Tumours and tumour-like conditions of the thymus other than thymoma; a practical approach.

Masses within the thymus and mediastinum are frequently biopsied to establish a diagnosis. The specimens sent for histopathological analysis are typically small (needle) biopsy specimens on which the pathologist is challenged to provide a classifying diagnosis. Despite the fact that the pathology of the anterior mediastinum is predominantly centred on the thymus and it thus might be expected that the range of possible diagnoses is limited, the list of disease entities is in fact extensive, encompassing hyperplastic conditions and both benign and malignant neoplasms. In addition to primary thymic epithelial tumours, a number of less common diseases typical for this location may be encountered, including extragonadal germ cell tumours, lymphomas, soft tissue tumours and a number of conditions which may simulate a tumour, such as cysts and inflammatory conditions. In this review, the disease entities that may be encountered in biopsy material from anterior mediastinal/thymic processes are discussed and an approach to tackle the differential diagnosis is provided.