The efficacy of neoadjuvant treatment in locally advanced rectal cancer with dMMR.

615 Background: The incidence of Mismatch repair gene deficiency (dMMR) was about 15% in colorectal cancer, but mostly in right side colon cancer, while in locally advanced rectal cancer, it is very rare. As is known, adjuvant chemotherapy with 5FU alone was not recommended in stage II colon cancer with dMMR or MSI. However, in locally advanced rectal cancer with dMMR or MSI, the efficacy of neoadjuvant treatment with 5FU was not yet known. Methods: We enrolled patients with locally advanced rectal cancer from three prospective clinical trials, including the FOWARC study (N = 309), the mFOLFOXIRI neoadjuvant chemotherapy trial (N = 106) and the total neoadjuvant treatment with FOLFOX and radiotherapy (N = 129). From the 544 patients, 35 (6.4%) patients were dMMR, 133 patients with unknown status of MMR. Among the 35 patients, 10 patients received 5FU concurrent with radiotherapy (group A), nine patients underwent FOLFOX concurrent with radiation (group B), and 12 patients received FOLFOX neoadjuvant chemotherapy alone (group C). Another four patients underwent mFOLFOXIRI neoadjuvant chemotherapy alone (group D), including one patient with nivolumab as neoadjuvant treatment after chemotherapy. Results: Totally, 4 (11.4%) patients achieved pathologic complete response, and 13 (37%) patients had tumor downstaging to ypT0-2N0M0 (stage 0-I). In group A, the pCR rate was 10% (1/10), the tumor downstaging rate was 20% (2/10); In group B, the pCR rate was 33.3% (3/9), the tumor downstaging rate was 55.6% (5/9); In group C, the tumor downstaging rate was 41.7% (5/12). In group D, only one patient achieved pCR, and it is the one who received nivolumab as neoadjuvant treatment. Conclusions: The efficacy of neoadjuvant in locally advanced rectal cancer seemed not affected by the MMR status. But further study was needed.