Disappearance of an activated EGFR mutation after treatment with EGFR tyrosine kinase inhibitors

A 34-year-old Japanese woman presented with left supraclavicular lymph node swelling. Computed tomography scans revealed a mass on the left lower lobe, pulmonary nodules, and pleural effusion. A lymph node biopsy revealed large-cell carcinoma with an epidermal growth factor receptor (EGFR) deletion mutation, L747-T751 in exon 19. Although malignant pleural effusions carried the same EGFR mutation, progressive pleural effusions after treatment with chemotherapy, gefitinib, and erlotinib did not show any EGFR mutation. A cell line established from the pleural effusion 3 days before the patient expired also did not harbor the EGFR mutation. Histological sections of the lymph node of the patient were similar to those of the xenograft tumor of the cell line. There may be genetic heterogeneity in EGFR mutant tumors.