I'm so sorry that Cooper has a malignant tumor.
I am glad that his quality of life has not been changed due to this though. Hopefully, he will continue to enjoy doing all the hamster things and be able to make more treasured memories with you for a while yet. I will be keeping you and Cooper in my thoughts and will pray that he will not be in any pain. Huge hugs to you, Gamina. Lots of comforting cuddles to Cooper, too.