Thymolipoma

Definition

• Mass forming lesion composed of circumscribed mature adipose tissue and unremarkable thymus

Diagnostic Criteria

• Circumscribed mass forming lesion
  • Usually over 500 gm
    • Smaller lesions may be found in thymectomy performed for myasthenia gravis
  • May form a discrete tumor in thymus or diffusely enlarge the thymus
• Predominantly adipose tissue containing interconnecting cords of thymus
  • Thymic tissue is essentially unremarkable with both cortex and medulla
• Rare variants are reported, with no known clinical difference
  • Thymofibrolipoma
  • Lipofibroadenoma
    • Interconnecting cords of epithelial cells and groups of adipocytes set in a loose fibrous component
    • 2 of 3 reported cases were associated with conventional thymoma
  • One case reported as thymohemangiolipoma
• Rare cases reported with prominent myoid cells
• Rare, mean age 20-30
  • May be seen in children
• Up to 50% in some series are associated with autoimmune disease
  • Myasthenia gravis is most common
    • Also aplastic anemia, hypogammaglobulinemia, lichen planus, Grave disease
    • Diseases and symptoms may improve following resection
• Benign
• Differential diagnosis is limited
  • Lipoma is ruled out by the presence of the thymic component
  • Atrophic thymus with adipose replacement is an incidental finding and does not form a mass

Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342

Classification / Lists

Intrinsic Tumors of the Thymus

• Thymoma
  • Type A
  • Type AB
  • Type B1
  • Type B2
  • Type B3
  • Other types
    • Micronodular with lymphoid stroma
    • Metaplastic
    • Sclerosing
    • With prominent signet ring like features
• Thymic carcinoma
  • Low grade
    • Well differentiated squamous
    • Basaloid
    • Adenocarcinoma
    • Adenosquamous
    • Low grade mucoepidermoid
    • Micronodular
  • High grade
    • Lymphoepithelial-like
    • Large cell undifferentiated
    • Clear cell
    • Sarcomatoid
    • Midline carcinoma with t(15;19)
• Thymic neuroendocrine carcinoma
  • Well differentiated (carcinoid)
  • Poorly differentiated
    • Small cell
    • Large cell
• Miscellaneous
  • Thymolipoma
  • Microscopic thymoma / nodular hyperplasia of thymic epithelium

Miscellaneous Thymic Lesions and Conditions

• Atrophy
• Cysts
  • Most are non-specific
  • Neoplasms most commonly associated with cysts
    • Thymoma
    • Seminoma
    • Hodgkin lymphoma
    • Extranodal marginal zone lymphoma
    • Basaloid carcinoma
• Castleman disease
• Dysplasia
• Hyperplasia
  • B cell/lymphoid/germinal center
  • Rebound
  • True thymic
• Microscopic thymoma / nodular hyperplasia of thymic epithelium
• (Ectopic hamartomatous thymoma)

Bibliography

• Shimosato Y, Mukai K, Matsuno Y. Tumors of the Mediastinum, Atlas of Tumor Pathology, AFIP Fourth Series, Fascicle 11, 2010
• Travis WD, Brambilla E, Muller-Hermelink HK, Harris CC eds. World Health Organization Classification of Tumors. Pathology and genetics of tumors of the lung, pleura, thymus and heart. IARC Press: Lyon 2004
• Moran CA, Zeren H, Koss MN. Thymofibrolipoma. A histologic variant of thymolipoma. Arch Pathol Lab Med. 1994 Mar;118(3):281-2
• Rieker RJ, Schirmacher P, Schnabel PA, Moser K, Hoffmann H, Dienemann H, Pfannschmidt J. Thymolipoma. A report of nine cases, with emphasis on its association with myasthenia gravis. Surg Today. 2010;40(2):132-6
• Ogino S, Franks TJ, Deubner H, Koss MN. Thymohemangiolipoma, a rare histologic variant of thymolipoma: a case report and review of the literature. Ann. Diagn. Pathol. 2000; 4; 236–239
• Gannon BR, Dexter DF, Petsikas D, Isotalo PA. Mediastinal thymolipoma: a rare occurrence with striated myoid cells. Tumori 2007; 93; 198–200
• Iseki M, Tsuda N, Kishikawa M et al. Thymolipoma with striated myoid cells. Histological, immunohistochemical, and ultrastructural study. Am. J. Surg. Pathol. 1990; 14; 395–398
• Moran CA, Zeren H, Koss MN. Thymofibrolipoma. A histologic variant of thymolipoma. Arch. Pathol. Lab. Med. 1994; 118; 281–282.
• Kuo T-T, Shih L-Y. Histologic types of thymoma associated with pure red cell aplasia: A study of five cases including a composite tumor of organoid thymoma associated with an unusual lipofibroadenoma. Int J Surg Pathol. 2001 9: 29-35