Soft tissue tumors: MR imaging and pathologic features according to the new WHO classification

The main features of the new WHO classification of soft tissue tumors includes:

• Tumors are classifyed according to the biologic behavior as :
• • Benign
  • Intermediate (locally aggressive)
  • Intermediate (rarely metastasizing)
  • Malignant
• Some tumors change its previous category
• Certain tumors change its name
• The new classification includes less tumors

(* Neurogenic tumors are included in this poster, although the WHO separates them on a separate volume of Neurogenic tumors)

1 Adipocytic Tumors

Due to the high incidence of lipomas, adipocytic tumors represent the largest single group of mesenchymal tumors. The WHO classification does no longer distinguish atypical lipoma (atypical lipomatous tumor) and well-differentiated liposarcoma, but has recognized these expressions as essentially synonymous (figure 1) .

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Figure 1

A major challenge for radiologist is to differentiate lipoma from a well-differentiated liposarcoma, intermediate (locally aggressive) mesenchymal tumor. Features for discrimination of simple lipoma (figure 2)

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Figure 2

from well-differentiated liposarcoma (figure 3)

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Figure 3

include: a deep (intramuscular) rather than subcutaneous location, a size of more than 10 centimeters in maximum diameter, the presence of nodular non-adipose components or thick septa, high signal intensity of septa or nodular soft tissue areas on T2-weighted or STIR images, and contrast-enhancement of non-adipose components (best seen on fat-suppressed T1-weighted images) (figure 3).

Lipomatosis arborescens is an overgrowth of fat from the subsynovium with a characteristic apperance on MRI (figure 4)

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Figure 4

Hibernoma is a very rare benign tumor. MR appearance of hibernoma shows marked variability. Most lesions are well-defined and demonstrate heterogeneous signal intensities which are similar but not identical to fat with prominent vascular structures (figure 5).

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Figure 5

In children a correct diagnosis of lipoblastoma/lipoblastomatosis can almost invariably be made on MR imaging because of the young age of the affected patients, whenever the lesion has myxoid component (figure 6).

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Figure 6

The various types of malignant liposarcomas shows on MR imaging unnspecific appearance which depend on the tissue components of fat, myxoid, cellular, vascular elements, and necrosis.

2 Fibroblastic/Myofibroblastic Tumors

Fibroblastic/Myofibroblastic Tumors represent a very large group of mesenchymal tumors, many of which contain fibroblastic as well as myofibroblastic elements. The WHO classification includes several new entities which previously had not been defined (figure 7).

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Figure 7

Conceptual changes are the clearer recognition of extrapleural solitary fibrous tumor and the reclassification of lesions that were formerly labelled as myxoid MFH (fibrohistiocytic) as myxofibrosarcoma (fibroblastic/myofibroblastic).

Nodular fasciitis (figure 8) , proliferating fasciitis, proliferative myositis (figure 9) and myositis ossificans (figure 10) are benign mass-forming fibrous proliferation. They are regarded as a variant of nodular fasciitis.

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Figure 8

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Figure 9

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Figure 10

The MR appearance vary with the histologic subtype of the lesion and the sate of development. The lesions may appear from a mass-like appearance (figure 9) through an ill-defined mass with edema (figure 10).

Elastofibroma appears virtually pathognomonic on MR imaging with almost specific location deep to the scapular tip (figure 11).

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Figure 11.

Superficial Fibromatosis of the hand (Dupuytren contracture) and plantar fibromatosis (Ledderhose disease) reveal subcutaneous nodules and cords lying superficial to the tendons on MR imaging. Relatively hypercellular lesions demonstrate intermediate to hyperintense signal on T2-weighted images, those with a higher collagen content and less cellular elements appear hypointense on both T1- and T2-weighted images (figure 12)

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Figure 12.

Desmoid-Type Fibromatoses depict infiltrative intermuscular masses which tend to grow along fascial planes on MR imaging (figure 13).

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Figure 13.

The WHO classification distinguishes between adult fibrosarcoma and myxofibrosarcoma, which are relatively common, and low grade fibromyxoid sarcoma and sclerosing epitheloid fibrosarcoma as extremely rare variants of fibrosarcoma. Myxofibrosarcoma has formerly been called myxoid MFH (Malignant Fibrous Histiocytoma). The MR appearance of fibrosarcoma is generally non-specific and varies with its histologic subtype (figure 14).

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Figure 14.

