Case report of radiographic pseudoprogression in a localized dMMR rectal cancer patient following treatment with neoadjuvant PD1-based immunotherapy

PD-1 based immunotherapy has shown significant efficacy in Deficient Mismatch Repair (dMMR) metastatic Colo Rectal Cancer (CRC), leading to FDA approval of several immunotherapeutic agents. Clinical trials have explored the use of immunotherapy in localized dMMR CRC, with promising results. This case report presents a challenging case of a patient with localized dMMR rectal cancer treated with a novel bispecific antibody (XmAb20717; vudalimab) targeting of PD1 and CTLA4. The patient initially experienced radiographic pseudoprogression but achieved a complete radiographic response after six months of therapy. The occurrence of pseudoprogression in localized dMMR rectal cancer is of relevance given the curative- intent setting and challenges it creates for treatment decision-making. In this case, endoscopic evaluation played a vital role in determining the presence of pseudoprogression, guiding treatment continuation, and enabling non-operative management. A multidisciplinary approach is needed for the management of locally advanced dMMR rectal adenocarcinoma treated with PD-1 based therapy to guide the appropriate treatment decision.