Reactive mesenchymal proliferation.

One hundred and thirteen cases in the files of the Netherlands Committee on Bone Tumors were diagnosed as heterotopic soft tissue ossification. Myositis ossificans was diagnosed in 62, ossifying hematoma in 21, and pseudomalignant osseous tumor of soft tissues in 30 cases. Antecedent trauma was present in 37%, 46% and 7%, respectively. Myositis ossificans arose in the large muscle groups of the thighs and upper arms; when closely related to the shaft of a bone, periosteal reactions were more outspoken. Pseudomalignant osseous tumor of soft tissues was located in the hands, feet, and pelvis, some cases were not in muscle groups and some were almost periosteal. Ossifying hematoma was located in the upper and lower legs and usually in close relation to bone. The three entities belong to the same kind of reactive mesenchymal proliferative process. The radiologic and histologic patterns are reflections of whether the lesions are closely localized to bone shafts or in soft tissues, e.g. in muscles and therefore the term "reactive mesenchymal proliferation" is preferred to myositis ossificans, pseudomalignant osseous tumor of soft tissues and ossifying hematoma. Diagnostic problems are encountered in early phases when cellularity, mitotic activity, and infiltrative spread suggest malignancy. Recognition of these reactions in early phases is important to avoid mutilating surgery.