Atypical fibroxanthoma: a histological and immunohistochemical review of 171 cases.

The clinical and histological features of 171 atypical fibroxanthomas (AFX) from a single institution in Western Australia are outlined. This area experiences high levels of solar radiation, and all assessable biopsies showed solar elastosis. Patients were aged between 41 and 97 years (median age 74), with 76% of tumors occurring in men (male to female ratio approximately 3 to 1). Most tumors were small, with a median diameter of 10 mm and a range of 4-35 mm. Only 5% exceeded 20 mm in diameter. Most AFX were well-circumscribed dermal lesions, with limited invasion of subcutis in a minority. Histological variants identified included keloidal (n = 8), clear cell (n = 3), and granular cell (n = 3), plaque like (n = 4), and myxoid (n = 1). Bland cytological appearances (spindle cell nonpleomorphic AFX) were noted in 5 tumors, with osteoclast-like giant cells in 2. Features suggesting regression were present in 22 cases. Two cases recurred locally, none metastasized. No tumors expressed melanocytic or epithelial markers. Seventy-four percent of cases expressed smooth muscle actin, typically strongly and diffusely. No AFX stained with desmin. Only 1 of 50 cases was CD117 positive. In conclusion, AFX may show a wide range of histological appearances, and a panel of immunohistochemical markers is essential to make the correct diagnosis. Histological mimics, such as poorly differentiated squamous cell carcinoma, must be carefully excluded. Specific diagnosis is important because there seems to be a very low risk of recurrence or metastasis despite the frequently alarming histology.