Richard, Hope

Abstract Follicular dendritic cell sarcoma (FDCS) is recognized as a diagnostically challenging neoplasm requiring rigorous exclusion of more common entities and high index of suspicion for diagnosis. Here, we report an aggressive case of FDCS arising in the mediastinum, which due to sampling limitations and the inherent difficulty of diagnosis required extensive workup with multiple samples prior to definitive classification. The patient was a 35-year-old woman who presented with progressive dyspnea and mediastinal mass on imaging studies. The initial sampling of the lesion had histologic features raising consideration of FDCS, but was too limited to allow definitive diagnosis, while progressing rapidly enough to require administration of chemotherapy even prior to classification. Repeat samples obtained via fine-needle aspiration and bronchoscopy enabled the diagnosis of FDCS, and high-throughput sequencing revealed apparently pathogenic NF2 mutations previously not reported in this disease. Unfortunately, the patient died of widely metastatic disease, which we comprehensively documented at autopsy. Together, these observations provide previously unreported insights into the end stage of this disease and experience relevant to its prospective recognition and workup.