Metastatic leiomyosarcoma of the thyroid: A rare entity.

Cancer metastasis to the thyroid gland from non-thyroid sites is a rare presentation in clinical practice. The most frequent primary cancers that metastasise to the thyroid are renal cell carcinoma, followed by colorectal, lung and breast. We report a case of a 64-year-old Malay lady who presented with anterior neck swelling 4 years after an initial diagnosis of uterine leiomyosarcoma. She had undergone a hysterectomy procedure four years ago. Fine needle aspiration cytology of the thyroid mass suggested undifferentiated thyroid carcinoma. After multi-disciplinary discussion, the patient underwent thyroidectomy and the final histopathological diagnosis was metastatic leiomyosarcoma of the thyroid. The diagnosis was aided by an immunohistochemistry panel of positive myogenic markers, negative epithelial markers as well as the previous medical history of uterine leiomyosarcoma. Metastatic leiomyosarcoma of the thyroid may mimic primary undifferentiated (anaplastic) thyroid carcinoma (UTC) with a sarcomatoid pattern, medullary thyroid carcinoma (MTC) with spindle cells morphology and spindle cell tumour with thymus-like differentiation (SETTLE). Hence, a multidisciplinary approach must be practised by pathologists, surgeons and radiologists to consider metastatic lesions of the thyroid gland, especially when a previous history of cancer exists or is suspected.