Background: Early-stage colon cancer is increasingly detected by colonoscopic screening. The development of adjuvant chemotherapy in patients with stage-TT colon cancer remains a challenge. Tn Thailand, the clinicopathological risk factors [CPR] are currently used as criteria for the selection of adjuvant chemotherapy in each patient. Previous reports showed that mismatch repair gene [MMR] status could be a prognostic factor for the decision on adjuvant chemotherapy.

Objective: To determine the characteristics of MMR status and CPR of Thai colon cancer cases, with treatment follow-up in stage-TT colon cancer by MMR and CPR.

Materials and Methods: This was a preliminary report of patients with stage-TT colon cancer who received treatment at Chulabhorn Hospital. MMR status was determined by microsatellite instability
[MST] testing and CPR was determined in each patient. Patients with deficient MMR and low CPR received post-surgery surveillance whereas those with proficient MMR and/or high CPR were treated
with adjuvant chemotherapy (5-FU/LV). The follow-up of adverse events, serious adverse events, disease-free survival [DFS], and overall survival [OS] was at the third and fifth years.

Results: During July 4, 2014 to December 31, 2016, there were 31 cases of stage-TT colon cancer. All of them were at the stage of T3NoMo (TTA). High CPR and low CPR were found in 20 cases (64.52%) and 11 cases (35.48%), respectively. There were 28 cases with MMR testing results. Proficient MMR (MST-low) was observed in 23 cases (82.14%). There were 3 cases with disease recurrence, all of which were in proficient MMR group and received adjuvant chemotherapy. Serious adverse events were found in 2 cases with infection during febrile neutropenia after chemotherapy but no
treatment-related death was observed. DFS and OS could not yet be evaluated.

Conclusion: Tncidence of proficient MMR (MST-low) in Thai patients with stage-TT colon cancer was comparable to that of other countries (80 to 90%). Treatment by adjuvant chemotherapy using MMR
status and CPR was feasible with low serious adverse events.

Keywords: Colon cancer, Mismatch repair gene, Clinicopathological risk factors, MST, Proficient MMR