Hemosiderotic fibrolipomatous tumor (HFLT) is a rare soft tissue neoplasm consisting of adipocytes, hemosiderin pigments, and fibrous tissue. The tumor arises characteristically in the foot or ankle region or the dorsum of hands of middle-aged women and tends to recur in 30–50 % after excision [54]. Malignant transformation into myxoinflammatory fibroblastic sarcoma has been described [143], an entity with the same characteristic chromosome translocation t(1;10) (p22;q24) as HFLT.

Radiography and ultrasound are usually nonspecific. MRI shows an often ill-defined tumor with fatty areas divided by reticular septations. On T1-weighted sequences, HFLT is iso- or hypointense to the muscle, while being hyperintense on T2-weigthed images, possibly due to the myxoid/fibrous content of HFLT. Contrast enhancement of the nonfatty tumor parts has been described (Fig. 18.14). Because of hemosiderin deposition in HFLT, susceptibility-weighted imaging sequences such as T2* can show characteristic blooming in foci of hemorrhage, which can be helpful to distinguish HLFT from other lipomatous lesions [121].

Atypical fibroxanthoma (AFX) is a rare mesenchymal tumor of intermediate malignancy affecting elderly patients and arising in the sun-exposed cutis of the head and neck region. AFX tends to show exophytic growth; ulceration and bleeding are also possible. Histologically, AFX consists of fibroblastic spindle cells as well as multinucleated epithelioid cells and giant cells. Despite the sometimes aggressive histological appearances, AFX has a benign clinical course, when diagnosed early and excised completely [89, 105].

There is only limited literature about the radiological appearance of AFX available. Ultrasonography and MRI can be used to determine the extent of the tumor or possible infiltration of subcutis. In one case study, AFX shows intermediate signal intensity on T1-weighted and T2-weighted images and diffusely heterogeneous enhancement after contrast-agent administration [96], while both malignant melanoma and squamous cell carcinoma as possible differential diagnoses tend to have a high signal intensity on T2-weighted images. Melanin within melanoma may also cause elevated T1 signal intensities.

Angiomatoid fibrous histiocytoma (AFH) is a rare, low-malignant soft tissue tumor. Usually, AFH presents as a well-circumscribed multilobular mass showing a fibrous pseudocapsule with typical lymphocytic infiltrates and consisting of fibroblast and histiocyte-like cells of low mitotic activity and blood-filled spaces [82].

AFH usually occurs in children and young adults. It predominantly affects the dermis and subcutaneous tissue in the extremities and the trunk. Metastasis is only rarely seen.

Correspondingly, CT and MRI show a lobulated soft tissue mass sometimes with fluid–fluid levels and focal low signal intensity in T1- and T2-weighted sequences as a sign of intralesional hemorrhage [97] (Fig. 18.15).

Ossifying fibromyxoid tumor (OFMT) is a rare soft tissue tumor of still unknown origin arising in the subcutaneous tissue of the extremities usually following a benign clinical course. Yet single cases showing cellular atypia, aggressive behavior, and metastasis have been described [87]. Typically, OFMT consists of small round cells and osteoid or lamellar bone [44, 58, 87, 136].

OFMT presents as a slowly growing lobulated, ossified, and encapsulated mass in the fifth to seventh decade of life with a male predominance. The tumor diameter does seldom exceed 10 cm.

CT can demonstrate the characteristic lamellar bone formation as well as a strong contrast enhancement corresponding to the good vascularization that is typical of OFMT. In MRI, focal areas of high signal intensity on T1-weighted images of the tumor as a sign of fatty marrow spaces seem to be characteristic [67, 125] (Fig. 18.16).

Phosphaturic mesenchymal tumor (PMT) is a rare neoplasm that causes systemic phosphate depletion and consequently an oncogenic osteomalacia by producing a phosphaturic substance, the so-called fibroblast growth factor 23 (FGF23). The tumor can occur in the soft tissue and bone; in almost half of the cases, the lower extremities are affected. There is no sex predilection and the age range of patients is wide. Clinically, muscle weakness, bone pain, and multiple fractures as well as renal phosphate wasting and elevated serum levels of FGF23 are characteristic. The tumor itself is a slow-growing ill-defined mass and can histologically be divided in three variants: osteoblastoma-like, non-ossifying fibroma-like, and the most common mixed connective tissue variant (PMTMCT). After complete excision, bone consistency and serum levels of fibroblast growth factor 23 get back to normal [48, 84].

