Mixed small cell neuroendocrine carcinoma and squamous cell carcinoma covered by tubulovillous adenoma in the rectum: a case report and detailed molecular analyses

Background Previously, only six cases of mixed neuroendocrine-non-neuroendocrine neoplasm (MiNENs) with squamous cell carcinoma (SCC) component have been described in the colorectum, and the molecular landscape of MiNENs is also poorly understood. Herein, we present a unique case in which the SCC developed as a component of a MiNEN in the rectum. Case presentation The patient was firstly diagnosed as rectal small cell neuroendocrine carcinoma (SCNEC) covered by tubulovillous adenoma, and then mixed SCNEC and SCC in the same site 6 months later. Representative samples from the three histologic subtypes were then sent for next-generation sequencing (NGS) separately. Multiple liver metastases occurred in the following month after the last surgery. The patient died of ketoacidosis 1 year after initial diagnosis of the tumor. Conclusion This is the first report of this exceedingly rare tumor type to include NGS of the 3 separate morphological entities. Our findings may expedite the understanding of combined tumors in the colorectum.