Extrapancreatic solid pseudopapillary neoplasm: report of a unique case of primary posterior mediastinum origin and review of the literature.

Solid pseudopapillary neoplasm (SPN) is a rare and low malignant potential neoplasm that traditionally occurs in pancreas. Herein, we report a mediastinal SPN in a 62-year-old woman. Clinically, the patient was asymptomatic. A mass in posterior mediastinum was detected by chest computerized tomographic (CT) scan during her annual checkup. The CT scan revealed a 30 mm solid nodule with well-defined outline in right posterior mediastinum. Histologically, the tumor was comprised of solid cellular nests as well as sheets of cells with an epithelioid appearance, and some pseudopapillary areas could also be identified. Immunohistochemically, the tumor cells were positive for β-catenin (nuclear and cytoplasmic), cyclin D1, CD56, CD10, CD99 (paranuclear dot-like), SOX11 (weak) and TFE3, while negative for cytokeratin (AE1/AE3), E-cadherin, WT-1, synaptophysin, chromogranin and progesterone receptor. SPNs can occur in aberrant locations and this is the first one reported in mediastinum, pathologists should learn about the rare case for a better differential diagnosis. The patient underwent a video-assisted thoracoscope tumorectomy. She has been followed up for 5 months with no recurrence or metastasis.