Transformation of NSCLC to SCLC after 1st- and 3rd-generation EGFR-TKI resistance and response to EP regimen and erlotinib: 2 CARE-compliant case reports.

Rationale: Genotypic and histological evolution of non-small-cell lung cancer (NSCLC) into small-cell lung cancer (SCLC) has been described as a mechanism of acquired resistance to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI) therapy. However, the number of clinical cases is rare.

Patient Concerns: Two lung adenocarcinoma patients with EGFR mutations who recurred after radical resection transformed into SCLC under treatment with the sequential first- and third-generation EGFR-TKIs.

Diagnosis: The 2 cases were both confirmed as SCLC by pathological rebiopsy after EGFR-TKIs resistance.

Interventions: Case 1 was treated with etoposide plus cisplatin (EP) regimen and erlotinib, while case 2 was treated with erlotinib and EP followed by oral etoposide.

Outcomes: Case 1 treated with EP only achieved 3-month progression-free survival (PFS), which is the first case that reported T790 M/C797S cis-mutation for osimertinib resistance before the SCLC transformation. However, case 2 treated with erlotinib and EP followed by oral etoposide, PFS lasted for 8 months.

Lessons: The cases highlighted the importance of rebiopsy that identified pathologically SCLC transformation after EGFR-TKI resistance, and suggested the treatment of erlotinib plus EP followed by etoposide, which could provide a reference for such phenotype.