EP485 - Large cell neuroendocrine carcinoma mimicking colon adenocarcinoma

Purpose/Background: Large cell neuroendocrine carcinoma of the colon (LCNECC) is an extremely rare tumor, accounting for only 0.2% of all colonic carcinomas. It is an aggressive cancer with a poor prognosis due to early metastasis. It is also challenging to diagnose due to its tendency to mimic colonic adenocarcinoma. We present a patient with no known significant past medical history who exhibited clinical signs and symptoms suggestive of perforated appendicitis with a cecal mass, discovered to be an unusual case of LCNECC.

Methods/Interventions: N/A

Results/Outcomes: A 57-year-old female with no relevant past medical or surgical history presented with 1 week of abdominal pain associated with fevers. Computed tomography (CT) of the abdomen and pelvis revealed an inflamed appendix with a 9.5 x 6.4 cm abscess contiguous with the right sidewall of the abdomen as well as cecal wall thickening extending into the proximal ascending colon, accompanied by a 7 x 4.7 cm irregular mass in the cecum. The patient's presentation was concerning for perforated appendicitis with associated malignancy. Given that the patient had no prior colorectal screening, a neoplasm could not be ruled out. The patient also noted significant weight loss over the past few months. However, her CEA was normal. At the time, the patient was started on IV antibiotics and underwent placement of a drain for the abscess. The fluid was purulent and sent for cytology; however, no malignant cells were seen. The patient was discharged with plans for a colonoscopy before an interval appendectomy.

The colorectal surgery service assumed care of the patient when she subsequently represented with an acute onset of worsening right lower quadrant abdominal pain after drain removal in the clinic. Labs were notable for leukocytosis. Repeat CT of the abdomen and pelvis showed increased pericecal stranding and edema with multiple locules of extraluminal gas in the right lower quadrant as well as above the liver and anterior right hemiabdomen with concern for a perforated cecal mass. The patient underwent an exploratory laparotomy. There was purulence encountered on entry, and a large mass was found in the cecum with involvement of the small bowel. The mass had perforated into the retroperitoneal space with gross spillage of enteric contents. Ultimately, the patient underwent a right hemicolectomy with resection of approximately 50 cm of ileum with end ileostomy creation. An estimated 310 cm of small bowel remained.

Pathology revealed an intact appendix with a mass involving the cecum, ileocecal valve, and ascending colon. Immunostains were positive for CDX-2 and INSM1 (scattered) while negative for chromogranin, synaptophysin, and CD56. Ki67/MIB1 was 80 - 90%. These findings were consistent with a large cell neuroendocrine carcinoma, staged pT4b, pN1. The patient is currently undergoing FOLFIRINOX chemotherapy.

Conclusion/Discussion: LCNECC is a highly malignant neoplasm that represents less than 1% of all gastrointestinal tumors. In this report, we present a rare case of LCNECC that initially appeared as perforated appendicitis with a cecal mass. As in this patient, at the time of its diagnosis, it is often found in advanced stages. This leads to challenges in treatment, which underscores the importance of early and accurate detection.

Figure 1. Cross-sectional imaging of a patient with LCNECC presenting with a cecal mass.
CT (A) axial and (B) coronal views.
Figure 2. Gross view of the resected specimen.
A large mass was found in the cecum with involvement of the small bowel.