Abscopal effect in a patient with solitary extramedullary plasmacytoma of the thyroid: case report

Background: The abscopal effect is a systemic antitumor response that occurs distant to the irradiation site and is induced by local irradiation. We herein describe a case of solitary extramedullary plasmacytoma (SEP) of the thyroid in which bone metastases disappeared after thyroidectomy and cervical radiotherapy (RT). Case Description: A 68-year-old man with a right cervical swelling for approximately 10 years visited a hospital with left elbow pain that persisted for 5 months. He was referred to our hospital for more detailed examinations and treatment. An immobile mass of approximately 10 cm in diameter was palpable on the right side of the patient's neck. Computed tomography (CT) showed osteolytic changes in the left humerus and right scapula, and a tumor in the right lobe of the thyroid gland. The patient was diagnosed with widely invasive follicular carcinoma and multiple bone metastases and underwent total thyroidectomy. During surgery, tumor infiltration into the trachea was observed. Shaving between the tumor and trachea was performed. Based on the results of a pathological examination, the patient was diagnosed with primary extramedullary plasmacytoma of the thyroid. After surgery, external beam RT was performed on the neck, which also reduced the pain in the left elbow. Positron emission tomography (PET)-CT showed the decreased accumulation of fluorodeoxyglucose (FDG) in the left humerus and right scapula, and the amelioration of osteolytic changes on CT.Conclusions: SEP of the thyroid is rare, and to the best of our knowledge, this is the first case report in which the abscopal effect was observed after thyroidectomy and cervical RT.