Papillary Thyroid Carcinoma with Spindle Cell Metaplasia: A Rare Encounter

A myriad of histological variants of papillary thyroid carcinoma (PTC) have been described, some of which can be diagnostically challenging due to their rarity and overlapping histomorphology with other entities. One of the scarce and poorly characterised variants is PTC with spindle cell metaplasia, of which fewer than 20 cases have been reported in the literature hitherto. Our patient was a 51-year-old woman with a four-month history of painless, gradually enlarging neck swelling. Physical examination revealed a solitary left thyroid nodule. Thyroid ultrasonography demonstrated a hypoechoic nodule with irregular borders and speckles of microcalcification at the periphery. Total thyroidectomy with central and lateral lymph node dissection was performed. Grossly, there was a poorly circumscribed mass occupying the entire left thyroid lobe measuring 30 mm in the largest dimension. Histopathological examination revealed features of a classical PTC. Incidentally, a well-circumscribed 9 mm nodule was identified within the tumour mass. The nodule comprised of spindle cells arranged in loose fascicles, displaying uniform bland looking nuclei. No mitosis, necrosis or nuclear atypia was observed. Immunohistochemically, the spindle cells were immunopositive to TTF-1 and thyroglobulin, indicating thyroid follicular cell lineage. p53 and BRAF V600E mutant protein immunoexpression were focally noted. They were negative for calcitonin, S100, and desmin. Loss of E-cadherin and CK19 were also demonstrated. A diagnosis of PTC with spindle cell metaplasia was rendered. The nature of spindle cell in PTC needs to be meticulously defined. Careful histomorphology examination and judicious use of immunohistochemistry stains are helpful in arriving at an accurate diagnosis.