Introduction: Neuroendocrine tumor (NET) that develops in the right-sided colon is relatively rare. Coexistence of adenocarcinoma and NET is extremely rare, and such cases are called mixed neuroendocrine-non-neuroendocrine neoplasm (MiNEN).
Presentation of case: Colonoscopy in an 85-year-old woman for an anemia examination indicated laterally spreading tumor-nongranular (LST-NG) in the transverse colon. Colonoscopy and endoscopic ultrasonography (EUS) showed that the depth of the tumor invasion was deep submucosal. The tumor localization was diagnosed as transverse colon close to the hepatic curvature by enema. Computed tomography (CT) showed no obvious lymph node or distant metastasis. Based on these findings, laparoscopic transverse colectomy with D2 lymphadenectomy was performed. Pathologically, most of the tumor was well-differentiated tubular adenocarcinoma, but some solid follicles of polygonal tumor cells with poor nuclear atypia were observed. Immunostaining was positive for synaptophysin and diagnosed as NET G1. This tumor consisted of adenocarcinoma and neuroendocrine tumor, so we diagnosed it as MiNEN and classified the tumor as fT1N0M0 fStage I (TNM Classification of Malignant Tumors, 8th Edition). Since it was an early stage cancer, postoperative adjuvant therapy was not performed. No recurrence has yet been noted.
Discussion: Although MiNEN is extremely rare, the detailed pathological specimen observation and diagnosis are important because long-term follow-up after surgery is needed, as is the adequate selection of postoperative adjuvant therapy.
Conclusion: It is important to elucidate the mechanisms involved in the coexistence of NET and adenocarcinoma and to apply these findings to future medical care.
Keywords: Adenocarcinoma; MiNEN (mixed neuroendocrine-non-neuroendocrine neoplasm); NET (neuroendocrine tumor).
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