Ancient hematoma is one of several lesions that require differentiation from soft tissue neoplasm. This hematoma, named as such by Mentzel et al., is a benign soft tissue tumor surrounded by a fibrous pseudocapsule with inflammatory cells present inside the capsule that gradually expands in size. Chronic expanding hematoma, designated by Reid et al., is a hematoma that increases slowly in size over more than a month after onset. Ancient hematoma is also called chronic expanding hematoma in the field of clinical medicine. It can mimic soft tissue sarcoma because of its increasing size after the initial hemorrhage. - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4738113/#B1