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
Original posting/updates: 9/24/10
Differential Diagnosis
Ectopic Hamartomatous Thymoma bears no resemblance to any type of thymoma
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
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
Thymoma
Thymic carcinoma
Low grade
Well differentiated squamous
Basaloid
Adenocarcinoma
Adenosquamous
Low grade mucoepidermoid
High grade
Lymphoepithelial-like
Large cell undifferentiated
Clear cell
Sarcomatoid
Midline carcinoma with t(15;19)
Thymic neuroendocrine carcinoma
Miscellaneous
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
