Type B1 Thymoma
Definition
- Thymoma exhibiting areas reminiscent of both normal cortex and medulla
Alternate / Historical Names
- Lymphocyte rich thymoma
- Lymphocytic thymoma
- Organoid thymoma
- Predominantly cortical thymoma
Diagnostic Criteria
- Predominant cortical areas with interspersed smaller medullary foci
- Lacks lobularity of normal thymus
- Usually thick fibrous capsule and septa
- Cortical areas closely resemble normal cortex: lymphocytes with inconspicuous epithelial cells
- Overwhelmingly lymphocyte rich ("impossible to count")
- Densely packed
- Small lymphocytes
- Immature T phenotype
- Inconspicuous epithelial cells
- Nuclei range from medium sized to large
- Nucleoli range from inconspicuous to prominent
- Long dendritic processes
- May be visible only on keratin stains
- Epithelial cell aggregates infrequent
- Tingible body macrophages may be scattered
- Overwhelmingly lymphocyte rich ("impossible to count")
- Medullary foci stand out as indistinctly circumscribed round pale zones
- On close inspection, they do not resemble normal medulla
- Pale appearance due to loose packing of lymphocytes
- Largely mature T phenotype
- Epithelial cells range from infrequent to occasional
- Hassall corpuscles range from absent to occasional
Supplemental Studies
- Epithelial cells
- Keratin positive
- Usually more prominent and dendritic in cortical areas
- EMA negative
- CD20 negative
- CD5 negative
- Keratin positive
- Lymphocytes
- Cortical areas: immature T phenotype (CD1a, CD99, TdT)
- Medullary foci: usually mature T phenotype
- Both types of T cells express most pan-T cell markers (CD3, CD5)
- Few B cells
Clinical
- Associations
- Myasthenia gravis
- Pure red cell aplasia
- Hypogammaglobulinemia
- Usually presents at low stage (75% of cases)
- May be aggressive
- Close to 90% long term survival
Differential Diagnosis
Type B1 Thymoma | Type B2 Thymoma |
---|---|
Predominantly lymphocytes with inconspicuous epithelial cells | Both lymphocytes and epithelial cell prominent |
Numerous scattered pale medullary foci | Only occasional medullary foci |
Type B1 Thymoma | Normal Thymus |
---|---|
Lacks regular thymic lobular structure | Lobular with regular pattern of superficial cortex and underlying medulla |
Thick fibrous capsule and septa produce large lobules | Thin normal thymic capsule |
Medullary foci typically have few epithelial cells other than Hassall corpuscles | Medulla may have many epithelial cells |
Hassall corpuscles variably present in medullary foci | Hassall corpuscles present in medulla of almost all cases |
Type B1 Thymoma | Precursor T Lymphoblastic Leukemia / Lymphoma |
---|---|
Scattered epithelial cells (may require anti-keratin stain) | Only keratin positive cells should be overrun thymus |
Scattered pale medullary foci | Lacks medullary foci |
Thick fibrous capsule and septa produce large lobules | Lacks thick fibrous capsule and septa and lobular growth pattern |
Typically older adult but may occur in children | Typically adolescent or pediatric |
No circulating blasts | Circulating blasts |
Variable expression of surface CD3, CD4, CD8 in a given case | Uniform phenotype in a given case |
Type B1 Thymoma | True Thymic Hyperplasia |
---|---|
Dendritic epithelial cells scattered throughout broad lymphocyte rich area with only scattered medullary like foci | Normal alternation of cortical and medullary areas |
Medullary-like areas rarely contain Hassal corpuscles | Medullary areas contain Hassal corpuscles |
Large lobules separated by fibrous bands | Small lobules surrounded by capsule and interstitium |
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, Burke AP, Marx A, Nicholson AG eds. World Health Organization Classification of Tumors. Pathology and genetics of tumors of the lung, pleura, thymus and heart. IARC Press: Lyon 2015.
- Nonaka D, Henley JD, Chiriboga L, Yee H. Diagnostic utility of thymic epithelial markers CD205 (DEC205) and Foxn1 in thymic epithelial neoplasms. Am J Surg Pathol. 2007 Jul;31(7):1038-44.
- Alexiev BA, Drachenberg CB, Burke AP. Thymomas: a cytological and immunohistochemical study, with emphasis on lymphoid and neuroendocrine markers. Diagn Pathol. 2007 May 11;2:13.
- Tateyama H, Eimoto T, Tada T, Hattori H, Murase T, Takino H. Immunoreactivity of a new CD5 antibody with normal epithelium and malignant tumors including thymic carcinoma. Am J Clin Pathol. 1999 Feb;111(2):235-40.
- Dorfman DM, Shahsafaei A, Chan JK. Thymic carcinomas, but not thymomas and carcinomas of other sites, show CD5 immunoreactivity. Am J Surg Pathol. 1997 Aug;21(8):936-40.
- Okumura M, Ohta M, Tateyama H, Nakagawa K, Matsumura A, Maeda H, Tada H, Eimoto T, Matsuda H, Masaoka A. The World Health Organization histologic classification system reflects the oncologic behavior of thymoma: a clinical study of 273 patients. Cancer. 2002 Feb 1;94(3):624-32.