Microsatellite Instability Incidence in Recurrent Colon Cancer Stage II and III

Objective: To evaluate the incidence of Microsatellite Instability (MSI) in patients with Colon Cancer in stage II & III in the Mexican population. Methods: This is a descriptive, retrospective and cross-sectional study performed through a review of 30 clinical charts of patients with the diagnosis of Colorectal Cancer and evaluation of Microsatellite Instability in surgical pathology specimens. Results: Males with 53.3% had a higher incidence than females. The most frequent site was the left colon (53%), followed by the right colon (16.6%), higher rectum (10%), mid rectum (10%), and lower rectum (10%). The majority of the patient were classified as moderately differentiated (86.7%), with 6.7% being poorly differentiated and the rest 6.7% well differentiated. For the presence of MSI in repair genes (MLH1, MSH2, PMS2), this was positive in 33% of the population studied. The clinical-stage most frequently affected was IIA with 36.7% of the positive cases followed by stage IIB with 20%, stage IIIA 20%, and finally, stage IIC and IIIB with 13% and 10% respectively. Among histologic subtypes, adenocarcinoma was found in 90% of the cases, mucinous carcinoma in 6.7%, and signet ring cell carcinoma in 3.3%. In regards to treatment, 50 % of patients underwent only surgery while the other 50% were treated with surgery followed by adjuvant chemotherapy. Clean surgical margins were achieved in 93.7% after initial surgery and the rest were taken back to surgery for a wider resection. The predominant tumor size was T2 (50%), T3 (33%), and T4a (16.7%). The most commonly found lymph node involvement was N1a with 63.3% of the cases followed by N1b with 16.7%. In terms of recurrent metastatic disease, M1a was the most frequent, found in 73.3% of the cases. Conclusion: In our study population in contrast with current literature from the rest of the world we found a higher presence of microsatellite instability (33% vs 15%) and a higher incidence in the left colon with a definitive impact in the survival of patients.