Malignant thymic neoplasms that may mimic benign conditions.

The thymus is a complex, highly specialized organ that is derived from the third and fourth pharyngeal pouches and contains elements that originate from all three germinal layers. As such, it shows certain specific and distinctive reaction patterns to injury that are frequently encountered in a variety of reactive and inflammatory conditions. These reaction patterns, however, can also often be associated with neoplastic diseases arising from this organ, and in some instances, their presence may contribute to obscure the malignant nature of the underlying process. We present a review of the main tumor entities that may be mistaken for benign conditions of the thymus, with emphasis on the three major reaction patterns to injury that may be associated with malignant neoplasms of this organ: prominent cystic changes, stromal fibrosis, and reactive and hyperplastic changes of thymic epithelium. Histologic clues helpful for diagnosis and the differential diagnosis of these lesions are discussed.