Since PMT usually do not exceed a diameter of 5 cm and can be located ubiquitary, diagnosis can be challenging. Radiography and CT scans often only show a general osteopenia and in case of intraosseous location lytic lesions with or without mineral deposition. Whole-body MRI is more helpful in finding the lesions, with T1 intensity depending on the vascularization and hyperintensity on T2-weighted images [48, 118]. In addition, whole-body scintigraphy with ¹¹¹In-octreotide and ⁶⁸Ga-DOTANOC PET/CT and ⁶⁸Ga-DOTATATE PET/CT is also excellent in detecting PMT [50] (Fig. 18.19).

Synovial sarcoma is a clinically and morphologically well-defined entity. It occurs primarily in the para-articular region, usually in close relationship with tendon sheaths, bursas, and joint capsules. It is, however, uncommon in joint cavities. On rare occasions, but widely reported in the radiological literature, it is also encountered in areas without any apparent relationship to synovial structures, such as in the pharynx, the larynx, the tongue, the maxillofacial region, the middle ear, precoccygeal and paravertebral regions, the mediastinum, the thoracic and abdominal wall, the heart, or even in intravascular and intraneural locations [23, 132, 140, 144]. Of note, only synovial sarcoma associated with the locomotor system is discussed in this chapter.

It is still uncertain whether trauma contributes to the development of synovial sarcoma or not. Although in many patients there is a definite history of trauma, the majority has no such antecedents. When reported, the interval between the episode of trauma and the detection of synovial sarcoma ranges from a few weeks to as much as 40 years. However, there are also reports of patients who sustain injuries after the presence of a mass had been noted, which suggests that, at least in some cases, the relationship between both is purely coincidental.

As mentioned before, synovial sarcoma occurs predominantly in the extremities (80–90 %), mostly near large joints, especially the knee joint (30 %) [45] (Table 18.2). These tumors are intimately related to tendons, tendon sheaths, and bursal structures, usually beyond the confines of the joint capsule. In less than 5 % of all cases, synovial sarcoma arises in the joint space itself.

The name of the lesion is derived from the microscopic resemblance of synovial sarcoma to normal synovium, although its origin is probably from undifferentiated mesenchymal tissue [45, 115]. Unlike most other types of sarcomas, the tumor is composed of two morphologically different types of cells that form a characteristic biphasic pattern: epithelial cells, which resemble those of carcinoma, and fibrosarcoma-like spindle cells. There are transitions between both types of cells, suggesting a close generic relationship. Depending on the relative presence of these types of cells and on their differentiation, synovial sarcomas can be classified into four different types.

The first type is the biphasic synovial sarcoma that is characterized by the coexistence of morphologically different but histogenetically related epithelial and spindle cells. Calcification with or without ossification is present in about 30 % of tumors. The degree of vascularity varies from a few scattered vascular structures to numerous dilated vascular spaces. In less well-differentiated tumors, hemorrhage may be prominent.

Multicystic formation is a common occurrence in synovial sarcoma. The cyst wall is thickened and composed of fibrous septa. Within the cyst, there are often several internal septa, consisting of tumor cells proliferating over the dense collagenous fibrous tissue without a lining pattern. Tumor cells may become detached from the septa in the internal lumen. Furthermore, dilated hemangiopericytomatous vasculatures can be prominent within those septa. Chromosomal rearrangements have been reported in association with synovial sarcoma, consisting of t(X;18) (p11–2;q11–2) translocation [31].

The majority of synovial sarcomas present on radiographs as round or oval, lobulated masses. They are usually located close to a large joint, particularly the knee joint. In 5–30 % of cases, there is periosteal reaction, bone erosion (related to pressure from the adjacent tumor), or even bone invasion. The most characteristic finding is the presence of multiple small densities caused by focal calcifications or ossifications. This feature is seen in about 20–30 % of cases. It may range from very fine stippling to marked calcifications or even bone formation, typically in the periphery of the lesion. When present, these opacities differentiate synovial sarcoma from liposarcomas and myxoid chondrosarcomas. The irregular shape of the calcifications helps to make the differentiation from hemangioma. In some cases, extensive ossification is present, resembling osseous or cartilaginous lesions such as soft tissue chondroma, extra-articular synovial chondromatosis, ossifying myositis, tumoral calcinosis, or osteosarcoma, both parosteal and extraskeletal. Cases with extensive calcification have been reported to have a better prognosis, with higher survival rates [117, 133].

