Not a surprise at all about the mutations. Mets often have different set of mutations than the Primary and even among themselves. I read a paper more than 16 years go on this subject I think it was. Hence the reason for multiple drug cocktails. I would stick with the Cetuximab (a monoclonal Antibody to EGFR) until there is no benefit from it as far as can be determined. I suspect it's still effective on that larger percent of cancer cells that are still KRAS wild. Do they know specifically what KRAS mutation you have? There are KRAS mutations that activate and a few that are still susceptible to an EGFr inhibitor I believe. Cetuximab is acting on the cells further down the signaling pathway from KRAS. You could be lucky and have the one Kras mutation that has a drug available to inhibit it.
My wife was KRAS wild but they didn't add Erbitux until after exhausting the standard FolFox, FolFiri, with added Avastin added to both. She tolerated chemo well. Erbitux seemed to work for several months as far as we could tell keeping her CEA in the normal range that is until it started to creep up. Using imaging to monitor cancer status was difficult as it hard to see the tumors developing in her Peri cavity and these typically are not as sensitive to chemotherapy (or for that matter surgery). Her liver Mets were well calcified, and her oncologist thought for all intents and purposes were dead. Spleen mets completely disappeared. It was in the final couple of months that these became large enough to visualize on scans and they more or less took over and ended up causing blockage of her colon from the outside and ureter from her kidney to her bladder. Up until the last month she had a good quality of life despite nearly 4 years of chemo. I personally think had they added Erbitux early on, she might have avoided the Peri Mets. YMMV.
Good luck in your journey
DW 53 dx Jun 2013
CT mets Liver Spleen lung. IVb CEA~110
Jul 2013 Sig Resct
8/13 FolFox,Avastin 12Tx mild sfx, Ongoing 5-FU Avastin every 3 wks.
CEA: good marker
7/7/14 CT Can't see the spleen Mets.
8/16/15 CEA Up, CT new abdominal mets. Iri, 5-FU, Avastin every 2 wks.
1/16 Iri, Erbitux and likely Avastin (Trial) CEA going >.
1/17 CEA up again dropped from Trial, Mets growth 4-6 mm in abdomen
5/2/17 Failed second trial, Hospitalized 15 days 5/11. Home Hospice 5/26, at peace 6/4/2017