Temporospatial heterogeneity of acquired resistance mechanisms in EGFR-mutant lung adenocarcinoma: A case of concurrent EGFR mutation and small cell transformation

Sequential small cell transformation and T790M mutation in an epidermal growth factor-mutant lung adenocarcinoma: A rare occurrence with significant management implications

BACKGROUND

Epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitor (TKI) resistance may be acquired via genotypic and/or phenotypic transformations. Herein, we report an extremely uncommon case with sequential small cell transformation and EGFR T790M mutation, in an elderly female with EGFR exon 21 L858R-mutant lung adenocarcinoma, following treatment with a first-generation EGFR-TKI.

CASE

A 67-year-old female, never-smoker, presented with cough and dyspnoea of two months' duration. Computerized tomography revealed a 39mm lesion in the upper lobe of right lung with pleural effusion. Pleural fluid cytology revealed metastatic lung adenocarcinoma and EGFR testing revealed exon 21 L858R mutation. She was started on gefitinib. After a progression-free survival of 31 months, she presented with disease progression and multiple extra-thoracic metastases. Fine needle aspiration cytology from a chest wall lesion revealed metastatic small cell carcinoma. EGFR testing on this aspirate revealed persistent L858R mutation only. In view of small cell transformation, chemotherapy (etoposide and carboplatin) was administered. After four months, ascitic fluid cytology revealed metastatic adenocarcinoma with persistent L858R mutation and an acquired T790M mutation (both detected on liquid biopsy as well) indicating amplification of the adenocarcinoma clone and regression of the small cell carcinoma clone. She was then initiated on osimertinib.

CONCLUSIONS

The index case highlights the significance of serial EGFR genotyping along with repeated tissue and/or blood sampling for the prompt detection of genetic and phenotypic resistance mechanisms to EGFR-TKIs. Furthermore, it lends evidence to the upfront treatment approaches targeting the heterogeneity of acquired EGFR-TKI resistance mechanisms.