MTC is a rare CT-secreting neoplasm that occurs in both familial and sporadic forms [6]. To date, only 16 cases of oncocytic variant of MTC have been reported in literature (Table 1) [7, 8]. It is the most aggressive differentiated thyroid carcinoma with 10 year survival rates of 40–50%. Peak age of familial MTC is younger (approximately 35 years) than sporadic MTC (40-60 years) [6, 9]. On histological examination, MTC consists of sheets of spindle-shaped, round or polygonal cells separated by fibrous stroma. The nuclei are usually uniform in shape with variable mitotic figures. The cytoplasm is eosinophilic with a finely granular appearance. Amyloid deposits are seen in about 75% of tumors [9, 10]. Most of MTCs show positive staining for CT and mCEA [11]. Oncocytic variant of MTC is extremely rare and is the only oncocytic tumor not derived from follicular cells [10]. According to Dominguez-Malagon et al at least 60-70% of the cells should be oncocytic for diagnosis [12]. The main criteria for malignancy included the presence of capsular and/or vascular invasion [10]. Our case revealed 80-90% cells having oncocytic morphology on cytology and histopathology along with nodal metastasis and capsular invasion, supportive classical immunohistochemical results and apple green birefringence on Congo-red stain.
Table 1
Review of Literature
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Tranchida et al.
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Harach et al.
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Domigez et al.
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Desai et al.
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Raikhlin et al.
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Chetty et al.
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Dedivitis et al.
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Canberk
et al.
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Present case
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FNA diagnosis
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Lymphnode metastasis of an oncocytic variant
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-
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-
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-
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-
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-
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-
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MTC,oncocytic variant
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Oncocytic variant of MTC
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Histopathological diagnosis
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-
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Oncocytic variant of MTC
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Oncocytic variant of MTC
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MTC with oncocytic change
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MTC with oncocytic change
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Oncocytic variant of MTC
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Oncocytic variant of MTC
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Oncocytic variant of MTC
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Oncocytic variant of MTC
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CT level is a key feature of both pre-operative diagnosis and post-operative follow up and known as most specific and sensitive marker of MTC for both the primary diagnosis and the postsurgical follow-up [13]. Serum CT level in our case was very high (1907 pg/ml).
A limited number of cases of oncocytic MTC have been reported in literature [14, 15]. Most case reports focus on mitochondrial DNA alterations that result in preferential survival in hypoxic conditions and are resistant to conventional treatments [5].
Oncocytic change is noted in many thyroid malignancies. The differentials include oncocytic variant of MTC (OV-MTC), oncocytic adenoma, oncocytic carcinoma, oncocytic variant of papillary thyroid carcinoma, and oncocytic variant of poorly differentiated thyroid carcinoma. Due to the distinct biologic behavior of oncocytic tumors, they should be evaluated in proper perspective in the thyroid. The IHC panel in this case showed positivity for CK7, CT, mCEA and an apple green birefringence on Congo-red stain while IHC for thyroglobulin was very focally weak positive, thus establishing the origin of tumor as from C cells of thyroid.
Oncocytic variant of MTC is a difficult diagnosis to make on cytology. Only one case of oncocytic variant of MTC could be diagnosed on FNAC of a lymph node in a known case diagnosed on thyroidectomy done 5 years earlier. To the best of our knowledge, this is the second reported case of oncocytic variant of MTC diagnosed on FNA and confirmed on histopathology [15].
As all oncocytic tumors are prone to infarction, it is important to correctly diagnose them on first FNAC. Surgical resection is the treatment of choice. Therefore, we present this case with aim of highlighting the importance of early diagnosis of MTC with varied morphology on FNAC and identification of a rare oncocytic variant thus providing correct management to the patient in due time leading to improved survival.