Prevalent Pseudoprogression and Pseudoresidue in Patients With Rectal Cancer Treated With Neoadjuvant Immune Checkpoint Inhibitors

BACKGROUND: Immune checkpoint inhibitor (ICI) treatment in patients with microsatellite instability-high/mismatch repair deficient (MSI-H/dMMR) tumors holds promise in reshaping organ preservation in rectal cancer. However, the benefits are accompanied by distinctive patterns of response, introducing a dilemma in the response evaluation for clinical decision-making. PATIENTS AND METHODS: Patients with locally advanced rectal cancer with MSI-H/dMMR tumors receiving neoadjuvant ICI (nICI) treatment (n=13) and matched patients receiving neoadjuvant chemoradiotherapy (nCRT; n=13) were included to compare clinical response and histopathologic features. RESULTS: Among the 13 patients receiving nICI treatment, in the final radiologic evaluation prior to surgery (at a median of 103 days after initiation of therapy), progressive disease (n=3), stable disease (n=1), partial response (n=7), and complete response (n=2) were observed. However, these patients were later confirmed as having pathologic complete response, resulting in pseudoprogression and pseudoresidue with incidences of 23.1% (n=3) and 76.9% (n=10), respectively, whereas no pseudoprogression was found in the 13 patients receiving nCRT. We further revealed the histopathologic basis underlying the pseudoprogression and pseudoresidue by discovering the distinctive immune-related regression features after nICI treatment, including fibrogenesis, dense lymphocytes, and plasma cell infiltration. CONCLUSIONS: Pseudoprogression and pseudoresidue were unique and prevalent response patterns in MSI-H/dMMR rectal cancer after nICI treatment. Our findings highlight the importance of developing specific strategies for response evaluation in neoadjuvant immunotherapy to identify patients with a good response in whom sphincter/organ-preserving or watch-and-wait strategies may be considered.