Thymoma is a tumour arising from the epithelial cells of the thymus — a gland located in the front of the chest that plays a crucial role in the immune system during youth. While the thymus normally shrinks in adulthood, residual thymic tissue can give rise to this tumour. Thymoma is notable for the paraneoplastic syndromes (tumour-related complications) it can produce, including myasthenia gravis (a condition causing muscle weakness, including difficulty swallowing) and a distinctive exfoliative dermatitis (severe flaking skin disease). The tumour itself is often slow-growing, and cats may present with breathing difficulty due to the mass occupying chest space. If the tumour can be completely removed surgically, the prognosis is often good. The paraneoplastic conditions may or may not resolve after tumour removal.