Angiography usually reveals a prominent vascularity, not only of the primary tumor but also of the metastases. This is especially true for the monophasic and poorly differentiated type of tumor [102]. There is extensive neovascularity and nonhomogeneous staining. Occasionally, the lesion is hypovascular.

Ultrasound characteristics are those of a well-defined, solid vascular tumor with prominent arterial and venous components. There may be internal cystic components due to internal hemorrhage [107]. Internal calcifications are noted in 20–30 %. Ultrasound appears useful as a detection method of local recurrence, in the guidance of fine-needle biopsy and in the examination of children [72]. In the early postoperative period (less than 6 months after surgery), inhomogeneous hypoechogenic lesions cannot be differentiated with certainty as recurrent tumor, hemorrhage, edema, granulation tissue, abscess formation, or any combination of these. Performed with high-frequency transducers, a lesion is considered to be a recurrent tumor when a discrete nodular hypoechogenic mass is present. Areas of diffuse abnormal echogenicity without evidence of a discrete nodule can be classified as non-tumoral.

CT shows a soft tissue mass, which may infiltrate adjacent structures, having a slightly higher density than muscle [115] (Fig. 18.20). Joint invasion is present when the soft tissue mass projects into the expected confines of a joint capsule or when an intra-articular ligament or tendon is involved. Although bony involvement can be identified on both MR and CT imaging, cortical bone erosion or invasion is better depicted on CT. Intratumoral calcification or ossification is also more easily seen on CT than on MR imaging. Because of its extensive vascular supply, synovial sarcoma enhances markedly after injection of contrast medium.

On MR imaging, most synovial sarcomas (>90 %) are hypointense relative to fat and nearly isointense relative to muscle on T1-weighted MR images [117]. In rare cases, the tumor may be mostly hyperintense due to extensive intratumoral hemorrhage. Small areas of high signal on T1-weighted images are more often encountered (45 %) [81, 117, 123]. They probably correspond with small foci of hemorrhage, since these areas are of high signal on T2-weighted images, too. Fluid–fluid levels, although not very frequent (15–25 %) and not specific, can be a striking finding. Areas of previous hemorrhage with fluid–fluid levels or high signal intensity on all pulse sequences may be associated with a worse prognosis [117] (Figs. 23.13–23.17). On T2-weighted images, marked inhomogeneity is the rule, and various degrees of internal septation may be noted [114] (Figs. 18.21 and 18.24). This is especially true for lesions more than 5 cm in diameter which show this heterogeneous signal pattern in more than 85 % of cases vs. 60 % for smaller lesions [81]. A triple signal pattern on T2-weighted images is described in one third of all synovial sarcomas [12, 81, 133]. It consists of high signal similar to fluid, intermediate signal intensity equal to or slightly hyperintense relative to fat, and low signal intensity closer to that of fibrous tissue. This triple signal pattern on T2-weighted images together with the small high signal foci on T1-weighted images is suggestive for synovial sarcoma (Figs. 18.22 and 18.23). More than half of all lesions are intimately related to bone. There seems to be no notable difference between the MR imaging characteristics of the mono- and biphasic pathologic subtypes.

Heterogeneous enhancement after injection of contrast material is generally seen. Hypervascular areas are strongly enhancing, whereas necrotic and cystic areas remain hypointense. Hypervascularity may be quantified by a dynamic contrast-enhanced MR examination. Early enhancement of tumor within 7 s after arterial enhancement is a reliable sign, occurring consistently in synovial sarcoma. Other previously described so-called malignant dynamic contrast-enhanced MR imaging features, such as early plateau or washout phase and peripheral enhancement are not always found in synovial sarcoma [155].

