Thymomas can be benign; benign but by virtue of expansion, invading beyond the capsule of the thymus ("invasive thyoma"), or malignant (a carcinoma). This classification is based on the appearance of the cells. A WHO classification also exists but is not used as part of standard clinical practice. Benign tumours confined to the thymus are most common; followed by locally invasive tumours, and then by carcinomas. There is variation in reporting, with some sources reporting malignant tumours as more common. Invasive tumours, although not technically malignant, can still spread () to other areas of the body. Even though thymomas occur of epithelial cells, they can also contain thymocytes. Treatment of thymomas often requires surgery to remove the entire thymus. This may also result in temporary remission of any associated autoimmune conditions.