Differential diagnosis
The diagnosis was reconsidered to be nodular fasciitis based on the cellular morphology and the growth pattern. The tumor consisted of plump fibroblasts with bland nuclear features and a low mitotic rate (4 per 10 HPFs, f.d. 0.57 mm) in a myxoid and collagenous stroma. Intersecting fascicles with a storiform pattern, high cellular density, pseudocyst formation, and an inflammatory infiltrate containing giant-cell macrophages suggest nodular fasciitis and help narrow the differential diagnosis.
While nodular fasciitis can be suspected based on histologic features, it must be differentiated from fibrosarcoma, smooth muscle tumors, malignant peripheral nerve sheath tumor, dermatofibrosarcoma protuberans, melanoma, and solitary fibrous tumor. Nodular fasciitis has no pathognomonic immunohistochemical markers. However, SMA and vimentin typically show diffuse intensive staining [3]. Desmin is only rarely positive; S100 and h-caldesmon are negative [4-5]. Other positive markers include calponin and CD10 [1,7] (Table 1). Proliferative index Ki67 is highly variable in nodular fasciitis and does not aid in diagnosis.
Fibrosarcoma and malignant smooth muscle tumors are in the differential, but the absence of necrosis, frank nuclear atypia, and atypical mitotic figures support this tumor’s benign nature. Immunohistochemistry can aid in differentiating these diagnoses by the presence or absence of desmin, h-caldesmon, and estrogen/progesterone receptor positivity (Table 1). Leiomyosarcoma is positive for desmin and h-caldesmon, but nodular fasciitis is negative for these markers. Fibrosarcoma can be CD34 positive in some cases, usually in dermatofibrosarcoma protuberans, but nodular fasciitis, the positivity is excitingly rare [6].
Malignant peripheral nerve sheath tumor that was initial diagnosis at the time of recurrence was excluded based on the relatively low mitotic count, absence of atypia, and invasion in surrounding tissue with negative S-100 staining on immunohistochemistry. Malignant melanoma was excluded due to the negative S-100 and melan-A staining in this case. Solitary fibrous tumors must show perivascular growth patterns and tend to be positive for CD34 and BCL-2 [8].