Malignant fibrous histiocytoma (MFH) was first described in the early 1960s and became widely accepted as a specific soft-tissue sarcoma type in the 1970s. Multiple subtypes of MFH, including storiform-pleomorphic, myxoid, giant cell, and inflammatory variants, were defined; subsequently, MFH was regarded as the single most common adult soft tissue sarcoma. [1, 2, 3, 4, 5, 6, 7] See the image below.
See Soft-Tissue Sarcomas: What You Need to Know, a Critical Images slideshow, to help identify and treat some of these malignant tumors of mesenchymal origin.
The term malignant fibrous histiocytoma implies that the tumor cells are of fibroblastic and histiocytic origin; however, the precise origin of MFH cells has been disputed and the concept of fibrohistiocytic differentiation has been challenged. [7, 4, 5, 6] Furthermore, the morphologic pattern seen with pleomorphic MFH is shared by a variety of poorly differentiated malignant neoplasms.
In his 1992 article, Fletcher noted that the majority of cases initially diagnosed as pleomorphic MFH could be reclassified as other diagnoses after lines of differentiation were discovered on testing with more sensitive techniques. [4, 8] Although the term malignant fibrous histiocytoma is still used by clinicians and pathologists, it is perceived as a misnomer by many investigators who recommend abandoning the term.
In 2002, the World Health Organization (WHO) reappraised and modified the terminology and classification of MFH and its subtypes.  Pleomorphic sarcoma is the alternate name advocated by the WHO to replace MFH, as it provides an accurate description of the tumor without implying the origin of the tumor cells. [4, 9] The WHO nomenclature now designates storiform-pleomorphic MFH as “undifferentiated high-grade pleomorphic sarcoma,” giant cell MFH as “undifferentiated pleomorphic sarcoma with giant cells,” and inflammatory MFH as “undifferentiated pleomorphic sarcoma with prominent inflammation.” These 3 undifferentiated pleomorphic sarcomas remain categorized as “so-called fibrohistiocytic tumors” in the WHO classification. Myxoid MFH, however, is seen as a specific entity, now termed “myxofibrosarcoma” by the WHO, and reallocated from the fibrohistiocytic category to the myofibroblastic tumor category.
Although the group of pleomorphic (MFH-like) sarcomas collectively represent the most common types of sarcoma in patients older than 40 years (incidence of 1-2 cases per 100,000 patients annually), the pleomorphic MFH/undifferentiated high-grade pleomorphic sarcoma subtype now probably accounts for 5% or less of adult soft-tissue tumors. Rare cases are seen in children, adolescents, and young adults, and there may be a slight male predominance.
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. Only vimentin was always expressed in MFH/undifferentiated pleomorphic sarcoma, while some of the tumors were positive for myogenic antigen and CD68. 
Most undifferentiated high-grade pleomorphic sarcomas and undifferentiated pleomorphic sarcomas with giant cells occur in the deep soft tissues of the extremities or trunk. The most common site of involvement of undifferentiated pleomorphic sarcoma with prominent inflammation is the retroperitoneum, although intra-abdominal and deep soft-tissue locations have been encountered.  MFH has been reported to occur in the lungs, liver, kidneys, 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. [11, 12]
The most common clinical presentation is an enlarging painless soft-tissue mass in the thigh, typically 5-10 cm in diameter. The majority of tumors are intramuscular. Rare signs and symptoms include episodic hypoglycemia and rapid tumor enlargement during pregnancy. Additionally, MFH has been associated with hematopoietic diseases such as non-Hodgkin lymphoma, Hodgkin lymphoma, multiple myeloma, and malignant histiocytosis.
Retroperitoneal tumors may present with constitutional symptoms, including fever, malaise, and weight loss. The tumor is often large 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. Early and complete surgical removal using wide or radical resection is indicated because of the aggressive nature of the tumor. [13, 14, 15]
As with other soft-tissue tumors, MRI is the imaging method of choice because of its ability to provide superior contrast between tumor and muscle, excellent definition of surrounding anatomy, and ease of imaging in multiple planes. [16, 17] Axial CT scanning may be performed in lieu of MRI if the patient is claustrophobic or if metal implants (eg, pacemakers, aneurysm clips) render the patient unsuitable for MRI. CT scanning is also useful for evaluation of calcifications. [18, 19, 20, 21, 22, 23, 24]
MFHs are displayed in the images below.
Although typically MRI is suited best for defining the anatomy of the tumor and its surrounding structures, the signal characteristics of pleomorphic sarcoma/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 scanning 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 pleomorphic sarcoma/MFH, and biopsy is usually necessary.
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 a malignant soft-tissue sarcoma.
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 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 undifferentiated pleomorphic sarcoma/malignant fibrous histiocytoma (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. [22, 23, 24]
See the images below.
Magnetic Resonance Imaging
MRI findings of undifferentiated pleomorphic sarcoma/malignant fibrous histiocytoma (MFH) include the following:
MRI typically reveals an intramuscular mass with heterogeneous signal intensity on all pulse sequences (see the images below)Transverse CT image following enhancement with intravenous contrast shows a heterogeneous and mildly enhancing soft tissue mass in the posteromedial right thigh.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.Preinfusion T1-weighted MRI reveals a large mass. 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.T2-weighted MRI obtained following gadolinium administration reveals heterogeneous enhancement of the malignant fibrous histiocytoma.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 myxofibrosarcoma/myxoid MFH, reflecting the high water content of these lesions 
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.