Digital fibromyxoma (superficial acral fibromyxoma): a detailed characterization of 124 cases.

Digital fibromyxoma (first described by Fetsch and colleagues as superficial acral fibromyxoma) is a distinctive soft tissue tumor with a predilection for the subungual or periungual region of the hands and feet. This report details the histologic, immunophenotypic, and clinical findings in 124 cases of digital fibromyxoma. The study group included 70 male and 54 female patients (1.3:1, M:F), ranging in age from 4 to 86 years (mean, 48 y; median, 49 y). Mean tumor size was 1.7 cm (range, 0.5 to 5 cm; median, 1.5 cm). Nearly half of the patients (41%) presented with a painful mass. Tumors arose on the hands (52%) or feet (45%), with rare tumors arising on the ankle or leg. Most tumors occurred on the digits (94% of hand tumors, 82% of foot tumors), with the majority growing in close proximity to the nail (97% on fingers, 96% on toes). Histologically, 80% of cases were poorly marginated; 70% infiltrated the dermal collagen, 27% infiltrated fat, and 3% invaded bone. In cases in which imaging studies were available, bone involvement by an erosive or lytic lesion was more frequent (9/25, 36%). All tumors were composed of spindle-shaped or stellate-shaped cells with palely eosinophilic cytoplasm and a random or loosely fascicular growth pattern. The tumor cells were separated by dense hyaline collagen alternating with myxoid stroma. Most (86%) of the tumors showed alternating areas of fibrous and myxoid stroma, 11% showed predominantly fibrous stroma, and 3% had predominantly myxoid stroma. Increased mast cells were noted in 88% of tumors. All tumors comprised cells with minimal atypia, occasionally showing scattered larger cells with so-called "degenerative change." Mitotic figures were infrequent, and all tumors lacked necrosis, pleomorphism, or neural/perineural infiltration. Multinucleate stromal cells were occasionally seen. Tumor cells were reactive for CD34 in 42/61 cases (69%), with rare tumors showing focal reactivity for EMA (3/40, 7.5%), smooth muscle actin (5/42, 12%), and desmin (1/18, 6%). All tumors were negative for S100 (0/66), MUC4 (0/11), GFAP (0/10), AE1/AE3 (0/4), Cam5.2 (0/2), PanK (0/2), Claudin (0/4), and NFP (0/3). Follow-up in 47 cases ranged from 1 to 252 months (mean, 35 mo). Ten tumors (24%) recurred locally (all near the nail unit of the fingers or toes) after a mean interval of 27 months. One tumor recurred twice. All recurrent tumors had positive margins on initial biopsy or subsequent excision and no other clinical or pathologic features correlated with recurrence/persistence. To date, no tumor has metastasized. Finally, sequencing of 8 digital fibromyxomas failed to reveal mutations in exon 8 or 9 of GNAS1, in contrast to intramuscular or cellular myxoma.