The I'm Yunity at the bottom of the article is the supplement I am suggesting you try.
To answer the original question, the tumor may have started many years ago. Hemangiosarcoma is an indolent cancer. Initially it is slow to develop and the dogs adjust to it with minimal discomfort. By the time it metastasizes, it has progressed very far, so prognosis is poor at that point, even with surgery. The mushroom supplement has shown some very positive results and further testing is underway at Penn State.View Thread