Perineuriomas are uncommon benign peripheral nerve sheath tumors that include four subtypes: soft tissue, intramural, sclerosing, and reticular. Soft-tissue perineuriomas are the most common subtype and show distinctive morphologic, ultrastructual, and immunophenotypic features that distinguish them from the much more common nerve sheath tumors, schwannomas, and neurofibromas. The tumors show a morphologic spectrum ranging from hypercellular lesions with collagenous stroma to hypocellular tumors with myxoid stroma. The immunohistochemical studies are often necessary for the diagnosis of soft-tissue perineurioma [3
To the best of our knowledge, MRI and CT images of soft-tissue perineurioma in the retroperitoneum have not been reported. The soft-tissue perineurioma in this report was similar to the one in the subcutis of the groin [5
]: well-defined margins with fibrous capsule, water density on plain CT, subtle enhancement on contrast-enhanced CT, low signal on T1
-weighted images, and hyperintensity with low-intensity capsule on T2
-weighted images. Only contrast enhancement on fat-suppressed T1
-weighted images was more prominent in our report. A dynamic MR study revealed the gradually enhancing nature of this myxoid tumor.
Nearly half of soft-tissue perineuriomas are hypocellular, and 20% are markedly hypercellular. The stroma can vary from a more collagenous to a myxoid appearance. Whereas the stroma in most tumors is collagenous, 40% show at least focally myxoid stroma, and 20% are nearly exclusively myxoid [2
]. The soft-tissue perineurioma in our report was hypocellular with abundant myxoid stroma.
Radiologic differential diagnosis of retroperitoneal soft-tissue perineurioma includes neurilemmomas, neurofibromas [6
], ganglioneuromas [7
], and cellular myxomas [8
]. The former two tumors are most common nerve sheath tumors, and a target-like enhancement pattern is the MR imaging characteristic. Moreover, they tend to demonstrate multiple cystic spaces of varying size that are caused by either cystic or myxoid degeneration within the tumors. Hemorrhage and calcification may be also seen [6
In contrast, the current case of soft-tissue perineurioma showed no definite focal degeneration or hemorrhagic areas. Cellular/intramuscular myxomas arise in the muscle, and are rarely found in the retroperitoneum [8
]. The most problematic differentiation may be between soft-tissue perineurioma and ganglioneuroma with abundant myxoid stroma. They both show rather homogeneous marked hyperintensity on T2
-weighted images. The tumor signal intensity on T2
-weighted images depends on the proportion of myxoid stroma to cellular components and collagen fibers. MR dynamic enhancement patterns of the two entities are similar: they lack early enhancement, but delayed enhancement increased gradually. However, one of the MR imaging characteristics of ganglioneuroma is curvilinear bands of low signal intensity on T2
-weighted images [7
], which were absent in the current soft-tissue perineurioma. The soft-tissue perineuromas can be either encapsulated or unencapsulated [2
]. Neurilemmomas are encapsulated, and neurofibromas are unencapsulated tumors [6
], and the possibility of neurofibroma can be excluded if the findings suggest the presence of a capsule [7
Other differential diagnoses include malignant tumors such as perineurial malignant nerve sheath tumor [9
], malignant fibrous histiocytoma with myxoid change, and low-grade fibromyxoid sarcoma.
Dynamic MR imaging better provides the gradually enhancing nature of a myxoid tumor. Soft-tissue perineuriomas with marked myxoid stroma may be mistaken for cysts on plain CT.