Cytodiagnosis of Metastatic Thyroid Follicular Carcinoma in Left Iliac Bone with Occult Primary: A Case Report

Bone metastasis of Follicular Thyroid Carcinoma (FTC) is an aggressive complication. Typically, follicular carcinoma remains confined to the thyroid gland, with distant metastasis occurring in 4-15% of cases. The lungs are the most frequent site of metastasis, followed by bone (2-13%). A peculiar clinical manifestation of FTC observed is the metastasis with the primary still remaining clinically occult. Solitary bone metastases are exceedingly rare and can easily be misdiagnosed as primary bone tumours on clinicoradiological assessment. This causes delays in the diagnosis and management. In these cases, the cytomorphology of cells on diagnostic Fine Needle Aspiration (FNA) was found to be useful in suggesting the primary origin of FTC. Distant bone metastases from follicular carcinoma on histopathological examination from biopsy specimens have been reported in the literature. The cytodiagnosis and cytomorphology of metastatic FTC by Fine Needle Aspiration Cytology (FNAC) are seldomly described in the literature. Involvement of the iliac bone in the metastasis of FTC is uncommon. The FNAC of such a bony lesion helps immensely not only in the diagnosis of the metastasis but also in suggesting the primary malignancy. Authors, hereby report a clinicopathological profile of one such rare case of a 74-year-old female with cytodiagnosis of FTC with no clinical thyroid swelling. The cytodiagnosis made from the primary lytic lesion involving the left iliac bone led to the retrospective radiological and histopathological work-up of the thyroid gland, confirming the occult primary carcinoma of the thyroid gland. The present case highlights the rare presentation and cytomorphological details of metastatic thyroid follicular carcinoma.