Superficial Acral Fibromyxoma: Clinicopathologic, 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, <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 males and 4 females aged 37 to 86years (median, 48). Mean size was 6mm (range, 4-20mm). 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 to 60months; median: 24months) 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 (IHC) showed expression of CD34 (9/10) and focal weak reactivity for EMA (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 9/10 cases (90%). All 7 cases with successful Rb1 FISH 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 IHC and FISH. 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.