Case report: small cell transformation and metastasis to the breast in a patient with lung adenocarcinoma following maintenance treatment with epidermal growth factor receptor tyrosine kinase inhibitors

BACKGROUND: Breast metastasis from lung cancer has been reported, but not from SCLC that is transformed from lung adenocarcinoma during maintenance treatment with epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI). Transformation to small cell lung cancer(SCLC), although uncommonly seen, has been associated with resistance to EGFR-TKI therapy in lung adenocarcinomas. CASE PRESENTATION: We describe a case of a 49-year-old man with lung adenocarcinoma harboring L858R point mutation at the exon 21 of the epidermal growth factor receptor (EGFR). During the maintenance treatment with EGFR-TKI, the patient presented with a right breast mass, which was accompanied by elevated serum neuron specific enolase (NSE) level. The histological examination of biopsies from the breast mass and enlarging lung mass revealed SCLC that was less sensitive to standard SCLC treatment. The breast tumor was positive for thyroid transcription factor-1 (TTF-1), consistent with a lung primary cancer. CONCLUSION: This is the first case report of small cell transformation and metastatic to the breast in a patient with lung adenocarcinoma following EGFR-TKI treatment. Repeat biopsy is important for evaluation of evolving genetic and histologic changes and selection of appropriate treatment. and serum NSE measurement may be useful for detection of small cell transformation in cases with resistance to EGFR-TKI therapy.