Well-defined or reasonably well-defined margins are seen in some cases of synovial sarcoma. This finding has been described as a probable sign of benignity [132, 133]. This may be one of the reasons why synovial sarcoma is the malignant tumor most frequently misdiagnosed as benign [17, 133]. Neurovascular involvement may be suspected in cases where the tumor margin abuts and displaces the neurovascular bundle. In one study, this was confirmed at operation in two out of five cases [114]. MR often fails to demonstrate small calcifications seen on plain films or CT. Therefore, the MR images of soft tissue and bone tumors should be interpreted with corresponding CT images and plain films.

The differential diagnosis on MR imaging of an inhomogeneous septate mass with infiltrative margins and located in close proximity to a joint, a tendon, or a bursa is very limited. Without the history of trauma or signs of infection, a malignant neoplasm is the most likely consideration. Especially in combination with soft tissue calcifications, the diagnosis of a synovial sarcoma should be preferred. However, a septate configuration can also be seen in two types of benign soft tissue masses: hemangioma and synovial or ganglion cysts. Typically, these benign tumors are sharply marginated and have a homogeneous hyperintense signal that is much brighter than that of the subcutaneous fat on T2-weighted images. The pattern of contrast enhancement may be helpful as well in the differential diagnosis (Figs. 18.24 and 18.25).

Treatment of synovial sarcoma consists of wide local excision or limb amputation, usually in combination with chemotherapy and radiation therapy. Despite this aggressive therapy, metastatic disease or local recurrence is found in approximately 80 % of patients. Metastases most frequently affect the lung (59–94 % of distant tumor spread). Additional sites of metastatic disease include lymph nodes (4–18 %) and the bone (8–11 %). Soft tissue metastases are rare. Local recurrence occurs in 20–26 % of patients and presents usually within 2 years after initial diagnosis. The 5-year survival rate is approximately 27–55 %. Favorable prognostic factors with synovial sarcoma include extensive calcification, younger patient age, lesions less than 5 cm in diameter, and neoplasms located in the extremities [117]. According to Tateishi, other prognostic MRI parameters include the presence of hemorrhage and cysts or the presence of a triple signal. Together with proximal tumor distribution, large tumor size, and the absence of calcification, these parameters are associated with high tumor grade and a less favorable prognosis [151]. The value of MRI in the early detection of tumor recurrence will be discussed in detail in Chap. 26, dealing with imaging posttreatment (Fig. 18.25).

The term “epithelioid sarcoma” was applied by Enzinger in 1970 to refer to a distinctive neoplasm that commonly involves the soft tissues of the extremities, most frequently of the hand. This tumor presents as a solitary or irregular multinodular mass located in the dermis or deep-seated and attached to tendons or fascia. The size of the lesion is variable and ranges from several millimeters to more than 15 cm in diameter. Central degeneration and necrosis are common features. Lesions located in the dermis often ulcerate through the skin. The tumor may spread along the neurovascular bundles, may invade the vessels, and, hence, may metastasize. In general, tumors located in the proximal extremity reveal a more aggressive course than those arising in the distal extremities [66]. A history of trauma is reported in up to 25 % [25, 129]; histogenesis, and the nature of this neoplasm remains unknown.

Epithelioid sarcoma represents the most common soft tissue sarcoma of the hand. Although it may occur at any age, the tumor is found most commonly in adolescents and young adults between 10 and 35 years of age. Children and older persons are only rarely affected. This tumor is twice as common in men as in women. The principal sites of involvement are the distal upper extremity, involved in 58 % of cases, particularly the hands and forearms. This main site of involvement is followed by the distal and proximal extremity, at 15 and 12 %, respectively. The trunk, head, and neck regions, with the exception of the scalp, are seldom involved [66]. Lesions may be located in the subcutis or are deep seated, usually attached to tendons, tendon sheaths, or fascial structures. Epithelioid sarcoma presents as a firm, hard nodule that may be solitary or multiple. These nodules are slowly growing and painless. Ulceration through the skin is common in intradermal lesions. Metastases, predominantly to the regional lymph nodes and lung, are observed in less than half of the cases. These develop mostly within the first year after diagnosis, but may be late and become apparent for many years after excision of the primary tumor. Recurrences, often multiple, are common and have been reported in 77 % of cases [25].