Large-Cell Neuroendocrine Cancer of the Cecum in Familial Adenomatous Polyposis

What happened, in summary

A woman in her late 40s went to the emergency department with abdominal pain, rapid heart rate, and an elevated white blood cell count. CT showed a mass near the cecum with possible perforation and enlarged abdominal lymph nodes. Colonoscopy found a partially obstructing cecal mass and more than 100 colonic polyps; upper endoscopy also found more than 100 gastric polyps. Germline testing identified a pathogenic APC mutation, confirming familial adenomatous polyposis. After an initial attempt at medical stabilization, she returned with fever, worsening pain, and an abdominal abscess. She underwent emergency right colectomy with a diverting loop ileostomy. Pathology showed an 8 cm large-cell neuroendocrine carcinoma centered in the cecum, with tumor in 3 of 23 lymph nodes. The tumor had a Ki-67 above 90% and was microsatellite stable with intact mismatch-repair proteins. Molecular testing also found APC, KRAS, and PALB2 alterations. After surgery, she began adjuvant cisplatin and etoposide. Twelve months later, the remaining left colon was removed as part of management of her extensive polyposis. At the most recent follow-up, 18 months after the initial colectomy, there was no pathologic, clinical, or imaging evidence that the neuroendocrine carcinoma had recurred.