A lung squamous cell carcinoma demonstrating spontaneous partial regression of the primary tumor with lymph node metastasis: a case report with analysis of the tumor immune microenvironment

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Background: Spontaneous regression (SR) of malignant tumors is a well-known but rare phenomenon in lung cancer. Previous studies have suggested that antitumor immune responses may contribute to SR. Case Description: We report the case of a 74-year-old man with a right lower lobe pulmonary nodule that demonstrated regression during observation, accompanied by mediastinal lymph node (LN) metastasis after nondiagnostic bronchoscopic and needle biopsies. Surgical resection confirmed squamous cell carcinoma with subcarinal LN metastasis. The primary tumor was largely replaced by fibrosis, with residual carcinoma cells comprising only 30% of the lesion. Immunohistochemistry (IHC) demonstrated marked infiltration of multiple T-cell subsets in the primary tumor, whereas the metastatic lesion exhibited reduced immune infiltration. Antigen-presentation-related markers, including β2-microglobulin (B2M), human leukocyte antigen-DR (HLA-DR), and programmed death-ligand 1 (PD-L1), were expressed in the primary lesion but were lost in the metastasis. Overall, the primary lesion showed dense infiltration of CD8+ and CD103+ T cells, whereas the metastatic LN exhibited reduced immune cell infiltration together with loss of antigen-presentation-related marker expression. Conclusions: By comparing the immune microenvironments in the primary tumor and the metastatic LN, this case suggests that differences in local immune context may underlie the distinct clinical behavior of the two lesions.

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