Diagnosis

Malignant fibrohistiocytoma (MFH)

Findings

Initial ultrasound over the area of maximal tenderness and erythema showed a round 3.5-cm-diameter hypoechoic lesion with subtle internal echoes (Figure 1), which was suggestive of an abscess. An ultrasound-guided abscess drainage was attempted, but no fluid was aspirated. The area over the hypoechoic mass was marked, and the patient was sent back to the emergency department for an incision and drainage procedure. The staff at the emergency department was unsuccessful in "draining" the hypoechoic lesion.

The surgery department was consulted and was also unsuccessful in obtaining any fluid from the mass. The patient underwent a second ultrasound examination, which again identified the round hypoechoic lesion but also found vascularity flow in the lesion, therefore indicating the solid nature of the mass (Figure 2). At that point, core biopsy specimens were obtained via ultrasound guidance. The results of the biopsy indicated malignant histiocytoma of the spindle cell type. Magnetic resonance (MR) imaging showed a round T1-intermediate, T2-hyperintense, heterogeneously enhancing mass in the proximal anteromedial right leg (Figures 3, 4, and 5). The adjacent tibia showed no involvement. Chest radiography and computed tomography (CT) revealed no pulmonary nodules or mediastinal adenopathy.

Discussion

Malignant fibrous histiocytoma is a pleomorphic sarcoma composed of histiocyte-like and fibroblast-like elements. It is differentiated from fibrosarcoma by the presence of giant cells. The subtypes of MFH include storiform ("spoke-like")/pleomorphic, myxoid, giant cell, inflammatory, and angiomatoid. Malignant fibrous histiocytoma is the most common soft-tissue sarcoma of late adult life and accounts for 20% to 30% of all soft-tissue sarcomas.¹ The peak incidence is in the fifth decade of life. Men are affected twice as often as women. The majority of MFH occur in the extremities (70% to 75%). The lower extremity is the most common site of involvement.² Other locations include the retroperitoneum (15%) and head/neck (5%).

Patients usually present with an enlarging painless mass with an average size of 5 to 10 cm.¹ In two thirds of the cases, the lesions are deep intramuscular masses. Soft-tissue MFH may be radiation-induced and is, in fact, the most common postradiation sarcoma. Storiform/pleomorphic MFH is the most frequent histologic type and accounts for 50% to 60% of lesions.¹ Classically, the spindle cells are arranged in cartwheel patterns around vessels.¹ This subtype is most common in the extremities, followed by the retroperitoneum.

Plain-film evaluations of soft-tissue MFH are usually nonspecific, showing only focal soft-tissue density. Abnormal mineralization (calcification or ossification) is seen in 5% to 20% of cases.² Malignant fibrohistiocytoma in the deep muscle is usually adjacent to a long-bone diaphysis. Erosion or invasion into the bone can be seen and is highly suggestive of this diagnosis.¹ Other soft-tissue sarcomas, such as liposarcoma (with the exception of synovial sarcoma), do not have this tendency for cortical involvement.

CT illustrates MFH as large, lobulated soft-tissue masses of attenuation similar to muscle. Areas of decreased density are frequently present centrally and correspond to myxoid regions, hemorrhage, or necrosis. Erosion of bone, periosteal reaction and pathologic fracture can be observed.² MR imaging of MFH show an intramuscular mass with intermediate intensity on T1-weighting and high signal intensity on T2-weighting.¹ Heterogeneous signal is often seen on all pulse sequences reflecting the presence of various histologic elements. Such areas include those characterized by prominent fibrous tissue or calcification (low signal intensity), hemorrhage (high signal on all pulse sequences), and necrosis (T1-hypointense and T2-hyperintense). Small areas of calcification or cortical involvement are best evaluated with CT. Soft tissue MFH is often large and lobulated, and tumor margins are relatively well defined.

Hemorrhagic components are common in soft-tissue MFH and are recognized on CT as increased attenuation, and on MR imaging as areas of increased signal intensity on T1- and T2-weighting. Hemorrhage can be extensive enough to obscure the underlying neoplasm, which leads to a very important tenet of soft-tissue imaging: "Any patient with an apparent spontaneous musculoskeletal hemorrhage has an underlying neoplasm until proven otherwise." Gadolinium-enhanced images may help identify small tumor nodules in such cases.

Prognosis for MFH is largely dependent on grade. A recent study has reported 10-year survival of 90%, 60%, and 20% for low-, intermediate-, and high-grade tumors, respectively.³

CONCLUSION

Malignant fibrous histiocytoma is the most common soft-tissue sarcoma of late adult life, and should be included in the differential diagnosis of any intramuscular mass in an adult patient. Though usually painless, it can occasionally present as a painful area of focal swelling in the lower extremities. In such cases, initial evaluation with sonography may show a predominantly hypoechoic mass, prompting the diagnosis of an abscess. However, diligent search for lesion vascularity and further imaging with CT or MR imaging should be performed to exclude a solid mass.

1. Kransdorf MJ, Murphey MD. Imaging of Soft Tissue Tumors.Philadelphia, PA: W.B. Saunders Co; 1997:192-209.
2. Resnick D. Diagnosis of Bone and Joint Disorders.3rd ed. Philadelphia, PA: W.B. Saunders Co; 1995:3964-3965, 4524-4529.
3. Gibbs JF, Huang PP, Lee RJ, et al. Malignant fibrous histiocytoma: An institutional review. Cancer Invest.2001;19:23-27.