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Hürthle cell carcinoma in an unattached thyroid lobe: A diagnostic challenge

To the Editor: Heterotopic thyroid tissue is a common abnormality and results from abnormal embryologic development and migration of the thyroid gland. Thyroid heterotopia can be found at a wide neck area defined as the W€ olfler area, spanning from the edges of the mandible through the neck to the aortic arch. There is no term for cases of unattached lobe to the rest of thyroid. Thyroid diseases can be, although rare, found in that setting. Herein, an exceptional case of H€ urthle cell carcinoma that developed in an unattached thyroid lobe is described. An 86 year old man was referred to the surgical department due to a right palpable cervical mass, hoarseness and respiratory distress. A subsequent ultrasound scan showed a cervical mass with solid and cystic anechoic areas and agenesis of the right thyroid lobe. On the MRI the right cervical mass was 68mm in diameter and extended from the level of the hyoid bone to the thoracic inlet. Multiple enlarged cervical and mediastinal lymph nodes were also found. Those findings raised the suspicion of an advanced well-differentiated thyroid neoplasm, a lymphoma or a metastatic neoplasm of unknown primary origin. Moreover the base of the mass was located at a distance of 1 cm from the thyroid isthmus. A small biopsy specimen of the neck mass revealed neoplasm with oxyphilic cells and raised the suspicion of a thyroid carcinoma, which was surgically managed. During surgery the left lobe and the isthmus were excised along with the right neck mass, considered to be a metastatic lymph node block. For histologic examination a specimen of thyroid gland was sent consisting of left lobe measuring 3 3.4 0.4 cm and isthmus measuring 1.3 1.7 0.4 cm. A nodular specimen with dimensions 7.7 4.3 3.5 cm corresponding to the right mass was also sent, which was thought to be metastatic lymph node, taking into consideration that most neck lumps are enlarged lymph nodes. On parallel sections a whitish lesion 6.5 cm at its greatest dimension was observed whereas a fine zone of thyroid parenchyma at the periphery was also apparent (Fig. 1a). Hematoxylin and eosin stained sections revealed a neoplasm of H€ urthle cells with monomorphic oncocytic morphology. Neoplastic cells were medium sized with eosinophilic granular cytoplasm and roundish or angular nuclei with evident nucleoli (Fig. 1b) and they were mainly arranged in groups in a fibrotic stroma and scarcely in follicles. Extensive areas of necrosis were observed. The lesion focally was surrounded by a fine capsule (Fig. 1c). Neoplastic emboli in vessels were found (Fig. 1d). On immunohistochemical study the neoplastic cells were positive to thyroglobulin, CK7, TTF1 antigens and negative to CK20, parathormone and calcitonin. MIB-1 mitotic index was almost 15%. Sections from thyroid parenchyma the adjacent to the neoplasm and from the left lobe and isthmus revealed nodular goiter lesions. The diagnosis of follicular carcinoma of the thyroid, oxyphilic type (H€ urthle cell carcinoma) was set. The postoperative period was uneventful; the patient's symptoms were alleviated and he was discharged at the seventh post-operative day. Oncocytic follicular carcinomas usually have a solid/ trabecular pattern of growth. Sometimes the cells group in well-defined nests or insulare. The solid and trabecular forms of these tumors tend to have scant or absent colloid. The nuclei tend to be hyperchromatic and pleomorphic and generally have prominent eosinophilic nucleoli. When colloid is present, there is a tendency for it to be basophilic with frequent concentric rims of calcification, resembling psammoma bodies. The cytoplasm of the tumor cells is deeply eosinophilic and granular clear cell change may be prominent in some tumors. The diagnosis of oncocytic follicular carcinoma depends on the demonstration of capsular and/or vascular invasion. The tumors are classified into minimally (also designated encapsulated type or angioinvasive carcinoma) and widely invasive types. As observed in the present case, oncocytic follicular carcinomas are typically positive for thyroglobulin and TTF1 and usually exhibit a cytokeratin 7 positive and cytokeratin 20 negative phenotype. The question that came up in this case was the origin of the right cervical mass. The absence of the right lobe on ECHO examination had been considered as agenesis of the right lobe, although left lobe agenesis is more common, accompanied by right cervical mass that possibly corresponded to metastatic lymph nodes. However, the presence of thyroid parenchyma at the periphery of the excised mass along with the absence of malignancy in the left lobe and isthmus, supported the case of H€ urthle cell carcinoma arose on heterotopic thyroid tissue. Based on embryology and current terminology on thyroid developmental anomalies a possible unattached right lobe