Introduction

Medullary thyroid carcinoma (MTC) is a rare neuroendocrine tumour that accounts for less than 1% to 2% of all thyroid carcinomas arising from the parafollicular or C cells of the thyroid gland, producing calcitonin.[1,2] MTC can manifest in 80% sporadic forms, 20% familial forms and hereditary MTC associated with germline gain-of-function mutations of the RET proto-oncogene which is rare human cancers and genetic screening can prevent precursor lesions (C-cell hyperplasia) and RET gene mutations, allowing for prophylactic surgery.[3.-5] Despite the existence of specific diagnostic markers like calcitonin, the FNAC diagnosis of MTC can be difficult due to its diverse cytomorphological patterns of growth like plasmacytoid cell type is the commonest cytological pattern of MTC, other variants, including spindle cell, small cell, follicular, tubular, and giant cell type.[5] The rare pure spindle cell variant of MTC presents in small proportion. [2] Fine-needle aspiration cytology (FNAC) is widely considered as a primary diagnostic tool for thyroid lesions. It can be challenging to diagnose the spindle cell variant of MTC specifically via FNAC often needs the use of ancillary studies for definitive confirmation. [2]

Case Presentation

A 42-year-old female presented with a gradually increasing, solitary, painless swelling on the left side of his neck, which he had noticed over the past 10 months. He also reported associated symptoms including hoarseness of voice, dysphagia, and dyspnoea that had developed over the last six months. On physical examination, a mobile, soft to firm in consistency, neck mass measuring approximately 3 x 2 x 2 cm, was identified in the left lobe of the thyroid gland, exhibiting movement with deglutition. No family history of similar tumours or other endocrine disorders anywhere else in body was noted in this patient.

Laboratory investigations indicated that the patient was clinically euthyroid. Ultrasonography of the neck revealed a hypoechoic nodule in the left lobe of the thyroid. Serum biochemical analysis showed raised levels of calcitonin.

Fine-needle aspiration cytology (FNAC) was performed with standard technique on the thyroid lesion using a 23-gauge needle. The aspirated material was used to prepare smears, which were subsequently stained with H & E, Papanicolaou (Pap) and May-Grunwald Giemsa (MGG) stains. Apart from this, Congo red stain was performed which confirmed the presence of amyloid.

Microscopic of the FNAC smears revealed high cellularity, composed predominantly of loosely cohesive clusters and dispersed singly. The cells were predominantly spindle-shaped, along with few admixed round cells. (Figure 1)The nuclei of the spindle cells were typically spindle-shaped, presenting with moderate anisokaryosis and were hyperchromatic. A neuroendocrine-type stippled nuclear chromatin pattern, described as granular with inconspicuous nucleoli, was noted in alcohol-fixed material. The cytoplasm was generally scant or moderate, eosinophilic, and granular, displaying prominent coarse intracytoplasmic red granularity.(Figure 2)

Figure 1: Smears show spindle cells in loose clusters in a background of haemorrhage (May Grunwald Giemsa, x 400)	Figure 2: Smears show cytoplasmic red granularity (May Grunwald Giemsa, x 400)
Figure 3: Gross specimen showing grey white solid area.	Figure 4: Section shows a tumour with predominantly spindle pattern of growth. (Haematoxylin and Eosin, x 100)
Figure 5: Section shows amorphous orangeophilic deposits consistent with amyloid. (Congo red,x 100)	Figure 6: Section shows apple green birefringence confirming amyloid. (Polarizing microscopy,x 100)

Total thyroidectomy was performed and specimen sent for histopathological examination. Cut surface showed a grey white, solid, homogenous nodule measuring 3 x 2 x 2 cm. (Figure 3) Histologically, the tumour presents with predominantly monomorphic spindle cells arranged in small nests, sheets, or short fascicles, separated by thin fibrovascular septa. (Figure 4) Cells were strongly positive for calcitonin and carcinoembryonic antigen (CEA).

Background showed presence of amorphous, eosinophilic (or pink/glassy) amyloid-like material and Congo red staining confirmed the presence of amyloid. (Figure 5) Under polarising light appears as apple-green birefringence. (Figure 6) Patient also presents with involvement of single regional lymph node. Based on these distinctive cytological features, a provisional diagnosis of spindle cell variant of Medullary Thyroid Carcinoma (MTC) was rendered, which was classified as Bethesda category VI (malignancy).

