A 24-year-old man presents with an anterior mediastinal mass that was incidentally found when he was treated for COVID-19. Intra-operatively, a giant fatty tumor measuring 35 x 22 x 10.5 cm was identified in the prevascular mediastinum extending toward the right chest causing compression atelectasis of the right lower lobe of the lung.
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Thymolipoma is an uncommon benign, slow-growing thymic tumor. They have been shown to be associated with autoimmune diseases such as myasthenia gravis, aplastic anemia, hypogammaglobulinemia, lichen planus, and Graves’ disease.  There have been several theories to explain the pathogenesis of these lesions, but the evidence so far points to it being a neoplasm of adipose tissue within the involuting thymus. Cytogenetic studies in one case report have demonstrated a t[12;14](q15q32) translocation involving the HMGA2 gene, which is also seen in about two-thirds of lipomas.. These lesions could also arise due to fatty replacement in a background of true thymic hyperplasia in the same manner as in a normal thymus.
Grossly thymolipomas are well-circumscribed, encapsulated, and the cut surface is lobulated and homogenous. Histologically, it is characterized by mature adipose tissue interspersed with cords of thymic tissue with cortex and medulla and Hassall corpuscles, although the thymic tissue is usually attenuated and seen in form of strands. In contrast to thymolipomas, thymomas are comprised of cellular lobules intersected by fibrous bands. The lobules are comprised of various ratios of neoplastic epithelial cells and reactive thymocytes. Lipofibroadenomas show similar features as thymolipomas and in addition harbor areas with fibrosis that appear similar to fibroadenomas of the breast.
Arpan Samaddar, M.B.B.S.
Resident, Anatomic and Clinical Pathology
Anja Roden, M.D.
Consultant, Anatomic Pathology
Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science