Myofibromas with atypical features: expanding the morphologic spectrum of a benign entity.

Typical myofibromas are biphasic tumors composed of a central zone of immature spindled to rounded cells arranged in a pericytic pattern and a peripheral zone of myoid nodules. Central necrosis is occasionally seen. A small but undefined subset of myofibromas displays atypical features that may lead to a misdiagnosis of sarcoma. To more completely characterize these tumors and define their behavior, we analyzed our experience with myofibromas having 1 or more atypical features including hypercellularity, absent or inconspicuous, poorly demarcated myoid nodules, infiltrative growth pattern, and perineural invasion. Of 266 cases of myofibromas, 24 cases were retrieved on the basis of pathology reports in which atypical features were mentioned. The tumors presented in 16 male and 8 female individuals (mean age 17 y; range, 2 wk to 62 y) as masses of variable size (mean 3.0 cm; range, 1.5 to 6.5 cm). Fourteen cases arose on the head and neck and 10 cases on the limbs. The referring or suspected diagnosis was sarcoma in 8 cases. The tumors were typically more cellular than ordinary myofibroma with levels of cellularity similar to that expected in fibrosarcoma (22/24). In addition, they displayed inconspicuous, loosely cohesive (22/24) or absent myoid nodules (2/24), infiltrating borders (19/24), intravascular growth (5/24), and perineural invasion/nerve entrapment (6/24). The mean mitotic rate was 5 mitoses/10 high-power fields, but no tumor showed significant cytologic atypia. The tumors were positive for actins (11/11) and CD34 (2/8). Follow-up in 14 patients revealed no distant metastases. We conclude that a small subset of myofibromas shows atypical features that complicate the diagnosis but do not adversely affect outcome.