Malignant Fibrous Histiocytoma Imaging 

Preferred examination

As with other soft-tissue tumors, MRI is the imaging method of choice for malignant fibrous histiocytoma (MFH) because of its ability to provide superior contrast between tumor and muscle, excellent definition of surrounding anatomy, and ease of imaging in multiple planes.

Axial CT may be obtained in lieu of MRI if the patient is claustrophobic or if metal implants (eg, pacemakers, aneurysm clips) render the patient unsuitable for MRI. CT is also useful for evaluation of calcifications.^{[1, 2, 3]}

Malignant fibrous histiocytomas are displayed in the images below.

Malignant fibrous histiocytoma (MFH), described by O'Brien and Stout in 1964, is the most common soft-tissue sarcoma of late adult life.^[4]

In 2002, the World Health Organization (WHO) modified the nomenclature for soft tissue neoplasms, with the most significant changes being in the nomenclature for fibrous and lipomatous malignancies.^{[5, 6, 7]} Originally, MFH was defined as a neoplasm showing both fibroblastic and histiocytic differentiation; however, more recent evaluation has shown no evidence of true histiocytic differentiation.

Furthermore, the morphologic pattern seen with pleomorphic MFH is shared by a variety of poorly differentiated malignant neoplasms. For these reasons, the WHO suggests new terminology for the various subtypes of MFH.

Guo et al assessed 33 previously diagnosed MFH cases based on the WHO classification to identify whether some of the MFH diagnoses would have the diagnosis changed. Among the 33 cases, 17 cases (51.5%) of MFH had their diagnoses changed, including 5 leiomyosarcomas, 3 malignant peripheral nerve sheath tumors, 1 fibrosarcoma, 1 inflammatory myofibrosarcoma, 1 giant cell tumor, and 1 angiomatoid fibrous histiocytoma. The remaining 16 cases (48.5%) were reconfirmed as MFH/undifferentiated pleomorphic sarcoma (UPS). Only Vimentin was always expressed in MFH/UPS, while some of the tumors were positive for myogenic antigen and CD68.^[8]

Anatomy

Malignant fibrous histiocytoma (MFH) occurs most commonly in the extremities (70-75%, with lower extremities accounting for 59% of cases), followed by the retroperitoneum. Tumors typically arise in deep fascia or skeletal muscle. MFH has been reported to occur in the lung, liver, kidney, bladder, scrotum, vas deferens, heart, aorta, stomach, small intestine, orbit, CNS, paraspinal area, dura mater, facial sinuses, nasal cavity, oral cavity, nasopharynx, and soft tissues of the neck.^{[9, 10]}

Clinical details

The most common clinical presentation is an enlarging painless soft-tissue mass in the thigh, typically 5-10 cm in diameter. Two thirds of tumors are intramuscular. Rare signs and symptoms include episodic hypoglycemia and rapid tumor enlargement during pregnancy. Additionally, malignant fibrous histiocytoma (MFH) has been associated with hematopoietic diseases such as non-Hodgkin lymphoma, Hodgkin lymphoma, multiple myeloma, and malignant histiocytosis.

Retroperitoneal MFH usually presents with constitutional symptoms, including fever, malaise, and weight loss. The tumor is often larger than 10 cm in diameter at presentation and may cause displacement of the bowel, kidney, ureter, and/or bladder. MFH may also occur secondary to radiation exposure and shrapnel injury and may be seen adjacent to metallic fixation devices, including total joint prostheses.^[11] Early and complete surgical removal using wide or radical resection is indicated because of the aggressive nature of the tumor.^{[12, 13]}

Limitations of techniques

Although, typically, MRI is suited best for defining the anatomy of the tumor and its surrounding structures, the signal characteristics of malignant fibrous histiocytoma (MFH) are not specific, and the true histologic nature of the tumor or other soft-tissue masses often cannot be ascertained by imaging alone, with few exceptions (eg, lipoma). Furthermore, patients with cardiac pacemakers and aneurysm clips may not be able to undergo examination with MRI. In these patients, CT can provide adequate information regarding the location and gross extent of the mass, although the contrast between tumor and muscle is often less than that seen with MRI. However, no single imaging technique can provide a specific histologic diagnosis of MFH, and biopsy is usually necessary.

Radiologists should keep in mind that although a large deep soft-tissue mass without visible fat in an extremity of an otherwise healthy older patient is statistically likely to represent MFH, the actual diagnosis can be made only via biopsy. Many malignant and several benign soft-tissue tumors may have imaging appearances identical to that of MFH.

If the radiologist is asked to perform a biopsy on a potentially malignant soft-tissue mass, the orthopedic surgeon resecting the mass must be consulted first. With certain tumors, the biopsy tract must be removed with the mass; a presurgical image-guided biopsy performed without appropriate orthopedic consultation may result in more extensive surgery (including amputation) than would have been necessary otherwise.

