e15601 Background: The duration of adjuvant oxaliplatin-combination treatment in stage III colorectal cancer (CRC) patients is still debatable. Circulating Tumor Cells (CTCs) have been proposed as a potential markers for detection of micrometastatic disease. The aim was to compare the elimination rate of CTCs in patients treated with three- and six-months treatment with FOLFOX or CAPOX, in stage III CRC patients. Methods: Peripheral blood samples were collected from 132 patients at baseline, after three (for all patients) and after six months (for those under six months treatment). Surgical specimens were collected from all patients for the evaluation of their genetic profiling using Sanger sequencing (Ras/Raf) and fragment analysis (microsatellite instability status). The detection and number of circulating tumor cells (CTCs) were evaluated using double immunofluorescence and a real time-polymerase chain reaction assay, at all three time points. Results: Median age of the patients was 62 years, 59.8% were males, 92.7% had a colon/sigmoid tumor location, 24.4% had tumor on the right colon and 99.2% had a good performance status (PS). Of all patients, 44.3% and 55.7% were treated with FOLFOX and CAPOX, respectively; patients received treatment for three and six months, were equally distributed. KRAS, NRAS and BRAFmutations were detected in 41.5%, 2.1% and 12.8%, respectively; whereas MSI-High status was detected in 9.8% of the patients. CTCs were detected in 43.2%, 42.4% and 54.2% of the patients, at baseline, after three and six months of treatment, respectively. Both positivity rate ( p< 0.001) and absolute number of CTCs ( p= 0.022) were decreased between baseline and three months of treatment. Moreover, a significant increase was observed both on the positivity rate ( p= 0.053) and absolute number of CTCs ( p< 0.001) between three and six months of treatment. Following Cox regression analysis, BRAF status and the detection of CTCs are significantly correlated with decreased time to progression. Conclusions: To conclude, the current study indicates that 3 months of adjuvant treatment is at least as efficacious as 6 months in eliminating CTCs. However, the increase of both the rate and number of positive CTCs after three months of treatment might highlight the emergence of resistant cancer cell clones.