Superficial acral fibromyxoma: clinicopathological, immunohistochemical, and molecular study of 11 cases highlighting frequent Rb1 loss/deletions

Superficial acral fibromyxoma (SAF) is an uncommon benign dermal mesenchymal lesion of adults with predilection for acral sites, in particular the nail region. To date, less than 300 cases have been reported. SAFs consistently express CD34, but other diagnostic markers or specific genetic alterations have not been established yet. We describe 11 SAFs occurring in 7 men and 4 women aged 37 to 86 years (median, 48 years). Mean size was 6 mm (range, 4-20 mm). Affected sites were fingers (n = 5), toes (n = 3), heel (n = 1), calf (n = 1), and unspecified digit (n = 1). None of 10 patients with available follow-up (2-60 months; median, 24 months) developed recurrence. Histology showed relatively hypocellular vaguely lobulated nodules composed of bland-looking spindled or stellate fibroblast-like cells arranged into storiform or loose fascicles within a variably myxoid, fibromyxoid, or collagenous vascularized stroma. Immunohistochemistry showed expression of CD34 (9/10) and focal weak reactivity for epithelial membrane antigen (2/11). None of the lesions expressed protein S100 (0/11), MUC4 (0/11), or STAT6 (0/11). Loss of Rb1 immunoexpression was observed in 9 (90%) of 10 cases. All 7 cases with successful RB1 fluorescence in situ hybridization testing showed RB1 gene deletions, which was variably associated with co-loss of the corresponding 13q12 signal (monosomy at the 13q region). To our knowledge, this is the first study investigating the expression status of the tumor suppressor Rb1 in SAF by immunohistochemistry and fluorescence in situ hybridization. Our results showed frequent Rb1 deficiency as a possible driver molecular event in SAF (seen in 90% of cases) indicating relationship of SAF to the RB1-deleted tumor family.