Plasmacytoma is a type of tumor that arises from plasma cells, which are a kind of white blood cell responsible for producing antibodies. It can occur in various parts of the body, most commonly in the bone or soft tissues. When it is found in the bone, it is often referred to as solitary plasmacytoma of bone, while soft tissue plasmacytoma occurs outside the bone. This condition is considered a form of multiple myeloma, a more widespread disease affecting plasma cells. Diagnosis typically involves imaging tests, biopsies, and blood tests to assess the presence of abnormal plasma cells. Treatment options may include radiation therapy, chemotherapy, or stem cell transplants, depending on the severity and location of the tumor.