Case reports in oncology
Radiologic Progression of Disease during Neoadjuvant Dostarlimab in Mismatch Repair-Deficient Lynch-Associated Locally Advanced Rectal Cancer Resulting in Pathologic Complete Response: A Case Report
By Pastorino A, Papadia FS, Pessino A and 5 others.
Abstract
Background
Neoadjuvant PD-1 blockade has demonstrated unprecedented activity in mismatch repair-deficient (dMMR) locally advanced rectal cancer (LARC), with high rates of clinical complete response and increasing adoption of organ-preservation strategies. However, response assessment during immunotherapy remains challenging as immune-mediated tumor regression may generate radiologic patterns that mimic persistent or progressive disease.
Case presentation
We report the case of a 40-year-old woman with Lynch-associated dMMR mucinous LARC (cT4N+) treated with nine cycles of neoadjuvant dostarlimab. Early treatment was associated with rapid normalization of carcinoembryonic antigen and nodal regression. Nevertheless, magnetic resonance imaging demonstrated persistent bulky mass with new diffusion-restricted areas, and fluorodeoxyglucose positron emission tomography revealed intense metabolic uptake (SUVmax 14.7). A rising carbohydrate antigen 19-9 level further raised concern for residual viable tumor. Surgical exploration confirmed a large, firm pelvic mass with apparent infiltration of adjacent structures, leading to posterior pelvic exenteration. Unexpectedly, final pathologic examination revealed complete pathological response (ypT0 ypN0), with extensive fibrosis and acellular mucin pools and no viable tumor cells in 61 examined sections.