3 So-called fibrohistiocytic tumors

The concept of fibrohistiocytic differentiation has been challenged and is now regarded as a poorly defined morphological descriptor of histiocytic differentiation. The new classification considers malignant fibrous histiocytoma (MFH) as an undifferentiated sarcoma and the term MFH is being substituted by MFH-like sarcoma corresponding to pleomorphic malignant fibrous hystiocytoma (figure 15). Myxofibrosarcoma (formerly known as myxoid MFH) is considered within the fibroblastic/myofibroblastic category (figure 7).

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Figure 15.

Giant cell tumor of tendon sheath (GCTTS) and pigmented villonodular synovitis (PVNS) encompasses a family of lesions from the synovium, bursae and tendon sheath. The MR features of both tumors are similar and reflect their histologic composition. Hemosiderin-laden synovial masses typically demonstrate low signal on T1-weigthed and T2-weighted images. (figure 16) T2*-weighted secuences depicts susceptibility effects caused by hemosiderin deposition (figure 17). There is diffuse contrast enhancement in both type of lesions.

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Figure 16.

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Figure 17.

Giant cell tumor of soft tissue (GCTST) is clinically and histologically similar to giant cell tumor of bone but occurs with less frequency. It has frequent peripheral mineralization. MR imaging shows low signal on T1 and T2-weighted due to haemosiderin (figure 18).

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Figure 18.

Pleomorphic malignant fibrous hystiocytoma was previously regarded as the most common type of adult soft tissue sarcoma. Today, this term is synonymous with undifferentiated pleomorphic sarcoma representing the group of pleomorphic sarcomas (MFH-like). Most of the tumors encompassed in the past as MFH should be incorporated into the category of fibrosarcoma (figure 7). MFH-like sarcomas are indistinguishable from the fibrosarcoma category on MRI and might show intermediate signal intensity on T1-weighted images and heterogeneous with low signal on T2-weighted images (figure 19).

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Figure 19.

4 Smooth muscle tumors

Smooth muscle tumors arising at non-cutaneous, non-uterine locations have been the focus of a considerable conceptual shift in recent years and this is ongoing. The WHO classification of soft tissue tumors includes in this chapter three entitites: angioleiomyoma, leiomyoma of deep soft tissue and leiomyosarcoma (figure 20).

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Figure 20.

Angioleiomyoma is usually small, well-defined lesions showing intermediate to low signal intensity on T1-weighted images and intermediate to high signal intensity on T2-weighted. Small curvilinear structures of low signal intensity may be seen on T1-weighted, probably representing tortuous muscular vascular channels. A hypointense rim on T2-weighted images surrounding the lesion may be seen, which corresponds to a fibrous capsule histologically (figure 21)

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Figure 21

Leiomyoma of deep soft tissue is a very rare type of leiomyoma. There have been few reports of the MRI appearances of deep soft tissue leiomyomas. Similarly to uterine leiomyomas, these lesions usually are isointense on T1-weighted images, and are known to show variable signal intensity on T2-weighted.

Leiomyosarcoma is a malignant tumor that occurs in the adult life or associated to Epstein Barr virus in young AIDS or after organ transplantation. Cutaneous or subcutaneous leiomyosarcomas are smaller than deeper ones, in the retroperitoneum. It shows often on MR imaging isointense signal to muscle on T1-weighted images and demonstrates a higher signal on T2-weighted images (figure 22).

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Figure 22.

5 Pericytic (Perivascular) tumors

The lesions in the pericytic/perivascular category show evidence of differentiation towards myxoid/contractile perivascular cells and tend to grow in a circumferential perivascular fashion. In the past it was included haemangiopericytoma in this group, but as there is no evident of pericytes and are more closely related to solitary fibrous tumor, has been classifyed as a fibroblastic tumor. The lesions remaining in the pericytic / perivascular category are shown in figure 23.

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Figure 23

Glomus Tumors appearance on MR imaging are variable but most tumors demonstrates low signal on T1-weighted images and high signal on T2-weighted images (figure 24). Because the glomus is richly vascularized, it shows marked contrast enhancement on MRI after the injection of gadolinium. MR angiography can be helpful to locate the tumor.

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Figure 24

6 Skeletal muscle tumors

The new WHO classification of soft tissue tumors in this chapter includes: (figure 25)

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Figure 25.

Rhabdomyoma is classified into cardiac and extracardiac types. Due to its rarity, MRI descriptions of extracardiac rhabdomyoma are anecdotical.

Rhabdomyosarcoma is a malignant tumor composed of cells showing distinct skeletal muscle features. MR imaging characteristics of rhabdomyosarcoma are non-specific and usually indistinguishable from those of other soft tissue sarcomas. Alveolar rhabdomyosarcoma, which produces fibrovascular septa that separates the tumor cells into discrete nets, may present a lobular appearance on MR contrast-enhanced images (figure 26).

