Is it wrong for a doctor to go into surgery on a tumor before a biopsy

[Ben]

unknowing whether it is malignant? My 21 year old sister died to cancer shortly after a doctor operated on her tumor. She had a tumor that was localized in her back and the doctor said it was most likely benign and the tumor was growing for many years. We got a call two weeks after the operation and they said the tumor was malignant. It ended up to be one of the most rarest painful cancers in the world and it spread throughout her whole body after the surgery. This is the horrible part. When she had the surgery the doctor left a piece of what he thought was a benign tumor to avoid causing nerve damage to her legs. However, the hospital calls my sister two weeks after the surgery and tells her about the cancer. When we go to the hospital, one of the doctors who decided to operate on her told her that there was nothing he can do to prevent her death. Is there anything wrong with this picture please anyone can i have some advice.