Is not quite as invasive as the Hemangioscarcoma. In general it stays localized, with surgery as the main treatment. As the following link discusses it's usually suspectable to high radiation therapy post surgery (if the margins are dirty).
Soft Tissue Sarcomas
My Daisy had one of these removed two months before a diagnosis of subsequent Lymphoma. In hindsight the haemangiopericytoma was an indicator of an already impaired immune system. Also it took almost 5 weeks for the incision to heal rather than the expected 2 weeks. So even after surgical recovery keep observing your puppy for any abnormal growths, or unexplain swellings.
You might want to start your dog on fish oil, and anti-oxidants post surgery.
