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Soft Tissue Pathology

Pathology of Ectopic Hamartomatous Thymoma

Dr Sampurna Roy MD                





Ectopic hamartomatous thymoma (EHT) is a rare benign neoplasm which was was named by Rosai et al. in 1984.

It arises from misplaced branchial pouch derivatives (which normally forms thymic tissue). Some studies suggest its origin from salivary gland.

According to recent studies it has been suggested that the tumour should be reclassified as a 'branchial anlage mixed tumor'.

There is no evidence that the tumour is in any way related to thymus.

Clinical presentation:

Common site is lower anterior part of the neck.

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: Image:

A well circumscribed, swelling measuring 5 cm or less. It is not encapsulated.

Microscopic features:  Image

Consists of variable proportions of :

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

ii) Islands of mature adipose tissue.

iii) Islands of epithelioid cells. The epithelioid component may be arranged in tubules or glands.

The lining epithelial cell varies from non-keratinizing squamous, cuboidal or glandular in type.

Some clear cells may be present.

Character of the cells are benign. Mitotic rate is very low or may be absent.

There is no necrosis.



Strong and 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.


Further reading:

Ectopic hamartomatous thymoma: a clinicopathologic and immuno histochemical analysis of 21 cases with data supporting reclassification as a branchial anlage mixed tumor.

Ectopic hamartomatous thymoma. A distinctive benign lesion of lower neck.Am J Surg Pathol.

Pitfalls in the diagnosis of ectopic hamartomatous thymoma.  

Ectopic hamartomatous thymoma: radiographic and clinicopathological features.

Ectopic hamartomatous thymoma is distinct from lipomatous pleomorphic adenoma in lacking PLAG1 aberration.

Ectopic hamartomatous thymoma: a case demonstrating skin adnexal differentiation with positivity for epithelial membrane antigen, androgen receptors, and BRST-2 by immunohistochemistry.

Ectopic hamartomatous thymoma growing in the sternocleidomastoid muscle masquerading as sarcoma.

Ectopic hamartomatous thymoma: a clinicopathological and immunohistochemical study of two cases.






Dr Sampurna Roy  MD

Consultant  Histopathologist (Kolkata - India)






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