Photodynamic therapy combined with systemic treatment achieves significant remission in microsatellite instability-high locally advanced colorectal cancer: A case report and literature review.

Patients with locally advanced colorectal cancer (LACRC) are at high risk of recurrence and metastasis and benefit little from neoadjuvant radiotherapy (nCRT) or neoadjuvant chemotherapy (nCT). Although the Microsatellite instability-high (MSI-H) status has been identified as a valid predictive marker for immunotherapy response, heterogeneity in the treatment response remains. In this study, we report a case of a 70-year-old woman with locally advanced rectal cancer with MSI-H status who had no chance of surgery due to tumor invasion of the uterine wall (cT3N2Mx) and who underwent a systemic treatment regimen of photodynamic therapy (PDT) in combination with a PD-1 inhibitor and antiangiogenic therapy. The treatment consisted of four tumor-directed PDT sessions and a 6-cycle systemic regimen (sintilimab/tislelizumab 200 mg/q3W combined with bevacizumab 7.5 mg/kg/q3W and capecitabine 1000 mg/m2). After six cycles, the tumor achieved near-complete remission, with negative mesorectal fascia (MRF) and extramural vascular invasion (EMVI) status. As of the most recent telephone follow-up prior to manuscript submission, the patient remained progression-free for 17 months. This case demonstrates the synergistic efficacy of PDT combined with immune-targeted therapy in MSI-H LACRC, offering a novel non-surgical treatment option for organ preservation in such patients.