Yes, you absolutely need to test for T790M in patients with EGFR exon 21 L858R mutations who progress on gefitinib.
In patients with lung adenocarcinoma harboring sensitizing EGFR mutations (including exon 21 L858R) who have progressed after treatment with gefitinib or other first/second-generation EGFR TKIs, physicians must use EGFR T790M mutational testing when selecting patients for third-generation EGFR-targeted therapy 1, 2.
Why T790M Testing is Mandatory
The T790M resistance mutation is the most common mechanism of acquired resistance to first- and second-generation EGFR TKIs like gefitinib, occurring in 41-63% of patients who progress after initial response 3. This mutation directly determines your next treatment decision:
- If T790M positive: Offer osimertinib (third-generation EGFR TKI) - this is a strong recommendation with high-quality evidence 1, 4, 5
- If T790M negative: Offer platinum-based chemotherapy with or without amivantamab 1, 4, 5
Critical Testing Requirements
Use a highly sensitive assay capable of detecting T790M mutations in as little as 5% of viable cells 6, 2. This is essential because:
- T790M often exists as a subclonal population that expands under selective pressure from EGFR TKI treatment 6
- Standard Sanger sequencing is inadequate - it lacks the sensitivity needed for this application 6
- Acceptable methods include: allele-specific real-time PCR, droplet digital PCR, or appropriately designed NGS panels 6
Testing Approach Algorithm
Preferred approach: Comprehensive NGS testing (tissue and/or blood-based) to assess for T790M and other potentially targetable resistance mechanisms 1, 4, 5
If tissue is limited or unavailable:
- Use plasma cell-free DNA (cfDNA) to identify T790M mutations 2
- If plasma result is negative, test tumor tissue if feasible, as liquid biopsy has a false-negative rate 2
Common Pitfalls to Avoid
- Don't skip T790M testing - it's not optional. The 2024 ASCO guidelines explicitly state "every effort should be made to assess for presence of new mutation by tissue and/or blood NGS testing" 1, 4, 5
- Don't use insensitive assays - laboratories must validate their methods to detect mutations at ≤5% allele fraction 6, 2
- Don't assume uniform resistance - intratumoral heterogeneity means some tumor sites may harbor T790M while others don't 6, 3
- Don't give immunotherapy - anti-PD-(L)1 agents with or without platinum chemotherapy are not recommended for EGFR-mutant disease that progresses on TKIs 1, 4, 5
Additional Considerations
While T790M accounts for approximately 50% of resistance mechanisms, testing should ideally capture other resistance mechanisms including 3:
- MET amplification (5%)
- HER2 amplification (12-13%)
- PIK3CA mutations (5%)
- BRAF mutations (1%)
- Small cell transformation (14%)
This is why broad NGS panels are preferred over single-gene testing when assessing progression 1, 4, 5.