Spontaneous remissions have been observed, and aren't really well understood, as far as I know. In this case, though, they had removed the "dark red and purple

Spontaneous remissions have been observed, and aren't really well understood, as far as I know. In this case, though, they had removed the "dark red and purple mass" of the original tumor, and the rat was a "survivor" since no overlooked bits of tumor or metastases turned up afterwards. For all that oncology is a deep and sophisticated branch of medicine, "just take it out" is still sometimes a good strategy.