Would you consider first line osimertinib to patients with an EGFR Exon 18 G719X mutation?

Yes, there is some retrospective data that osi (or afatinib) are reasonable options here

Yes, I would consider osimertinib. Afatinib is the only EGFR TKI that is FDA-approved for the rare sensitizing EGFR mutations based on pooled data from LUX-Lung 2/3/6. These studies included 18 patients with G719X mutations with a RR 78%, PFS 14 mo., and OS 27 mo. (Yang et al., PMID 26051236) Other small series demonstrate varying RRs with the lowest appearing to be 14% (n=7) (Heigener et al., PMID 26354527).

There is less data for osimertinib, but it does seem active and in the same ballpark as afatinib. A very recent report in 19 patients with G719X noted a RR of 53% and PFS of 8.2 mo. (Cho et al., PMID 31825714).

So, either afatinib or osimertinib are reasonable options. Since the scant data seem to be similar (wide CIs with small numbers)...