studied in laboratory and animal models of tumor growth, and in people mainly for symptoms of cancer and its treatment; there is no evidence that any cannabinoid product changes the course of a tumor in humans.