Neuroendocrine Differentiation in Mature Cystic Teratomas in Adults: A Case Report and Review of the Literature.

Neuroendocrine tumors (NETs) arising within mature cystic teratomas (MCTs) represent a rare form of malignant transformation, with limited data guiding prognostication and management. Retroperitoneal MCTs with NET transformation are exceptionally rare, with only 4 cases previously reported in literature. The authors have reported the case of a 19-year-old male patient who presented with abdominal pain and was found to have a retroperitoneal cystic mass. Histopathologic examination following surgical resection demonstrated an MCT containing a well-differentiated NET. The neoplasm exhibited classic neuroendocrine morphology arising within teratomatous tissue, with immunohistochemical positivity for neuroendocrine markers and a Ki-67 proliferation index of 2%. No evidence of metastatic disease was identified on staging evaluation. The patient underwent robotic surgical resection without adjuvant therapy and remained disease-free at 6 months of follow-up. The authors performed a focused review of the literature identifying reported NETs arising in MCTs, most of which were well-differentiated NETs (previously termed carcinoid tumors) originating from small bowel epithelium within the teratoma. Most cases have demonstrated indolent behavior following complete excision, with low chance of recurrence. This case explored the clinical implications of NET transformation within MCTs, highlighted favorable prognosis of low-grade NET transformation within MCTs, supported surgical resection as definitive management, and underscored the need for standardized reporting and long-term follow-up.