Solitary extramedullary plasmacytoma of the thyroid gland associated with multinodular goiter: case report and review of the literature.

Solitary extramedullary plasmacytoma (SEP) is a rare malignant neoplasm arising from plasma cells most commonly occurring in the nasal cavity, nasopharynx and larynx. Thyroid involvement is rare and less than 75 cases of SEP of the thyroid gland have been reported to date. A 74-year-old woman with an history of multinodular goiter presented with dysphonia and painful neck swelling, related to a rapidly growing nodule in the right thyroid lobe. Thyroid function tests showed subclinical hypothyroidism; no evidence of Hashimoto's disease was found. Ultrasound confirmed the presence of an isoechoic nodule, 35 mm in diameter, with a CDIII vascular pattern. FNAC showed a monotonous population of atypical cells, interpreted as suspicious for malignant neoplasia (Thyr. 4). The patient underwent total thyroidectomy. Histopathological examination showed a unencapsulated neoplasm composed of atypical tumour cells characterized by abundant cytoplasm and eccentric nuclei. At immunohistochemistry, tumour cells revealed diffuse reactivity for CD138 and CD45RB and predominant staining for kappa chains. Pan-cytokeratins, TTF1, thyreoglobulin, calcitonin, CD20 and CD79a were negative. Clinically, a complete multiple myeloma workup was negative. On this basis, a definitive diagnosis of SEP was made. At 16 months follow-up, the patient showed good clinical conditions without evidence of multiple myeloma. In conclusion, SEP should be considered in the differential diagnosis of a rapidly enlarging thyroid nodule. Clinical correlation and immunocytochemistry are crucial in avoiding pitfalls. Surgery remains the best modality of treatment whenever the lesion is localized and easily removable.