KRAS mutation status impacts diagnosis and treatment decision in a patient with two colon tumours: a case report.

KRAS mutation status predicts response to anti-EGFR therapy in colorectal cancer patients. Here we report an interesting case of discordant KRAS mutation status in a patient with two separate tumour foci. Tumour A in sigmoid colon invaded through muscularis propria into the subserosal fat with metastatic disease in regional lymph nodes (pT3N2b). Tumour B in ascending colon had a relatively lower stage and no metastasis (pT2N0). Both tumours showed similar morphology, immunohistochemical staining and microsatellite instability pattern. KRAS mutation, however, was detected only in tumour A. These findings indicate distinct clonal nature of these two tumours. The discordance of KRAS mutation status also suggests that a combination of anti-epidermal growth factor receptor and chemotherapy is likely the best treatment option for this patient. This case exemplifies a notion that comprehensive pathological work-up comprising molecular testing is critical to guide the diagnosis and treatment decisions for colorectal cancer patients with multiple tumours.