Spindle cell proliferations originating in follicular derived thyroid neoplasms are rare and known to cause diagnostic conundrums. We describe a unique case of a spindle cell variant of follicular thyroid carcinoma (FTC) in a 48-year-old female without relevant past medical history, who was being followed for a 1.4 cm left thyroid nodule for the past 15 months. A fine needle aspiration (FNA) of the nodule was interpreted as benign (Bethesda II). On follow-up ultrasound the nodule demonstrated a slight increase in size (to 1.5 cm) and the appearance of coarse calcifications A repeat FNA was performed 12 months later and interpreted as malignant neoplasm (Bethesda VI), containing a population of spindle and epithelioid cells that could not be further classified. A left subtotal thyroidectomy showed an encapsulated tumor mainly composed of fibroblast-like spindle cells, extensive foci of calcifications and focal ossification, with minimal tumor capsule invasion without vascular invasion. Tumor cells expressed vimentin, ERG and SMA (focal), while being negative for pancytokeratin, thyroglobulin, TTF-1, Pax-8, calcitonin, CEA and other lineage-specific mesenchymal, neuroendocrine and melanocytic markers. Importantly, a few residual thyroid follicles were identified within the nodule, and a diagnosis of minimally invasive FTC with extensive spindle cell changes, calcification and osseous metaplasia was rendered. This is only the second cytologic report of a pure spindle cell FTC. The rarity of this neoplasm and its potential broad differential diagnosis create diagnostic difficulties both on cytology and histology.

中文翻译：

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滤泡性甲状腺癌梭形细胞变异：一例极为罕见的病例及文献复习

源自滤泡性甲状腺肿瘤的梭形细胞增殖是罕见的，并且已知会引起诊断难题。我们描述了一名 48 岁女性滤泡性甲状腺癌 (FTC) 梭形细胞变异的独特病例，既往无相关病史，在过去 15 个月内因左侧 1.4 厘米的甲状腺结节被随访。结节的细针穿刺 (FNA) 被解释为良性 (Bethesda II)。在后续超声检查中，结节大小略有增加（增至 1.5 cm）并出现粗大钙化 12 个月后再次进行 FNA 并被解释为恶性肿瘤（Bethesda VI），包含大量梭形细胞和上皮样细胞无法进一步分类。左侧甲状腺次全切除术显示有包膜的肿瘤，主要由成纤维细胞样梭形细胞组成，广泛的钙化灶和局灶性骨化，肿瘤包膜侵犯最小，无血管侵犯。肿瘤细胞表达波形蛋白、ERG 和 SMA（局灶性），而对泛细胞角蛋白、甲状腺球蛋白、TTF-1、Pax-8、降钙素、CEA 和其他谱系特异性间充质、神经内分泌和黑素细胞标志物呈阴性。重要的是，在结节内发现了一些残留的甲状腺滤泡，诊断为微创 FTC，伴有广泛的梭形细胞变化、钙化和骨化生。这只是纯梭形细胞 FTC 的第二份细胞学报告。