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.

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].

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.

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been removed from the classification of “extrapleural solitary fibrous tumor” because it is well established that this outdated term included tumors that represented cellular examples of SFT [5].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.

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.

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.”

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].