Ectopic hamartomatous thymoma (EHT) is a rare benign neoplasm which was was named by Rosai et al. in 1984.Ectopic hamartomatous thymoma. A distinctive benign lesion of lower neck.Am J Surg Pathol. 1984 Jul;8(7):501-13.

It arises from misplaced branchial pouch derivatives (which normally forms thymic tissue).

Some studies suggest its origin from salivary gland. Pitfalls in the diagnosis of ectopic hamartomatous thymoma.Histopathology. 1996 Dec;29(6):549-55.

According to recent studies it has been suggested that the tumour should be reclassified as a 'branchial anlage mixed tumor 'Ectopic hamartomatous thymoma: a clinicopathologic and immuno histochemical analysis of 21 cases with data supporting reclassification as a branchial anlage mixed tumor.Am J Surg Pathol. 2004 Oct;28(10):1360-70.

Clinical presentation:

The tumour arises in adults, mostly male, as a slowly growing, sub-fascial swelling in the supra-sternal or supra-clavicular region.

The tumour usually does not recur.

Gross:

A well circumscribed swelling measuring 5 cm or less.

Microscopic features:

Consists of variable proportions of :

i) Bland spindle cells with pale cytoplasm & tapering nuclei, arranged in fascicles. 

ii) Island of mature adipose tissue

iii) Epithelial components are arranged in tubules or glands with epithelium that varies from non-keratinizing squamous, cuboidal or glandular elements.

Character of the cells are benign and mitosis is rare.

Immunohistochemistry:

Strong & diffuse positivity for cytokeratin and epithelial membrane antigen (EMA) in the spindle cell areas suggested thymic nature.

According to recent studies immunohistochemical analysis of the tumour revealed a complex immunophenotype with a diverse keratin profile. The plump spindled cells revealed a myoepithelial phenotype, as evidenced by the coexpression of cytokeratins , alpha-smooth muscle actin and CD10.

The tumour is negative for S-100 protein, glial fibrillary acidic protein, and CD34.

Differential diagnosis:

The differential diagnosis includes mixed tumours of skin adnexal or salivary gland origin, synovial sarcoma, glandular malignant peripheral nerve sheath tumor and cystic teratoma.

 Visit: Myxoid Tumours of Soft Tissue ;  Soft Tissue Tumours of Uncertain Differentiation ;   Neural tumours ;  Vascular tumours ; Fibroblastic/Myofibroblastic tumours ;  Myofibroblastic tumours ; Myogenic tumours ;  ChondroOsseous tumours

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