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Metastatic papillary thyroid carcinoma in the thigh with unusual cytologic features

Dear Dr. Bedrossian, We herein report a case of metastatic papillary thyroid carcinoma (PTC) in the thigh with unusual cytologic features that was sampled by fine-needle aspiration (FNA). The patient was an 81-year-old Caucasian woman with a history of PTC 2.5 years ago. She underwent a complete thyroidectomy with paratracheal, superior mediastinal, and right neck dissection. Surgical resection specimen showed high-grade PTC with squamous differentiation and extrathyroidal extension; a few neck lymph nodes were involved by metastatic disease. She subsequently received external beam radiation therapy. Her serum thyroglobulin was undetectable at that time, but recently, became detectable. A palpable nodule was found in the right posterior thigh. Real-time ultrasound evaluation of this lesion demonstrated a hypoechoic, wellcircumscribed 1.3 3 1.2 3 1.8 cm nodule in the subcutaneous fat with significant peripheral vascularity but minimal internal vascularity or calcifications. The patient then underwent ultrasound-guided FNA of the thigh nodule. The aspirate smears were hypercellular and consisted of loosely cohesive or dispersed single tumor cells (Figs. 1A and B). The cells were pleomorphic with some showing epithelioid or plasmacytoid morphology and others showing short or long spindle in shape. The cells had moderate to abundant cytoplasm with generally fine chromatin and occasional distinct nucleoli. Mitotic figures are readily identified. During immediate assessment, differential diagnosis included metastatic PTC in light of the patient’s known history, as well as other metastatic carcinoma with spindle cell features, sarcoma and melanoma because of the epithelioid/spindle cell component, the presence of nuclear pleomorphism, discohesive arrangement of the tumor cells, and the site of the lesion being the thigh. Cell block section was subsequently obtained showing corresponding histologic features (Fig. 1C), which was different from classic features of the most PTC. We performed immunostains on cell block sections and found tumor cells to be diffusedly positive for cytokeratin, TTF-1, thyroglobulin and PAX-8, and negative for MART-1, calcitonin, and monoclonal CEA (Figs. 1D–F). These immunoprofile confirmed a diagnosis of metastatic poorly differentiated carcinoma from known primary of PTC. A few months later, she underwent resection of the thigh lesion and the tumor was found to involve subcutaneous adipose tissue and focally deep dermis. She was well and asymptomatic in the last follow-up. PTC is the most common type of thyroid cancer, comprising approximately 75%–85% of all thyroid cancer cases, and usually occurs in women between the ages of 30–40 years old. Although metastases to cervical lymph nodes are common, and hematogenous spread (mainly in the lung and bone) has been documented in a small percentage of patients. Skin and soft tissue metastasis are infrequent, including arm muscle, trapezoid muscle, cutaneous tissue in the thigh and in the breast. PTC in elderly patients is often clinically aggressive and associated with more advanced disease and metastasis to bone and lung. The current case is unique in that not only the location of the metastatic tumor is thigh and the age of the patient is old, but also the cytologic features are unusual. We did not appreciate the characteristic cytologic features of PTC such as monolayered cohesive sheets or papillary structures, pale chromatin, nuclear grooves, and intranuclear cytoplasmic inclusion. The diagnosis of such lesion can be challenging if patient’s history is unavailable. This case indicates that PTC can show uncharacteristic cytologic features and can metastasize distantly to soft tissue of the thigh. Clinical and radiologic correlation is important for guiding differential diagnosis and immunoperoxidase workup to reach a correct diagnosis.