Appendiceal carcinoma associated with microsatellite instability.

Approximately 15% of colorectal cancer (CRC) cases exhibit microsatellite instability (MSI), which appears to be associated with unique biological behavior. The present study presents a case of appendiceal carcinoma associated with MSI that responded well to adjuvant chemotherapy. Clinical, pathological and immunohistochemical (IHC) characteristics have been described. The 60-year-old male patient had suffered from recurrent lower abdominal pain associated with abdominal distention for 6 months; then, following an acute attack, he was subjected to laparoscopic appendectomy. The histopathological examination revealed moderately differentiated appendiceal adenocarcinoma with mucinous areas, without lymphovascular or perineural invasion. The IHC examination was positive for keratin-20 and caudal type homeobox 2, and negative for MutL Homolog 1, MutS Homolog (MSH) 2 and MSH-6. A postoperative colonoscopy revealed diverticulosis, without the presence of polyps or tumors. However, an abdominal axial computerized tomography scan revealed thickening of the distal portion of the appendix, increased density of the greater omentum, and metastases to the liver capsule, spleen and peritoneum. The treatment of choice was right hemicolectomy with peritoneal debulking, followed by 10 cycles of chemotherapy with 5-fluorouracil, leucovorin and oxaliplatin (FOLFOX regimen). After 5 years of follow-up, the patient remains in good condition, without clinical or radiological signs of recurrence. The good response to chemotherapy corresponds with the observations made in other colon cancers with MSI. Therefore, testing for MSI in appendiceal carcinomas may provide useful information on prognosis and predict response to chemotherapy.