Abscopal effect induced by conventional fractionated radiotherapy following anti-PD-1 immunotherapy in pulmonary metastatic thymic squamous cell carcinoma: a case report and literature review.

Background: Radiotherapy (RT) can enhance immune control of distant metastases, known as the abscopal effect (AE), but it doesn't significantly alter the immunosuppressive tumor microenvironment (TME), resulting in low AE incidence. Combining RT with immunotherapy (especially anti-PD-1/PD-L1 agents) has increased AE occurrences, though questions remain about this approach, particularly in tumors with low immunogenicity such as thymic squamous cell carcinoma (TSCC). Case description: A 73-year-old woman with advanced TSCC and multiple metastases experienced disease progression after two therapies. Following palliative conventional fractionated radiotherapy (CFRT) (40Gy) for thoracic metastases, her pleural lesions outside the radiation field significantly reduced, indicating an AE. Despite subsequent immunotherapy and antiangiogenic drugs, treatment efficacy was unsatisfactory due to severe lymphopenia, possibly contributing to disease progression. Conclusion: The rise of immunotherapy challenges traditional RT. To enhance AE occurrence in practice, factors like radiation dose, irradiation site, timing with ICIs, ICI drug choice, patient health, disease stage, and tumor traits must be considered. This case demonstrates that CFRT can induce an AE in TSCC but also highlights the associated risk of severe lymphopenia that may limit its durability. Monitoring and mitigating lymphopenia are crucial in optimizing combined therapy outcomes. This case provides new clinical evidence for treating recurrent TSCC with combined therapy, though more research on its immunological mechanisms is needed.