WHO Classification of Soft Tissue Tumors

Tumor category

Major changes


Mixed-type liposarcoma removed


DFSP and giant cell fibroblastoma included for the first time


“Hemangiopericytoma” removed as a synonym for SFT

Recognition of nodular fasciitis and variants as true neoplasms

So-called fibrohistiocytic

“Malignant fibrous histiocytoma” removed

Smooth muscle

Angioleiomyoma reclassified as pericytic tumor


Angioleiomyoma reclassified as a pericytic tumor

Myofibroma now classified as pericytic tumor

Skeletal muscle

Spindle cell/sclerosing rhabdomyosarcoma classified together and separated from others subtypes


Pseudomyogenic (epithelioid sarcoma-like)

Hemangioendothelioma added as a new entity

Gastrointestinal stromal

GIST included in the volume on STT for the first time

Nerve sheath

Peripheral nerve sheath tumors included volume on STT for the first time

New hybrid benign nerve sheath tumors included (schwannoma/perineurioma)

Tumors of uncertain differentiation

New tumors: acral fibromyxoma, hemosiderotic fibrolipomatous tumor differentiation

Phosphaturic mesenchymal tumor

Atypical fibroxanthoma now included

PNET removed as synonym for Ewing’s sarcoma

Undifferentiated/other category

Includes tumors that cannot be classified into any unclassified sarcoma

Abbreviations: WHO World Health Organization, DFSP dermatofibrosarcoma protuberans, SFT solitary fibrous tumor, GIST gastrointestinal stromal tumor, STT soft tissue tumor, PNET primitive neuroectodermal tumor

11.2 Adipocytic Tumors

Due to the high incidence of lipomas, angiolipomas, and liposarcomas, adipocytic tumors represent the largest single group of mesenchymal tumors. The WHO classification persists to consider atypical lipoma (atypical lipomatous tumor) and well-differentiated liposarcoma as essentially synonymous (Table 11.2) and considered locally aggressive with no potential for metastasis [1]. The terms “mixed-type liposarcoma” and myxolipoma have been deleted.

Table 11.2
Group 1 – Adipocytic tumors




Lipomatosis of the nerve



Myolipoma of the soft tissue

Chondroid lipoma

Spindle cell lipoma/pleomorphic lipoma


Intermediate (locally aggressive)

Atypical lipomatous tumor/well-differentiated liposarcoma


Dedifferentiated liposarcoma

Myxoid liposarcoma

Pleomorphic liposarcoma

Liposarcoma, not otherwise specified

In children diffuse lipoblastoma is now the preferred term for lipoblastomatosis.

The definition of dedifferentiated liposarcoma has been slightly modified and is currently subdivided into four groups (see Table 11.1). Chondroid lipoma may resemble myxoid liposarcoma. Unlike myxoid liposarcoma chondroid lipoma usually shows calcifications, which are best demonstrated by radiographs, CT, or US [3].

11.3 Fibroblastic/Myofibroblastic Tumors

Fibroblastic/myofibroblastic tumors represent a very large group of mesenchymal tumors, many of which contain fibroblastic as well as myofibroblastic elements (Table 11.3). The current WHO classification recognizes that nodular fasciitis, proliferative fasciitis, and proliferative myositis (Fig. 11.1) are neoplastic [4], whereas previously they were believed to be reactive lesions.

Table 11.3
Group 2 – Fibroblastic/myofibroblastic tumors


Nodular fasciitis (proliferative fasciitis/proliferative myositis)

Myositis ossificans


Fibromatosis colli

Fibrous hamartoma of infancy

Juvenile hyaline fibromatosis

Inclusion body fibromatosis

Fibroma of the tendon sheath

Desmoplastic fibroblastoma

Mammary-type myofibroblastoma

Calcifying fibrous tumor



Gardner fibroma

Calcifying fibrous tumor

Intermediate (locally aggressive)

Superficial fibromatosis (plantar/palmar)

Desmoid-type fibromatosis


Giant cell fibroblastoma

Intermediate (rarely metastasizing)

