Hi, I'm am not on Keytruda but am on Opdivo. There are very similar, they attack the same part of the cancer. With your mother having a high level of PD-1 expression she has a very good chance of this Keytruda doing her some good. Watch her for shortness of breath ( inflammation of the lung lining) upper abdomen pain ( inflammation of the liver) severe diarrhea ( inflammation of the bowel) and report ANY symptoms she has to her doctor. Remember that where ever there is life there is hope. When I started on immunotherapy I had tumors in my right lung, chest, abdomen, right and left adrenals, by my spine, muscle of my butt, and in my breast. I now have 1 tumor in my right lung that has not grown in a year and a half and no other tumors. So she is not to far gone. Best wishes to you and her, keep us posted.