대한흉부영상의학회 Korean Society of Thoracic Radiology

Discussion

Diagnosis With Brief Discussion

Diagnosis

Circumscribed Myositis Ossificans

Radiologic Findings

CT scan before contrast enhancement shows ovoid soft tissue mass located in the right anterior chest wall. It is isodense to the adjacent musculature and contains plump and multifocal small calcifications. Contrast-enhanced CT scan shows diffuse enhancement of the soft tissue mass. There is no evidence of rib destruction. Axial MR images show ill-defined soft tissue mass which is isointense on T1WI, heterogeneously hyperintense on T2WI and enhanced diffusely on contrast-enhanced T1WI.
Under the impression of malignant tumor resection of the mass was done, which was confirmed as myositis ossificans.

Brief Review

The World Health Organization defines myositis ossificans (MO) as "a non-neoplastic conditions, sometimes associated with trauma. The lesion may occur on the external surface of a bone or in soft tissue at a distance from the periosteal surface. The abnormal tissue is characterized by proliferation of fibrous tissue and by formation of large amounts of new bone. Cartilage may also be present."(1)

From an etiologic point of view, heterotopic soft-tissue ossification is divided into several categories :
1. An hereditary background with progressive, generalized evolution, such as fibrodysplasia (myositis) ossificans progressiva. (2)
2. Circumscribed lesions that can be related to different causes :
a) Trauma as a causative factor(most common). Usually, repeated microtrauma is responsible.
b) Organic diseases, such as poliomyelitis, tabes, syringomyelia, paraplegia, tetanus, hemophilia, etc. Often these entities produce MO by trauma resulting from passive exercises rather than the usual neuropathic factors.
c) Non-traumatic cases. Under intensive questioning, the patient cannot confirm a history of antecedent trauma.

They appear usually in patients between 10 and 80 years of age ; most of them occur in the 3rd and 4th decades of life. Typical symptoms consist of focal pain with partial limitation of movement and a localized soft-tissue expansion that may be accompanied by fever, elevated sedimentation rate, and leukocytosis. For half the cases, the lesions are found in six specific anatomical sites : adjacent to the anterior superior iliac spine; at the superior lip of the acetabulum; at the greater trochanter; at the linea aspera in the midportion of the femur; at the mid-part of the humeral shaft; and in the area of the first and second metacarpals.

Histopathologically, the most prominent feature in the diagnosis of this entity is the recognition of the "zone phenomenon" described by Ackerman(3). It consists of a tumoral structuring in three areas, that reflects different degrees of cellular maturation: The central zone is composed of undifferentiated mesenchyma with high-grade mitotic activity : the intermediate zone in which there is a variable amount of osteoid intermingled with fibroblasts and osteoblasts; and the peripheral zone, in which the osteoid undergoes calcification and evolves into mature lamellar bone. The maturation period occurs between 1 and 7 weeks. (4)

References

1. Schajowicz F, Ackerman LV, Sissons HA, Sobin LH, Torloni H(1972) Tipos histologicos de tumores oseos. Organizacion Mundial de la Salud (WHO), Geneva. p50
2. Smith R, Rusell RGG, Woods CG. Myositis ossificans progressiva, Clinical features of eight patients and their response to treatment. J Bone Joint Surg 1976;58:48
3. Ackerman LV. Extra-osseous localized non-neoplastic bone and cartilage formation (so-called myositis ossificans). Clinical and pathological confusion with malignant neoplasms. J Bone Joint Surg 1958;40:279
4. Mirra Jm, Picci P, Gold RH. Bone tumors. Clinical, radiologic and pathologic correlations. Lea & Febiger, Philadelphia, 1989; p1550

Keywords

Rib, Chest wall, Non-infectious inflammation,