Dermatofibrosarcoma protuberans

Solitary fibrous tumor

Inflammatory myofibroblastic tumor

Myofibroblastic sarcoma/atypical myxoinflammatory fibroblastic tumor

Infantile fibrosarcoma


Adult fibrosarcoma


Low-grade fibromyxoid sarcoma

Sclerosing epithelioid fibrosarcoma


Fig. 11.1
Proliferative myositis of the thigh. Axial T2-WI image shows a well-defined mass (arrow) within the adductor muscle with inhomogeneous mild high signal intensity with areas of low signal within the lesion due to the fibrous content

Other changes were the inclusion of the closely related giant cell fibroblastoma and dermatofibrosarcoma protuberans (DFSP), formerly included in volume on skin lesions. DFSP is categorized as a rarely metastasizing (intermediate) tumor, although it should be noted that metastatic potential is gained only when a component of a fibrosarcomatous change is present [5]. Giant cell fibroblastoma is classified in the locally aggressive (intermediate) category because it recurs in approximately 50 % of cases, but does not metastasize [6]. Hemangiopericytoma has been removed from the classification of “extrapleural solitary fibrous tumor” because it is well established that this outdated term included tumors that represented cellular examples of SFT [5]. Lipomatous hemangiopericytoma and giant cell angiofibroma have also been included as SFT. The term “atypical myxoinflammatory fibroblastic tumor” was introduced for as synonym for “myxoinflammatory fibroblastic sarcoma.” This new term better reflects the extremely low potential for metastasis for this tumor type.

11.4 So-Called Fibrohistiocytic Tumors

As the concept of fibrohistiocytic differentiation has been a matter of debate [7], malignant fibrous histiocytoma (MFH) has been finally deleted from the current WHO classification. MFH and its subtypes have been reclassified in the new separate group of unclassified/undifferentiated sarcomas (group 12). Thus, group 3 now does not include malignant tumors (Table 11.4). It includes the common tenosynovial giant cell tumors, the uncommon giant cell tumor (Fig. 11.2) of soft tissues, and the plexiform fibrohistiocytic tumor.

Table 11.4
Group 3 – The so-called fibrohistiocytic tumors


Tenosynovial giant cell tumor

 Localized type

 Diffuse type


Deep benign fibrous histiocytoma

Intermediate (rarely metastasizing)

Plexiform fibrohistiocytic tumor

Giant cell tumor of the soft tissue


Fig. 11.2
Giant cell tumor of the soft tissue with Paget’s disease. Axial T2-WI image shows an intermediate signal nodular lesion in the pretibial subcutaneous space (thin arrow). Note the cortical thickening due to Paget’s disease (thick arrow)

11.5 Smooth Muscle Tumors

Smooth muscle tumors arising at non-cutaneous, non-uterine locations have been the focus of a considerable conceptual shift in the past but there were no major changes in this category (Table 11.5). The only change is that angioleiomyoma (vascular leiomyoma) was reallocated to the category of pericytic (perivascular) tumors (group 5). There are no changes on the deep leiomyosarcoma, although some tumors classified as undifferentiated pleomorphic sarcoma closely resemble a subset of leiomyosarcoma, which may suggest the existence of “dedifferentiated leiomyosarcoma.”

Table 11.5
Group 4 – Smooth muscle tumors


Leiomyoma of the deep soft tissue


Leiomyosarcoma (excluding the skin)

11.6 Pericytic (Perivascular) Tumors

The lesions in the pericytic/perivascular category were first included in the 2002 edition. At the same time, hemangiopericytoma was removed from this group and listed as synonym for solitary fibrous tumor (group 2). Although rare, the most common tumor analyzed on imaging in this group is glomus tumor [8], composed of cells resembling the modified smooth muscle cells of the normal glomus body. It has been acknowledged that myofibroma/myofibromatosis represent morphological features along the spectrum of myopericytic neoplasms (Table 11.6) instead of fibroblastic/myofibroblastic tumors (group 2), which were included in the past. Angioleiomyoma (vascular leiomyoma) which typically presents as a painful lesion in the distal extremities of middle-aged adults has now been included in this group [9, 10].
Nov 22, 2017 | Posted by in MUSCULOSKELETAL IMAGING | Comments Off on WHO Classification of Soft Tissue Tumors

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