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