Magnetic resonance imaging downstaging, pathological response, and microsatellite instability status in patients with signet-ring cell carcinoma rectum undergoing preoperative long-course chemoradiation

Aim and Objective: To assess the magnetic resonance imaging (MRI) downstaging, pathological response, and the relationship between microsatellite instability (MSI) and radiotherapy response in signet-ring cell carcinoma rectum. Materials and Methods: Twenty two patients were recruited prospectively and retrospectively in this observational study. Six weeks following radiotherapy, the response was assessed using an MRI pelvis, and patients who were operable underwent total mesorectal excision followed by adjuvant chemotherapy. The outcome of radiotherapy was correlated with post radiation MRI downstaging, pathological response, and MSI status. Results: The post radiotherapy response assessment MRI showed tumor regression grading (TRG) 5 in 5 patients. TRG 4 seen in 12, TRG 3 in 3, and TRG 2 in 1 patient. Fifteen patients were operable and post-operativ histopathology showed that 40% had pathological complete response (pCR) and 26.7% had near-complete response. Even those who had no response in MRI had pCR. MSI done 17 patients were stable. Of the 6 patients who had complete pathological response, two were MRI TRG 5, three were MRI TRG 4, and one was MRI TRG 3. The median survival was 23 months. The 2-year and 3-year disease-free survival was 46% and 38%, respectively. Conclusion: The predictive value of MRI downstaging in these tumors following neoadjuvant long-course chemoradiation therapy is not often in concurrence with the histopathological response and needs to be interpreted carefully. Even though the pCR rate seen in this cohort is encouraging, this needs to be evaluated in studies with large cohorts.