Clinical features :
Female patient, 48 years old. Plurinodular euthyroid goiter

Gross features :
2cm, firm, well delimited, left nodule in plurinodular hyperplasia

Microscopy :
This lesion is made of trabecular solid pattern, ribbon-like, or cribriform microfollicular clumps, with palissading disposition of the cells (fig A and fig B).
	fig. A
	fig. B
Those cells are variable in shape : most often elongated, fusiform, sometimes polygonal or round, with clear borders and eosinophilic, granular cytoplasm (fig C).
	fig. C
The nuclei are ovoid, with small nucleoli, small grooves, and frequent pseudo inclusions (fig D). The stroma is abundant, situated at the periphery of the epithelial trabecular structures, eosinophilic, hyalinized, suggesting amyloid deposits. Some calcifications sometimes psammoma-like are seen ; there is a thin and regular capsule around the lesion.
	fig. D
Immunohistochemical profile demonstrate the follicular origin of this lesion : the cells are positive with Thyroglobulin (fig E), negative with Calcitonine
	fig. E

Diagnosis :
Hyalinizing trabecular tumor

Differential diagnosis :
-    medullary thyroid carcinoma : fusiform cells in trabecular pattern, pseudo amyloid stroma : diagnostic with immunohistochemical profile : CT positive, TG negative
-    paraganglioma : lobular or alveolar pattern, positivity with Chromogranin and S100 Protein (sustentacular cells).
-    papillary carcinoma mostly in FNA (pseudo inclusion)

Comments :
Carney, who described this lesion in 1987, considered it as benign :" hyalinizing trabecular adenoma ".
During the next decade, malignant forms with metastasis or with capsular or/and vascular invasion has been described as " hyalinizing trabecular carcinoma ".
In addition, in 1/3 of cases, papillary carcinoma were found in the same thyroid gland associated with TTH .
Recent works have shown Ret protooncogene rearrangment in TTH in 28,8% in cases, the same frequence as in papillary carcinomas.
For Livolsi, the question is : " is the hyalinizing trabecular adenoma not an adenoma at all but the unusual histologic variant of papillary carcinoma. "  The question is debated in literature :
-    some authors consider all TTH as malignant papillary carcinoma.
-    For Livolsi, an encapsulated tumor showing only classical hyalinizing trabecular pattern (Carney 's description) is to consider as very low malignant potential tumors.
The lesion presented here, belongs to this classical form.

References :

CARNEY J.A., RYAN J., GOELLNER J.R.
Hyalinizing trabecular adenoma of the thyroid gland.
Am. J. Surg. Pathol. - 1987 - vol. 11 - n° 8 - p. 583-591.

Li VOLSI Virginia
HTT of the thyroid : adenoma, carcinoma or neoplasm of uncertain malignant potential ?
Am. J. Surg. Pathol. 2000 vol 24 1683-84