Cauda Equina Neuroendocrine Tumors: Distinct Epithelial Neuroendocrine Neoplasms of Spinal Origin

The tumor formerly known as "cauda equina paraganglioma" was recently renamed as cauda equina neuroendocrine tumor (CENET) based on distinct biological and genetic properties. Nevertheless, it remains insufficiently understood. For this study, we retrieved CENETs (some previously reported), from the pathology files of 3 institutions; we examined their immunohistochemical profile, including common neuroendocrine tumor-associated hormones and transcription factors. We identified 24 CENETs from 7 female and 17 male adult patients, with a median age of 47 years. Six included neurofilament-positive ganglion cells. All tumors tested were positive for INSM1, synaptophysin, chromogranin A, SSTR2, and CD56 as well as at least 1 keratin (AE1/AE3, CAM5.2); CK7 and CK20 were negative. Glial fibrillary acidic protein was negative, except for peripheral nontumoral elements. S100 protein was variable but mainly expressed in scattered sustentacular cells. All but 1 tumor tested were positive for HOXB13; several stained for SATB2, and all tumors were consistently negative for GATA3. All tumors tested were negative for transcription factors found in various other epithelial neuroendocrine neoplasms including TTF1, CDX2, PIT1, TPIT, SF1, and PAX8; staining for T-brachyury was negative. Four of 5 CENETs tested had at least focal tyrosine hydroxylase reactivity. Serotonin expression was detected in all 21 tumors tested; it was diffusely positive in 5 and had variable positivity in the remainder. A few tumors had scattered cells expressing gastrin, calcitonin, pancreatic polypeptide, and peptide YY, while glucagon, adrenocorticotropic hormone, and monoclonal carcinoembryonic antigen were negative. PSAP expression was found focally in 4 of 5 tumors examined. SDHB was consistently intact; ATRX was intact in 14 tumors and showed only focal loss in 3. The median Ki-67 labeling index was 4.5% (range: 1% to 15%). We conclude that CENET represents a distinct neuroendocrine neoplasm; the subset with ganglion cells qualifies for designation as composite gangliocytoma/neuroma-neuroendocrine tumor (CoGNET) as defined in the 2022 WHO classification of neuroendocrine neoplasms. In addition to INSM1, chromogranin, synaptophysin, and keratins, the most characteristic finding is nuclear HOXB13 expression; a subset also express SATB2. Serotonin is the most common hormone expressed. The cytogenesis and pathogenesis of these lesions remains unclear.