Histologic Features

The fat around the tumor contained areas with this appearance. What does it represent? Answer

Clinical summaryDiscussion

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Answer: This tissue (A) is a remnant of thymic tissue as it appears in the adult. It indicates that the tumor probably arose in the thymus. Although the tumor could represent a metastasis from another site, such primary tumors are rarely occult. Neither bronchoscopy nor chest CT revealed another primary site in this patient.

Compare this section of thymus with the normal thymus of a young child below (B). The atrophic thymus has relatively few lymphocytes, and the epithelial component is prominent. The lymphoid tissue in this patient shows a follicle (arrow) that is usually not present in the normal thymus. Follicles can also be seen in the thymus of patients with myasthenia gravis, collagen vascular diseases, or thyrotoxicosis [1].

A

Normal thymus (B): During gestation the thymus migrates into the antero-superior mediastinum from the 3rd and 4th pharyngeal pouches. It is an encapsulated organ that is divided into lobules composed of an outer, dark blue, densely cellular cortex (C) and an inner, light blue, less cellular medulla (M). Both portions contain a mixture of epithelial cells and lymphocytes, which can be distinguished from each other morphologically or by immunohistochemistry [2]. The medulla also contains whorls of keratinized epithelial cells--Hassall's corpuscles. Eosinophils, myoid cells with cross striations, and occasional neuroendocrine cells (Kultschitsky cells) are also present [3]. Fat accumulates with age as lymphoid tissue atrophies, but remnants of thymic tissue can be found at autopsy in adults by sampling fat anterior to the great vessels. The gland is important in T-cell development. As bone marrow-derived T lymphocytes pass through the cortex and the medulla, they undergo selection and maturation in response to thymic antigen-presenting cells [4]. Absence or abnormal development of the thymus is associated with several syndromes, e.g., DeGeorge or SCID. Thymectomy in the adult is not associated with immunologic abnormalities [5].

B

References:

1. Shimosato Y, Mukai K. Tumors of the Mediastinum. Atlas of Tumor Pathology, Third series, Fascicle 21. Washington, D.C., Armed Forces Institute of Pathology, 1997, 23-31.

2. Kornstein M. Controversies regarding the pathology of thymomas. Path Annu 1992; 27 Part 2:1-15.

3. Rosai J, Levine G. Tumors of the Thymus. Atlas of Tumor Pathology. Second series, Fascicle 13. Washington, D.C., Armed Forces Institute of Pathology, 1976, 1-21.

4. Klein J, Sato A. The HLA system. N Engl J Med 2000; 343:702-709.

5. Morgenthaler T, Brown L, Colby T, Harper Jr C, Coles D. Thymoma. Mayo Clin Proc 1993; 68:1110-1123.

Clinical summaryDiscussion

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