Rare Combined Small Cell Lung Carcinoma and Lung Squamous Cell Carcinoma Response to PD-1 Inhibitor as Third-Line Therapy: A Case Report

Background: Combined small cell lung cancer (c-SCLC) is a relatively rare subtype of SCLC, especially when SCLC is initially diagnosed and recurrent lesions are non-small cell lung cancer (NSCLC). Moreover, SCLC combined lung squamous cell carcinoma (LUSC) has few been reported. Case Presentation: Here, we report a 68-year-old man pathologically diagnosed as stage IV SCLC of right lung. With cisplatin and etoposide, the lesions were significantly reduced. It was not until three years later that a new lesion was found in his left lung, pathologically confirmed as LUSC. The patient was initiated with sintilimab based on high tumor mutational burden (TMB-H). Both lung tumors were stable, and PFS was 9.7 months. Conclusion: This case provides a meaningful reference for the third-line treatment of SCLC combined LUCS patients. This case also provides valuable information on the response to PD-1 inhibition of patients with c-SCLC based on TMB-H and better understanding of PD-1 therapy applications in the future. Keywords: c-SCLC, LUSC, PD-1 inhibitor, high tumor mutational burden