Discussion

Medullary thyroid carcinoma (MTC) is a rare neuroendocrine tumour that originates from the parafollicular C cells of the thyroid gland. [1] It is present in 10% of all thyroid carcinomas. MTC can manifest in either sporadic or familial forms, with sporadic cases accounting for the majority (70-80%) [1] Familial MTC is inherited as an autosomal dominant trait, either as familial medullary thyroid carcinoma (FMTC) or as part of multiple endocrine neoplasia (MEN) type 2A or 2B syndromes.[1,6] Familial forms are associated with germline gain-of-function mutations in the RET proto-oncogene. [1] Preoperative diagnosis of MTC is critically important due to its implications for clinical management with screening for associated MEN syndromes and pheochromocytoma. Total thyroidectomy performed at the initial surgical procedure without the need for frozen section is the mainstay of treatment. [1, 6]

Fine-needle aspiration cytology (FNAC) is considered as primary diagnostic test for evaluating thyroid lesions. [1,7] Along with early diagnosis, facilitated by FNAC and ancillary studies, can significantly proves the probability of cure and long-term survival [8] This spindle cell variant may confuse with its differential diagnoses like fibroblastic tumour, benign or low-grade soft tissue tumours, spindle cell melanoma, or even anaplastic carcinoma. In nodular or colloid goitre, fibroblasts from supporting stroma or granulation tissue, hyalinizing trabecular adenoma and nodular fasciitis variant of papillary may mimic spindle cells of M. [1,2] The spindle cell tumour may mimic a fibroblastic tumour or even a melanoma which proves out to be helpful with measurement of serum calcitonin levels [4]

For definitive diagnosis neuroendocrine (granular) appearance of nuclear chromatin, neurosecretory cytoplasmic red granules, varying from 160 to 300 nm confirms at cytology spindle cell variant of MCT. Ancillary tests and histopathologic examination were also performed. [2] MTC presents with variety of cytomorphological patterns of growth, which can make diagnosis challenging. It includes plasmacytoid, spindle cell, small cell, follicular, tubular, and giant cell variants. [5]The most common presents as the plasmacytoid cell type.[2] FNAC smears shows plasmacytoid cells in MTC which are cellular, arranged as dispersed tumour cells with eccentric nuclei, "neuroendocrine type" chromatin, moderate to abundant amphophilic cytoplasm, binucleation, and multinucleation. [4] Intranuclear inclusions may also be observed [6] The cytoplasm appears faintly granular in fixed material but show conspicuous red granules in air-dried May-Grünwald Giemsa (MGG)-stained smears [2] The Spindle Cell Variant of MTC is an unusual morphological pattern where neoplastic cells resemble spindle cells.[3] Pure spindle cell MTC is rare, spindle cells are mixed with other cell types, like epithelioid cells.[6]. In some cases, the smears show predominantly spindle cells, prompting the provisional diagnosis of this variant [2]

Amyloid Deposition is a relevant diagnostic finding in MTC, mainly useful in spindle cell variants, show presence of amyloid-like amorphous material. [2] Amyloid may be confused with colloid or connective tissue and in the stroma of hyalinising trabecular adenoma and present as acellular material in the form of strings or as round to oval shaped fragments surrounded by tumour cells which gets stain in variable shades of magenta with MGG and greyish-orange with Pap. Congo Red staining is crucial helps to differentiate amyloid from colloid or hyaline fragments. Calcitonin proves out to be most reliable tumour marker as it is highly specific and sensitive. [2, 5]

Giard, Orell, Sangalli et al study explored pure spindle cell variant of MTC as rare case comprising of predominately spindle cells along with mixed other types of cells.[1]. It includes plasmacytoid, epithelioid, and small cell which are common finding on cytologic smears along with spindle cells and on FNAC report termed as “spindle cell variant. Amyloid is present in 43-81% of MTC cases. [3]

On Histopathological finding spindle cell variant MTC comprising of irregularly arranged bundles or interlaced spindle cells separated by fibrous stroma or amyloid. The tumour cells present with short or long spindle nuclei, abundant cytoplasm, inconspicuous nucleoli, as well as clear boundaries with surrounding tissues. Mild to moderate anisokaryosis is frequently noted. [1] These findings can mimic some spindle cell mesenchymal tumours occurring in the thyroid gland like leiomyoma, peripheral schwannoma, and spindle cell melanoma which are negative for neuroendocrine markers. [3]

Conclusion

Fine-needle aspiration cytology (FNAC) is a simple, outpatient-based procedure remarkable aid in the definitive preoperative diagnosis of MTC, despite its inherent cytological variability. For the spindle cell variant of MTC, accurate diagnosis on FNAC requires a thorough comprehension of rare cytomorphological features, complemented by ancillary studies such as serum calcitonin measurement, immunohistochemistry (IHC) for specific markers (calcitonin, chromogranin A, synaptophysin), and relevant electron microscopy (EM). This integrated diagnostic approach facilitates early and precise diagnosis, leading to appropriate patient management and potentially improved long-term survival. Present case attests the need for heightened cytological awareness in evaluating atypical thyroid neoplasm.

References

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