Gastric neuroendocrine carcinomas (NECs) exhibit aggressive features, such as rapid growth, higher rate of metastasis, and a generally unfavorable prognosis compared to gastric adenocarcinoma. As a result, therapeutic options for NECs remain limited, contributing to the poor prognosis of patients. Immunotherapy has emerged as a promising treatment strategy and demonstrated the potential to partially improve the survival and prognosis of patients with NECs. Nevertheless, the unique clinical response termed pseudoprogression (PsP) has garnered considerable attention in the context of immunotherapy.

Presented here is a case of NEC recurrence five and a half months after radical gastric surgery. The 45-year-old male patient underwent combination treatment involving a PD-1 blocker and tyrosine kinase inhibitors and encountered two instances of PsP during treatment. The patient ultimately achieved a durable treatment response without altering his treatment regimens, resulting in a substantial therapeutic benefit.

This case report aimed to provide the authors' experience with the diagnosis of PsP and treatment strategies for PsP in ongoing immunotherapy.