Background: Osimertinib, a highly selective third generation EGFR tyrosine kinase inhibitor (TKI) became the standard front-line therapy for EGFR-mutant NSCLC. However, therapeutic options are limited for TKI resistance which commonly occurs. Therefore, overcoming acquired resistance to osimertinib remains an important high unmet need in the field of precision oncology. Herein, we present the first case of advanced adenocarcinoma of the lung that showed notable response with the addition of selpercatinib after acquired resistance to osimertinib monotherapy. Methods: Case presentation. Results: A 37-year-old woman with stage IVB adenocarcinoma of lung with osseous, hepatic and brain metastases initially received one cycle of carboplatin, pemetrexed and pembrolizumab. Based on the EGFR exon19 deletion detected from ctDNA NGS assay (Guardant 360) [variant allele frequency (VAF) 62.7%], the treatment regimen was changed to osimertinib monotherapy (80mg PO daily). Bevacizumab was empirically added given CNS involvement. She maintained overall stable disease for 10 months before subsequent CT showed disease progression. The treatment regimen was switched to atezolizumab, bevacizumab, paclitaxel and carboplatin combination therapy. She tolerated 6 cycles of the regimen in 4 month before a subsequent brain MRI revealed progression of the metastatic brain disease with new leptomeningeal disease. Whole brain radiotherapy was performed and decision was made to start combination TKI treatment of selpercatinib (120mg BID) added to the osimertinib (80mg daily) monotherapy based on her repeat ctDNA NGS assay result showing concurrent acquired CCDC6RET fusion (VAF 0.05%) and EGFR exon 19 deletion (VAF 10.0%). The 6 week follow-up CT demonstrated significant decrease in the largest lung mass (33.95*24.22mm->32.50*16.07mm). Repeat ctDNA NGS assay at one week after selpercatinib use showed disappearance of RET fusion and significant decrease in EGFR clone (VAF 10.0% to 0.05%). Conclusions: It has been reported that co-occurring RET fusions in NSCLC patients with EGFR mutations may contribute to acquired resistance to EGFR inhibitors. Several successful cases of cabozantinib, a non-selective RET inhibitor, or pralsetinib, a selective RET inhibitor combined with EGFR inhibitor, have been reported to aid in overcoming the acquired resistance to EGFR inhibitors. To date, there has been no report of clinical benefit in adding a RET inhibitor based on ctDNA detection of RET fusion with minute variant allele frequency. We for the first time report the case of overcoming acquired resistance to osimertinib by adding selpercatinib, a selective RET inhibitor in NSCLC patients with acquired RET fusion detected in ctDNA at VAF of 0.05%.