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Ectopic Hamartomatous Thymoma

Definition

  • Subcutaneous tumor composed of bland spindle cells, nests of epithelioid cells and adipocytes

Alternate/Historical Names

  • Branchial Anlage Mixed Tumor (see below)

Diagnostic Criteria

  • Triphasic histologic appearance, components may vary in proportions
  • Predominantly composed of spindle cells
    • Vary from plump to thin
    • Bland, inconspicuous nucleoli
      • Mitotic rate generally <3/50 hpf, no pleomorphism or necrosis
    • Smooth muscle actin, CD34, keratin, p63 positive
    • Rare findings
      • Palisading
      • Entrapped nerve
      • Focal differentiated smooth muscle
      • Psammoma-like calcifications
  • Haphazard nests and anastomosing cords of epithelioid cells
    • Frequently appear entrapped
    • May be squamous or glandular
    • Occasionally clear cells or tubules
    • Bland
    • Keratin and p63 positive
  • Variable population of mature adipocytes
  • Lacks any population of immature T cells
  • Circumscribed but not encapsulated
  • Occurs in subcutis of lower anterior neck
  • There is no evidence that this tumor has any relationship to the thymus
    • It has never been reported in association with thymic tissue
    • It does not occur in the mediastinum
    • It shows no evidence of thymic differentiation
    • The spindle cell phenotype (actin, keratin, CD34 and p63+) is not seen in other thymic lesions
    • Fetsch et al. have proposed Branchial Anlage Mixed Tumor as a better name
  • One report (Weinreb) describes associated skin adnexal differentiation
    • Sebaceous (EMA+) and apocrine (BRST2 and androgen receptor+) cells
    • Eccrine-like EMA luminal staining

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

Original posting/updates: 9/24/10

Differential Diagnosis

Ectopic Hamartomatous Thymoma bears no resemblance to any type of thymoma

Spindle Cell Lipoma Ectopic Hamartomatous Thymoma
Lacks epitihelioid nests Has epithelioid nests
Has ropy collagen Lacks ropy collagen
Keratin, p63 and smooth muscle actin negative Keratin, p63 and smooth muscle actin positive
Both contain spindle cell and adipose components and both are CD34 positive in the spindled component

 

Extragonadal Teratoma Ectopic Hamartomatous Thymoma
In the neck, most are associated with the thyroid or ectopic thymus Located in subcutaneous fat
May have a variety of differentiated tissues present Restricted to spindled cells with squamous or glandular epithelium and fat
A variety of differentiated epithelial patterns may be present Epithelium in nests or anastomosing cords
Spindle cell component may have a variety of phenotypes Spindle cell component has a myoepithelial phenotype

Classification / Lists

Miscellaneous Thymic Lesions and Conditions

Non-Thymic Neoplasms that May Arise Primarily in the Thymus

  • Hematopoietic
    • Primary mediastinal large B cell lymphoma
    • Extranodal marginal zone lymphoma
    • Precursor T lymphoblastic lymphoma/leukemia
    • Hodgkin lymphoma
  • Germ cell tumors
    • Seminoma (Germinoma)
    • Embryonal carcinoma
    • Yolk sac tumor
    • Teratoma
    • Choriocarcinoma
    • With associated somatic type malignancy
      • Embryonal rhabdomyosarcoma
      • Angiosarcoma
      • Others
    • With associated hematologic malignancy
      • Acute myelogenous leukemia
      • Acute megakaryoblastic leukemia
      • Others

Intrinsic Tumors of the Thymus

Bibliography

  • Shimosato Y, Mukai K, Matsuno Y. Tumors of the Mediastinum, Atlas of Tumor Pathology, AFIP Fourth Series, Fascicle 11, 2010
  • Rosai J, Limas C, Husband EM. Ectopic hamartomatous thymoma. A distinctive benign lesion of lower neck. Am J Surg Pathol 1984;8:501- 13
  • Weinreb I, O'Malley F, Ghazarian D. Ectopic hamartomatous thymoma: a case demonstrating skin adnexal differentiation with positivity for epithelial membrane antigen, androgen receptors, and BRST-2 by immunohistochemistry. Hum Pathol. 2007 Jul;38(7):1092-5
  • Kushida Y, Haba R, Kobayashi S, Ishikawa M, Doi T, Kadota K. Ectopic hamartomatous thymoma: a case report with immunohistochemical study and review of the literature. J Cutan Pathol. 2006 May;33(5):369-72.
  • Fetsch JF, Laskin WB, Michal M, Remotti F, Heffner D, Ellis G, Furlong M, Miettinen M. Ectopic hamartomatous thymoma: a clinicopathologic and immunohistochemical analysis of 21 cases with data supporting reclassification as a branchial anlage mixed tumor. Am J Surg Pathol. 2004 Oct;28(10):1360-70
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