1 November 2007 – Case of the Week #99


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We thank Dr. Keloth E. Pradeep, Wrexham Maelor Hospital, United Kingdom, for contributing this case.  To contribute a Case of the Week, please email info@PathologyOutlines.com with the clinical history, your diagnosis and microscopic images in JPG, GIF or TIFF format (send as attachments, any size, we will shrink if necessary).  Please include any other images (gross, immunostains, etc.) that may be helpful or interesting.  We will write the discussion (unless you want to), list you as the contributor, and send you a check for $35 (US dollars) for your time after we send out the case.  Please only send cases with a definitive diagnosis, and preferably cases that are out of the ordinary.


Case of the Week #99


Clinical History


A 70 year old man had a calcified mediastinal mass on chest Xray.  A core needle biopsy was taken.


Micro images:  #1#2#3#4#5

Immunostains:  CD3CD20keratinCD68 


What is your diagnosis?




























Thymoma, B1 type




Thymoma is the most common primary anterior mediastinal neoplasm.  It is often associated with myasthenia gravis and other immune-mediated disorders.


Morphologically, thymoma is a combination of bland epithelial cells and non-neoplastic lymphocytes.  This case was classified as type B1 (WHO) based on the large number of T lymphocytes (CD3+) mixed with epithelial cells (keratin+).


The numerous classification systems of thymoma have been extensively discussed, most recently by Suster and Moran (Am J Clin Pathol 2006;125:542).  For prognostic purposes, it is important to distinguish thymoma types that typically have benign behavior (types A, AB, B, B1 and B2), from those with atypical behavior (type B3, Ann Thorac Cardiovasc Surg 2005;11:367) and those that are cytologically malignant (type C, also called thymic carcinoma, AJSP 2002;26:1605).


The differential diagnosis of this lymphocyte rich type of thymoma is primarily lymphoblastic lymphoma and thymic lymphoid hyperplasia.  Lymphoblastic lymphoma typically presents in teenagers or young adults.  In contrast to thymoma, the lymphocytes are malignant and the epithelial component, if present, represents trapped normal epithelial cells.  Careful use of immunostains is necessary to avoid misdiagnosis.  Thymic lymphoid hyperplasia is also associated with myasthenia gravis, but typically does not produce a mass.  Lymphoid follicles are present with normal germinal centers, but the pattern of epithelial cells may be disrupted.  In some cases, the differential diagnosis of thymoma may also include thymic carcinoid tumor (if rosettes are present), or Castleman’s disease.


As an aside, we are pleased that American Journal of Clinical Pathology is now providing articles that are free full text, no registration, after 1 year from publication.  Other free full text, no registration pathology journals that we find particularly useful are Archives of Pathology and Laboratory Medicine (always free) and Modern Pathology (free after 1 year from publication).


Additional references: Mediastinum chapter of PathologyOutlines.com




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