Solitary Extramedullary Plasmacytoma of the Thyroid Gland Following Head and Neck External Beam Radiotherapy

ABSTRACT Objective We report a case of a solitary plasmacytoma of the thyroid gland that was diagnosed 17 years after external beam radiotherapy for treatment of tonsillar squamous cell carcinoma in a patient on long-term immunosuppression. Although radiation exposure has been reported to be associated with solitary bone plasmacytoma, this is the first case report of an association with external beam radiotherapy and long-term immunosuppression with plasmacytoma of the thyroid. Methods A 54-year-old man presented to the outpatient endocrinology clinic with a 2-year history of a fluctuating but progressively enlarging tender goiter. Past medical history included tonsillar squamous cell carcinoma treated with surgical resection and head and neck external beam radiotherapy 17 years previously, external beam radiotherapy for metastatic squamous cell carcinoma to jugular digastric lymph node, and long-term immunosuppression for liver transplant. Results Fine-needle aspiration biopsy revealed numerous plasmacytoid cells. Core biopsy revealed clonal plasma cell proliferation. Computed tomography of the neck, chest, abdomen, and pelvis showed no evidence of lymphadenopathy, splenomegaly, bony lytic lesions, or other abnormalities to suggest underlying systemic lymphoma or multiple myeloma. A bone marrow aspirate and biopsy showed no evidence of a plasma cell neoplasm or B cell lymphoma. A total thyroidectomy was performed and pathology revealed diffuse plasma cell proliferation consistent with a plasmacytoma. The lesion appeared to be restricted to the thyroid gland. Conclusion Plasmacytoma of the thyroid should be considered in the differential diagnosis of an enlarging thyroid mass in patients with previous exposure to head and neck external beam radiotherapy.