i'm on medicare for 4b serous endometrial. i was on trastumzab with the first infusion and medicare covered it with no problem. are you sure about your insurance or did you just talk to some idiot? i just finished 6 (carbo, platin, trastumzab (actually i'm getting Keytruda but same thing). when i get the EOB's from insurance, the herceptin is about $10K (medicare gives them about $1K i think).

from the keytruda website:
a kind of uterine cancer called advanced endometrial carcinoma in adults:
when a laboratory test shows that your tumor is mismatch repair proficient (pMMR) or not microsatellite instability-high (MSI-H), and
you have received anti-cancer treatment, and it is no longer working, and
your cancer cannot be cured by surgery or radiation.

sounds like it should be approved. maybe you need a second opinion (they take a LONG time to get, however, at least in the SF bay area). I'm pMMR (there is something called Jemperli for dMMR).