The clinicopathologic and immunohistochemical features of 69 pediatric examples of infantile digital fibroma/fibromatosis (IDF) were analyzed. Thirty males, 26 females, and 1 child (sex unstated) ranging from newborn to 120 months of age (median, 12 mo) manifested 74 lesions (5 identified in follow-up) involving the toe or finger (n=71) and the hand or foot (n=3). Tumors ranged in size from 3 to 35 (median, 10) mm. All but 4 study members presented with a solitary lesion. Metachronous IDFs developed in 7 patients within 17 to 82 months. Microscopically, a cytologically bland, fibroproliferative lesion was observed forming a dome-shaped/polypoid nodule directly beneath the epidermis and invading dermal adnexa. Mitotic figures per 20 high-powered fields ranged from 0 to 7 (median, 1). Paranuclear cytoplasmic inclusions were identified in 57 tumors. Tumor cells immunohistochemically expressed calponin (11 of 11 tumors), desmin (9/9), alpha-smooth muscle actin (11/11), CD99 (11/11), CD117 (6/8), heavy caldesmon (2/11 and scattered cytoplasmic inclusions in 4 tumors), CD10 (1/9), nuclear beta-catenin (2/11), and CD34 (1/11), but not muscle actin (HUC1-1), keratins, estrogen/progesterone receptor proteins, or activated caspase-3. Twenty-eight of 38 patients (74%) experienced recurrent/persistent disease (single in 22; multiple in 6) (median, 4 mo after surgery). One recurrent tumor spontaneously regressed and the size of another remained unchanged for almost 17 years before reexcision. All 23 patients with >5 years follow-up are currently disease free (median disease-free interval, 23 y). Minor postoperative functional/cosmetic complaints were reported in 47%. No patient with adequate clinical data developed the digitocutaneous dysplasia syndrome or a conventional fibromatosis, or relayed a family history of IDF/conventional fibromatosis. Our results indicate that IDF is a unique myofibroblastic process separable from conventional fibromatoses and from histologic mimics. Conservative excision or observation after biopsy (with additional surgery employed as necessary) are recommended treatment options.