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Figure 26.

7 Vascular tumors

Benign vascular tumors are very common and most frequently occur in the skin. It is often difficult to determine whether represent malformations, true neoplasms or reactive processes. Changes of the new classification includes lesions of intermediate malignancy such as kaposi and haemangioendothelioma tumors (figure 27).

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Figure 27.

Hemangiomas, localized vascular malformations, are subcategorized as lymphatic, capillary, venous, arteriovenous, and mixed malformations on the basis of their histology. On T2-weighted MR images hemangiomas are generally composed of multiple lobules of high signal intensity resembling a 'bunch of grapes' (figure 28).

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Figure 28.

Other vascular lesions such as Kaposi sarcoma and Epithelioid hemangioendothelioma are not specific on MR imaging and the appearance might be identical to that a solitary hemangioma (figure 29).

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Figure 29.

Angiosarcoma of soft tissue is a rare malignant tumor which develops usually in the cutaneous tissue. MR imaging might show a mass in the edematous subcutaneous fat tissue with heterogeneous signal on T2-weighted images. Dynamic MR imaging following intravenous administration of gadolinium can help to demonstrate the large vessels of this tumor.

8 Chondro-Osseous Tumors

In the new classification the concept of chondro-osseous soft tissue tumors has in so far changed , that only soft tissue chondroma as a benign entity and the malignant entities mesenchymal chondrosarcoma and extraskleletal osteosarcoma are further categorized under this heading. Myositis ossificans is now regarded as a (myo)fibroblastic lesion (figure 7), and extraskeletal myxoid chondrosarcoma has been placed in the miscellanous category (Chapter 10), since it shows little evidence of cartilaginous differentiation (figure 30).

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Figure 30.

Soft Tissue Chondroma (chondroma of soft parts) is a benign cartilage-forming tumor which occurs in extraarticular soft tissue locations unattached to bone. On MR imaging soft tissue chondroma usually show a multilobulated appearance with low to intermediate signal intensity on T1-, high signal intensity on T2-weighted images, and peripheral and/or septal contrast enhancement (figure 31).

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Figure 31.

Mesenchymal chondrosarcoma occurs less frequently than the myxoid variant, but nearly half of the mesenchymal chondrosarcomas are extraskeletal in location. The MR features of extraskeletal mesenchymal chondrosarcoma demonstrates a lobulated soft tissue mass with high signal intensity on T2-weighted images with complex, heterogeneous enhancement following administration of gadolinium (figure 32).

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Figure 32.

Extraskeletal Osteosarcoma MR features are unspecific and vary with the differentiation of the lesion, its amount of matrix production, and the presence of hemorrhage or necrosis (figure 33).

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Figure 33.

9 Neurogenic tumors

In this chapter are included the following groups: (figure 34)

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Figure 34.

Morton neuroma is not a true tumor. It is a reactive sclerosing process. The term 'neuroma' is a misnomer. It is typically in the 2nd-3rd intermetatarsal space. More common in woman. Gadolinium is not helpful (figure 35).

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Figure 35.

Neurofibroma and schwannoma are the two types of benign peripheral nerve sheath tumors. Both types arise from Schwann cells, and is more common shwannoma. Both types of peripheral nerve tumors are suggested by the exiting and/or entering nerve.

Neurofibromas usually appear as smooth, well-marginated masses in the distribution of a peripheral nerve. In about half of cases, there is a 'target' pattern on T2-weighted sequences with a high signal peripheral rim and a central zone of lower signal intensity. Other MR findings of neurofibromas, which are shared by other peripheral nerve sheath tumors, are the fat split sign (i.e., a rim of fat surrounding the lesion) and the fascicular sign (i.e., multiple small ring-like structures with peripheral higher signal intensity on either T2- or proton density weighted images). (figure 36).

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Figure 36.

Schwannoma, which also been termed neurinoma and neurilemoma, is an encapsulated neoplasm arising from the Schwann cells of a peripheral nerve sheath. There may be areas of hemorrhage, cystic degeneration, and necrosis. On MRI, schwannomas are often difficult to differentiate from neurofibromas.

Malignant peripheral nerve sheath tumor (MPNST) is now the accepted nomenclature for a spindle cell sarcoma arising from nerve or neurofibroma or demonstrating nerve tissue differentiation. This term encompasses entities formerly named neurofibrosarcoma, neurogenic sarcoma and malignant scwhannoma. Signal intensity features on MR are non-specific, showing intermediate to low signal intensity on T1-weighted images, and medium to high signal intensity on T2-weighted images. (figure 37).