Radiographs may reveal a nonspecific soft-tissue mass (see the image below), often greater than 5 cm in diameter. Deep intramuscular tumors often lie adjacent to the diaphysis of a long bone.

Secondary osseous involvement, including periosteal reaction, cortical erosion, and pathologic fracture, is uncommon but suggestive of malignant fibrous histiocytoma.

Calcification or ossification can be detected in 5-20% of patients. Calcifications within the tumor may be punctate, curvilinear, and/or poorly defined.

Heterotopic bone formation may be present in the periphery of the mass.

CT scan findings include the following:

• CT typically reveals a nonspecific, large, lobulated, soft tissue mass of predominantly muscle density, with nodular and peripheral enhancement of solid portions
• Central areas of low attenuation may be present, corresponding to myxoid regions, old hemorrhage, or necrosis
• Fat attenuation is not observed in the tumors; this fact can be useful in distinguishing MFH from some well-differentiated liposarcomas
• CT may be used to evaluate potential internal matrix and/or cortical erosion
• Retroperitoneal tumors manifest as heterogeneous masses with areas of hemorrhage and/or necrosis and occasionally focal or diffuse coarse calcifications (approximately 10%); the tumors may invade the abdominal musculature but do not invade the renal veins or inferior vena cava

MRI findings of malignant fibrous histiocytomas include the following:

• MRI typically reveals an intramuscular mass with heterogeneous signal intensity on all pulse sequences (see the images below)TI-weighted image obtained following intravenous contrast administration reveals a soft tissue mass within the posteromedial right thigh. Mild enhancement is noted. The mass is relatively isointense. T2-weighted MRI reveals a hyperintense mass within the posteromedial right thigh (same patient as in the previous image). The mass proved to be a malignant fibrous histiocytoma. T2-weighted MRI demonstrates the tumor much more conspicuously (same patient as in the previous image). Preinfusion T1-weighted MRI reveals a large mass in the anterior thigh (same patient as in the previous image). Although slightly heterogeneous, the tumor is predominantly isointense to muscle. T1-weighted MRI obtained following intravenous administration of gadolinium reveals circumferential enhancement of the tumor (same patient as in the previous image). T2-weighted MRI reveals heterogeneous signal of the malignant fibrous histiocytoma (same patient as in the previous image). Central low signal intensity may be the result of old blood products. T1-weighted MRI obtained following gadolinium administration reveals heterogeneous enhancement of the malignant fibrous histiocytoma (same patient as in the previous image). Preinfusion T1-weighted MRI of a patient with a medial left thigh mass proven at biopsy to represent a malignant fibrous histiocytoma. Although slightly heterogeneous, the mass is predominantly isointense to muscle. Malignant fibrous histiocytoma. T2-weighted MRI reveals predominantly increased signal intensity in the tumor mass (same patient as in the previous image).
• As with other soft-tissue neoplasms, the signal intensity pattern is nonspecific, usually low to intermediate on T1-weighted images and intermediate to high on T2-weighted images; low signal intensity of T1-weighted images and prominent high signal intensity on T2-weighted images may be a feature of myxoid MFH, reflecting the high water content of these lesions {Ref4}
• Regions of prominent fibrous tissue (high collagen content) may demonstrate low signal intensity on both T1-weighted and T2-weighted images
• Calcification may present as foci of low signal on both T1-weighted and T2-weighted sequences
• Subacute hemorrhage should be considered when regions of high signal are noted on both T1-weighted and T2-weighted images
• Areas of necrosis demonstrate a signal pattern similar to that of fluid
• As with CT, solid components of MFH typically reveal nodular and peripheral enhancement
• Tumor margins appear relatively well defined on MRI; a low signal intensity margin may be observed, representing a pseudocapsule

Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.

The radiologist should keep in mind that the diagnosis of MFH is made using histopathology, not imaging; however, MRI remains invaluable for delineating tumor extent.

Sonography typically reveals a well-defined heterogeneous mass that contains hyperechoic areas of cellularity and hypoechoic regions of necrosis. The appearance of tumors on ultrasound is nonspecific; however, sonography may be used to evaluate tumor volume. Retroperitoneal tumors tend to appear as hypoechoic solid masses with scattered regions of heterogeneity. (See the images below.)

Technetium-99m bone scintigraphy often shows increased uptake by tumor, regardless of the presence of calcium within the tumor or invasion of adjacent bone. Hypervascular lesions show increased radionuclide uptake on both dynamic and blood pool images. The mildly increased uptake observed on static images is also probably related to hypervascularity and may be more prominent with internal calcification. Bone scans usually are not ordered to evaluate the primary tumor but may be obtained if osseous metastases are suggested. Gallium-67 scans may also demonstrate increased activity. (See the images below.)

Angiographic findings are nonspecific. The tumor may be hypovascular or, more commonly, hypervascular with early venous return (see the image below). Similarly, retroperitoneal tumors may be either hypovascular or hypervascular, with blood supply from the lumbar, celiac, iliac, renal, renal capsular, and/or inferior adrenal arteries.