Introduction: Limited data exists for non-small cell lung cancer (NSCLC) patients harbouring de novo T790M mutation.

Methods: NSCLC patients, with de novo T790M, who registered at our institute between 01/03/2015 and 31/12/2019, were considered for retrospective analysis of treatment pattern and clinical outcomes, i.e., progression-free survival (PFS) and overall survival (OS).

Results: Of 1,542 epidermal growth factor receptor (EGFR)-mutated patients, 40 (2.59%) had de novo T790M. Most were male (27, 67.5%) and smokers (23, 57.5%). The commonest site of metastasis was the lungs (31, 77.5%), while 7 (17.5%) had central nervous system (CNS) involvement. Additional EGFR gene mutations and anaplastic lymphoma kinase (ALK) positivity were observed in 20 (50.0%) and 4 (10.0%) cases, respectively. The first-line systemic therapy and the number of patients receiving it were as follows: osimertinib by 14 (35.0%), first-generation EGFR tyrosine kinase inhibitors (TKIs) by 10 (25.0%), gefitinib + chemotherapy by 3 (7.5%), chemotherapy by 7 (17.5%) and gefitinib + bevacizumab by 2 (5%). One patient defaulted before starting any treatment. Hence, 39 were considered for survival analysis. The median PFS and OS for the entire cohort were 10.4 (95% CI = 7.6–19.7) months and 24.9 (95% CI = 15.7–NA) months, respectively. The median PFS for patients on osimertinib was 19.8 (95% CI = 11.6–28.0) months versus 8.8 (95% CI = 6.6–10.9) months for those on other systemic therapy. No CNS involvement, use of osimertinib or first-generation EGFR TKI plus chemotherapy or ALK inhibitor in ALK-positive cases prognosticated better PFS. When compared to other systemic therapies, osimertinib improved PFS in patients with or without additional EGFR mutations, although it was statistically significant for the former group only (p = 0.002).

Conclusion: The incidence of de novo T790M is low. Osimertinib in frontline therapy provides promising outcomes.