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Figure 37.

10 Tumors of uncertain differentiation

In this category are included tumors with unknow clear line of differentiation. Extraskeletal myxoid chondrosarcoma is included in this category as there is no convincing evidence of cartilaginous differentiation (figure 38).

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Figure 38.

Intramuscular myxoma is a benign soft tissue tumor with abundant myxoid stroma and areas of increased vascularity. MRI shows low signal intensity on T1-weighted images and high signal on T2-weighted images. Theres is peripheral or patchy enhancement after injection of gadolinium. In some cases has been demonstrated the presence of fat and as a result may be indistinguishable from that of liposarcoma (figure 39).

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Figure 39.

Deep ('aggressive') angiomyxoma is an uncommon slowly growing neoplasm with a predilection for pelvic and perineal regions and tendency to local recurrence. The MR features are characteristic on T2-weighted images as having a swirled or layered apperance due to the fibrovascular stroma. The tumor is isointense on T1-weighted images and there is moderate enhancement after injection of gadolinium. The lack of high fat content is a key feature in differentiating aggressive angiomyxoma from myxoid liposarcoma. (figure 40).

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Figure 40.

Ossifying fibromyxoid tumor of soft tissue is a well circumscribed lobulated hard tumor covered by a thick fibrous pseudocapsule. Usually originates from the deep subcutaneous tissue of the extremities. Lesions are usually of longstanding duration. MR demonstrates isointense signal on T1-weighted images and isointense with areas of low signal on T2-weighted images. Irregular calcifications within the mass and surrounding it are depicted. (figure 41).

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Figure 41

Synovial sarcoma is a misnomer, it is not derived from true synovial cells. Occurs para-articular, usually close to tendons, bursae and joint capsules. On MR imaging most synovial sarcomas are isointense to muscle on T1-weighted images and high inhomogeneous signal on T2-weigthed images. Because of its extensive vascular supply, synovial sarcoma enhances markedly after injection of contrast medium (figure 42).

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Figure 42.

Epithelioid sarcoma is the term applied to an aggressive sarcoma affecting mainly adolescents and young adults involving frequently the hand. The MR findings described shows isointense signal on T1-weighted images, heterogenous and hyperintense on T2-weighted images with hypointense and serpiginous hyperintensity in the peripheral portion

Alveolar soft part sarcoma is a rare tumor affecting mainly adolescents and young adults. The tumors is highly hypervascular and frequently surrounded by thick, tortuous blood vessels and because of it hemorrhage might occur. In spite of the benign appearance alveolar soft part sarcoma is one of the most malignant soft tissue tumors. On MRI the tumor shows bright signal on both T1- and T2-weighted images, with multiple areas of signal void and septations. The lesion might mimick arteriovenous malformations due to the tortuous vessels.

Clear cell sarcoma of soft tissue is a very rare tumor of young adults with melanocytic differentiation, thus it is also known as malignant melanoma of soft parts. Clear cell sarcoma in contrast to malignat melanoma of the skin is more deeply located and mostly in the extremities. On MRI present as an homogeneous mass with slight high signal intensity on T1- and T2- weighted images. Although migh be expected low signal on T2-weighted images due to the paramagnetic effect of melanin, it has been suggested that the hyperintensity could be caused by high amount of myxoid stroma or abundance of loose connective tissue. The majority of cases show enhancement after gadolinium.

Extraskeletal myxoid chondrosarcoma is a malignant tumor with no convincing evidence of cartilaginous differentiation. It is primarily a tumor of adulthood. Usually contains cystic and hemorrhagic areas. The lesion shows a fibrous capsule and fibrous septations separating the multiple lobules each other. It might appear on MRI as an homogenous or heterogenous mass with variable signal on T1- and T2- weighted. Highly myxoid tumors resemble a cyst or a myxoma on T2-weighted images (figure 43).

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Figure 43. Extraskeletal myxoid chondrosarcoma. Sagital T2-gradient echo image shows a large homogeneous well-marginated mass, “pseudocystic”, with septae due to the myxoid component.

Extraskeletal Ewing 's sarcoma/Primitive neuroectodermal tumor (PNET) are rare soft tissue tumors defined as round cell sarcomas. MRI shows a well-circumscribed mass within the involved muscle. On T1-weighted images the tumor shows intermediate signal and heterogeneous high signal on T2-weighted images (figure 44).

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Figure 44