How quickly do angiosarcomas of the breast grow? Had vascular area on u/s, biopsy negative, area is diminishing 5 months later. Can they shrink?

Behave differently. Most angiosarcomas of the breast are induced, primarily from previous radiation to the area of breast involved. The lesion is aggressive and can become fatal if allowed to grow and metastasize. It usually takes 1-2 years for the lesion to breast away and metastasize so if possible and if solitary, the lesion should be resected.
Seek resolution. Angiosarcomas of the breast are treacherous. While these questions remain in your mind, there are a couple of things you can do to help resolve them. Whenever there's a question of a breast angiosarcoma, ask that a few other pathologists review the case. If you are at all concerned that the lesion was missed on the first biopsy, you can request rebiopsy; this would be reasonable. Good luck.