It is well known that a wide variety of superficial soft-tissue solid masses can be seen in daily clinical practice, however, some of which can be easily diagnosed and others would be difficult. According to our experience, a systematic analysis would be helpful to give a definitive diagnosis or narrow a differential diagnosis. Generally, most cutaneous tumors arising from the skin appendage are easily seen and are liable to make a diagnosis accurately in clinic, so imaging examination is limited. However, some large mesenchymal tumors arising from the cuntaneous and subcutaneous tissue, especially with malignant tendency, should be evaluated the area and depth pre-operatively. In this article, imaging features of a spectrum of histologically proven superficial soft-tissue solid mesenchymal tumors from our institution will be presented. It is intended as a comprehensive review with emphases on relatively more common masses within the superficial soft-tissue, and the diagnosis may be suggested by MRI.

1. Adipocytic tumors are quite common and represent the largest single group of mesenchymal tumors. Lipoma represents the most common benign adipocytic tumor, and liposarcoma stands for the common malignant type. Typical findings of lipoma are identical to fat on MR images, and thin fibrous septa are occasionally seen. MR images of fatty masses are frequently sufficiently characteristic to suggest a specific diagnosis of lipoma (Fig.1). However, when a mass does not meet the diagnostic criteria for a lipoma, a presumptive diagnosis of liposarcoma may be suggested.

2. Vascular tumors are common soft tissue masses, and MRI often demonstrates characteristic features that allow imaging diagnosis. Hemangiomas are among the most frequent tumors to involve the soft tissue. MR imaging is the best modality to evaluate soft tissue hemangiomas. Angiosarcoma is a malignant vascular tumor, and most frequently involves the skin and deep soft tissues. The intrinsic characteristics of angiosarcoma involving the skin and subcutaneous tissues are usually nonspecific on MRI(Fig.2).

3. Fibroblastic/myofibroblastic tumors are a heterogeneous group of neoplastic lesions in clinical practice and are seen in all age groups, the superficial types of which mainly include nodular fasciitis, elastofibroma, and palmar/plantar fibromatoses. Nodular fasciitis is a self-limiting benign fibrous neoplasm that usually occurs in subcutaneous tissue. The upper extremity is the most frequently involved site. Lesions are typically subcutaneous and presents as a subcutaneous nodule, attached to superficial fascia. Although MR imaging is nonspecific, the linear extension along the superficial fascia (fascial tail sign) on MRI helps suggest the diagnosis of subcutaneous lesions, and mild surrounding edema may be seen.

Dermatofibrosarcoma Protuberans (DFSP) is a superficial, with low to intermediate grade malignant potential, locally aggressive fibroblastic neoplasm, which arises from the dermis. Large lesions may invade underlying structures, ulcerate, bleed, or become painful. Local recurrence is common. Typical imaging manifestations of DFSP show a subcutaneous protuberant mass with a lobular or nodular architecture involving the skin and subcutaneous adipose tissue. MR images are well suited to demonstrate this location and the distinct lobular or nodular architecture. More importantly, the relationship of the lesion to the underlying structures is well delineated(Fig.3). The intrinsic signal intensity of the lesion on MRI is nonspecific. However, in our experience, linear extension along the skin surface is suggestive of the diagnosis. MRI is useful in identifying the extent and depth of DFSP tumor infiltration, facilitating preoperative planning for adequate margins and tumor clearance.

4. Nerve sheath tumors are very common soft tissue tumors which arise from the nerve. Schwannoma (Fig.4) and neurofibroma (Fig.5) are the most common types of benign peripheral nerve sheath tumors (BPNST), and malignant peripheral nerve sheath tumor (MPNST) accounts for 5% to 10% of all soft tissue sarcomas. Imaging characteristics, including fusiform shape with entering and exiting nerve, target sign, split-fat sign, fascicular sign and associated muscle atrophy, often suggest neoplastic lesions of neurogenic origin. Differentiation of BPNST from MPNST is often very difficult. However, imaging features favoring malignancy include size larger than 5 cm, prominent enhancement, marked heterogeneity with central necrosis, rapid growth, and infiltrative margins in a nonplexiform lesion.

5. Undifferentiated pleomorphic sarcoma (UPS), previously called malignant fibrous histiocytoma (MFH) is the most common soft tissue sarcoma of late adult life. Only 5% to 10% of UPSs involve the subcutaneous tissue. Subcutaneous UPS reveals an intrinsically nonspecific soft tissue mass replacing the normal subcutaneous fat on MR images. These lesions commonly invade the underlying muscle and may cause overlying skin ulceration. MR images often show heterogeneous signal intensity on all pulse sequences, which depends on variable amount of collagen, myxoid tissue, necrosis, and hemorrhage.