Cytological diagnosis of oncocytic variant of medullary thyroid carcinoma with histological and immunohistochemical correlation Running title: Cytolomorphology of oncocytic variant of medullary thyroid carcinoma

Abstract

Medullary thyroid carcinoma (MTC) is a sporadic or familial tumor of the parafollicular or C-cells that secrete calcitonin. Oncocytic tumors of the endocrine system are a group of rare entities described in several organs, including the thyroid gland and seen in a variety of thyroid conditions that are associated with a broad differential diagnosis like benign (Hürthle cell adenoma, granular cell tumor) and malignant (Hürthle cell carcinoma) neoplasms, variants of papillary thyroid carcinoma (PTC) (tall cell variant, oncocytic variant, and Warthin-like variant) and the oncocytic variant of medullary carcinoma. Oncocytic variant of medullary carcinoma is exceptional not only because of being extremely rare but also being the only oncocytic tumor that is not derived from follicular cells of the thyroid gland. We report here an 18-year-old female patient who presented with complaints of passage of loose stool for 4 months and was on examination detected to have a right thyroid nodule. FNAC of the nodule was done and an oncocytic tumour was diagnosed. The differentials considered were Hürthle cell lesion and medullary carcinoma thyroid (oncocytic cell variant). This was followed by total thyroidectomy along with modified radical neck dissection. Histopathological examination and immunohistochemistry were performed in addition to serum calcitonin levels and a diagnosis of oncocytic variant of medullary carcinoma thyroid was confirmed. We hereby conclude that MTC should be considered as one of the differentials for accurate diagnosis in unusual thyroid carcinomas with predominance of oncocytic cell changes.

Introduction

Medullary thyroid carcinoma (MTC) is an uncommon primary thyroid malignancy arising from the C cell or parafollicular cells and accounts for 5–7% of all thyroid malignancies[1]. Both sporadic and hereditary forms of MTC are known to occur, accounting for 80% and 20% of all the cases respectively [1–3]. The biochemical activity of MTC includes production of calcitonin (CT) and cacinoembryogenic antigen (CEA). The stage and age at diagnosis are the most important prognostic factors. Early detection and diagnosis is important, and fine-needle aspiration cytology (FNAC) of the thyroid gland/ lymph nodes plays a major role in the diagnosis of MTC.

The variants of MTC include glandular, papillary, small cell, spindle cell and giant cell type. Less common are the clear cell, melanotic (pigmented), oncocytic (oxyphilic), squamous, amphicrine and paraganglioma-like variants [4]. Oncocytes (oxyphilic cells, Hürthle cells) are characterized morphologically by enlarged size, distinct cell borders, abundant granular acidophilic cytoplasm, large nucleus and prominent nucleolus. The pathogenetic basis of oncocytic change is fascinating because of mitochondrial richness which contain their own separate and parallel DNA. Alterations in mitochondrial DNA result in preferential survival in hypoxic conditions and explains the malignant behaviour of these tumors which are resistant to conventional treatments [5]. Any misdiagnosis will cause delay in aggressive surgery or other modalities of therapy causing infarction and resistance to conventional treatment.

Case History

An 18-years-old young female presented to the outpatient department of Surgery with the chief complaints of loose stools since four months. Patient also complained of heat intolerance, palpitations and weight loss. On examination she had a right sided thyroid nodule and a provisional clinical diagnosis of toxic thyroid nodule with thyrotoxicosis was considered. Her thyroid function tests were within normal limits. FNAC done from the thyroid nodule revealed highly cellular smears showing a monomorphic population of oncocytic tumor cells with large, eosinophilic, loosely granular, finely vacuolated cytoplasm, well-defined cytoplasmic borders and eccentrically placed nuclei admixed focally with spindle cells [Fig. 1]. Oncocytic cells had a salt-and-pepper-textured chromatin and prominent nucleoli which were arranged in clusters and microfollicular patterns. Binucleation, intracytoplasmic red granules and multivacuolation were remarkably noted. Background showed scanty to absent colloid. Based on the oncocytic morphology and presence of spindle cells, provisional differentials of Hurthle cell lesion and MTC were given and serum CT level was performed and found to be raised (1907 pg/ml; normal < 5 pg/ml).

Ultrasound abdomen showed normal study with no evidence of adrenal mass lesion. Contrast enhanced computer tomography (CECT) neck showed a well defined enhancing nodule in the right lobe of thyroid with multiple enlarged lymphnodes in the neck. Paratharmone (PTH) level was 60.8 (15–65 pg/ml), urinary vanillylmandelic acid (VMA) 4.32 (1.6–7.3 mg/24hrs) and metanephrine 61.92 (276.1 µg/24hrs) excluding multiple endocrine neoplasia (MEN) syndrome. Positron emission tomography (PET) scan was suggestive of an active malignant disease involving right lobe of thyroid and multiple fluorodeoxyglucose avid in bilateral level II, right level III, right level IV cervical lymphnodes. ⁹⁹Tc scan thyroid showed vascular cold nodule in right lobe of thyroid gland, mid pole-lateral [Fig. 2a].

To confirm the diagnosis, immunohistochemistry was performed on the cytological smears and showed strong granular positivity for CT in most cells and focally weak positivity for thyroglobulin (TG) in few cells [Fig. 1]. Based on cytomorphological, immunohistochemical and clinical findings, a final FNAC diagnosis of MTC, Bethesda VI was given. Due to the predominant population of oncocytic cells, the subtype of oncocytic variant was suggested. Patient underwent total thyroidectomy with modified radical neck dissection.

On gross examination, right lobe of thyroid was enlarged with a hard nodule showing tan brown appearance measuring 3x2 cm [Fig. 2b]. Multiple lymphnodes were attached to the adherent soft tissue component. Microscopically, the tumour was encapsulated and compressing the surrounding residual thyroid tissue with focal capsular invasion. Tumour cells were arranged in solid, trabecular and nested pattern separated by a fibrous stroma [Fig. 3]. Prominent eosinophilic amorphous extracellular substance was seen within the tumour [Fig. 3]. IHC for monoclonal carcinoembryogenic antigen (mCEA), chromogranin-A and cytokeratin 7 (all Thermo-scientific, RTU) were performed on paraffin sections and showed strong positivity [Fig. 3c-3e]. Congo-red stain showed apple green birefringence of eosinophilic amorphous material on polarization thus confirming amyloid [Fig. 3f]. Nine out of eleven lymphnodes isolated from right side showed presence of similar morphology tumor deposits, while left sided nodes were negative. Based on the above histopathological and IHC workup, a diagnosis of oncocytic variant of MTC with nodal metastasis was further confirmed.

Discussion

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. 

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. 

Conclusion

FNAC is a simple, OPD based procedure which greatly helps in definitive preoperative diagnosis of MTC. In spite of cytologic variability, we were able to diagnose this rare variant of oncocytic type of MTC accurately on FNAC. A high index of suspicion on FNAC combined with immunocytochemistry and serum calcitonin levels helps in diagnosing this rare tumor with accuracy and helps in timely management of this rare condition.  

Declarations

Authors' contributions – 

Gupta RK: Concept of work, Intellectual content and finer editing, literature search (Corresponding Author),

Srivastava R: Collection of data and drafting the manuscript

Majumdar K: Intellectual content and finer editing 

Batra VV: Intellectual content and finer editing

Ghuliani D: Clinical work-up and surgical intervention 

Competeting interests- None to be declared

Ethics approval – In accordance with Institutional Ethics policy

Consent to participate – An informed consent was obtained to publish the data without disclosing patient’s identity

Data availability- Not applicable

Funding- Nil

Acknowledgment- None

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