Primitive neuroectodermal tumor (PNET) is an extremely rare malignancy thought to be derived from fetal neuroectodermal precursor cells. It usually occurs in central and peripheral nervous system or soft tissue and bone, while intravenous or intracavitary PNET is considered as an extremely rare tumor. We reported a case of a 44-year-old woman who presented with the left unilateral facial and neck swelling. Magnetic resonance imaging revealed a tape-shaped solid mass within left subclavian vein, left brachiocephalic vein, superior vena cava, and right atrium; the proximal end proportion occupied almost the entire right atrium with a pedicle flip protruded into the right ventricle. Ultrasonography revealed an irregular hypoechnoic mass arising from the left subclavian vein, which extended along the left brachiocephalic vein and superior vena cava into the right atrium and up to the right ventricle. Positron emission tomography-computed tomography revealed several hypermetabolic thyroid nodules with no evidence of intravenous hyperactive lesion. The patient underwent tumor resection under cardiopulmonary bypass. At 15 days postoperatively, total thyroidectomy and resection of the left subclavian vein were simultaneously performed. The patient received chemotherapy and radiotherapy later. Histologically, the neoplasm displayed small, round, blue cells with hyperchromatic nuclei and scant cytoplasm. The neoplastic cells showed a strong immunopositivity for CD99, synaptophysin, CD56, CD57, and friend leukemia integration 1, thus confirming a diagnosis of the PNET. Histopathological examination of the thyroid showed papillary carcinoma. Thus, this PNET had no definitive organ or tissue of origin, which primarily originated from the left subclavian vein with tumor extension along the superior vena cava to the right ventricle.
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