My dog Boris passed away from hemangiosarcoma October 18, 2008, aged almost 12. A few weeks before, I noticed his abdomen was distended, and took him that day for a checkup. My vet just grimaced, and examined him with ultrasound, and there was a big tumor on his spleen. The next day she removed his spleen, and told me his liver was riddled with cancer and asked me for permission to put him down. I just couldn't do it. My older dog Asja had died a few months previously, and I wasn't ready to lose Boris too. He recovered from that surgery, but only lived three more weeks, and passed away a few days after my birthday. On my actual birthday, he perked up that day and was playful and energetic, but the next day crashed, and never recovered again,and died a few days later. I was devastated. But I knew I did everything I could for him, there was nothing more I could do. There was no warning.
Sondraham, your dog was likely sick the past month or so, but doubtful longer than that. Eight months ago when your dog had GDV surgery, I highly doubt the tumors were there. Hemangiosarcoma is fast growing aggressive cancer with few symptoms. My dog also had surgery five months before he passed away, and all tests were normal and nothing obviously wrong with him. I'm sorry. I know how hard it is to lose a beloved dog.
Klaus, and all the other responders, it's so sad to lose our dog to such a horrid cancer. I wish there was more hopeful treatment, or an explanation of why this cancer happens. In my dog's case, the year before, he had a very bad infection with Rocky Mountain Spotted Fever, a tick-borne infection. It took him months to recover. Two vets I saw for his hemangiosarcoma thought the severity of the RMSF infection likely led to hemangiosarcoma, both spread through blood. But this isn't proven, but it seem likely the two illnesses are related.