Superficial Acral Fibromyxoma and Other Slow-Growing Tumors in Acral Areas

The standard treatment of SAFM is complete local resection of the tumor, though some patients have been treated with partial excision or biopsy and partial or complete digital amputation.¹ Local recurrence may occur in up to 20% of cases; however, approximately two-thirds of the reported recurrences in the literature occurred after incomplete tumor excision.^1,2 It may be more appropriate to consider these cases as persistent rather than recurrent tumors. Superficial acral fibromyxoma is considered a benign tumor, with no known cases of metastases.⁴

Figure 5. A firm flesh-colored papule on the hyponychium of the right thumb prior to biopsy. Figure 6. Intradermal nodular proliferation of bland spindle cells arranged in loose fascicles and bundles and embedded in a myxoid stroma (H&E, original magnification ×40).

A broad differential diagnosis exists for SAFM and it can be difficult to differentiate it from a wide variety of benign and malignant tumors that may be seen on the nail unit and distal extremities (Table). Myxoid neurofibromas typically present as solitary lesions on the hands and feet. Similar to SAFM, myxoid neurofibromas are unencapsulated dermal tumors composed of spindle-shaped cells in which mast cells often are conspicuous.^2,7 However, tumor cells in myxoid neurofibromas are S-100 positive, and the lesions typically do not show vasculature accentuation.^4,7

Sclerosing perineuriomas are benign fibrous tumors of the fingers and palms. Histopathologically, bland spindle cells arranged in fascicles and whorls are observed in a hyalinized collagen matrix.⁸ Immunohistochemically, sclerosing perineuriomas are positive for EMA and negative for S-100, but unlike SAFM, these tumors usually are CD34 negative.⁸

Superficial angiomyxomas typically are located on the head and neck but also may be found in other locations such as the trunk. They present as cutaneous papules or polypoid lesions. Histopathologically, superficial angiomyxomas are poorly circumscribed with a lobular pattern. Spindle-shaped fibroblasts exist in a myxoid matrix with neutrophils and thin-walled capillaries. The fibroblasts are variably positive for CD34 but also are S-100 positive.^1,9

Myxoid dermatofibrosarcoma protuberans is a rare, locally aggressive, mesenchymal tumor of the skin and subcutis² that typically presents on the trunk, proximal extremities, or head and neck; occurrence on the fingers or toes is exceedingly rare.^2,10 Histopathologically, a myxoid stroma contains sheets of bland spindle-shaped cells with minimal to no atypia, sometimes arranged in a storiform pattern. The tumor characteristically invades deeply into the subcutaneous tissues. CD34 is characteristically positive and S-100 is negative.^2,10

Low-grade myxofibrosarcoma is a soft tissue sarcoma easily confused with other spindle cell tumors. It is one of the most common sarcomas in adults but rarely arises in acral areas.² It is characterized by a nodular growth pattern with marked nuclear atypia and perivascular clustering of tumor cells. CD34 staining may be positive in some cases.¹¹

Similar to SAFM, myxoinflammatory fibroblastic sarcoma has a predilection for the extremities.⁴ However, it typically presents as a subcutaneous mass and has no documented tendency for nail bed involvement. Also unlike SAFM, it has a remarkable inflammatory infiltrate and characteristic virocyte or Reed-Sternberg cells.¹²

Acquired digital fibrokeratomas are benign neoplasms that occur on fingers and toes; the classic clinical presentation is a solitary smooth nodule or dome, often with a characteristic projecting configuration and horn shape.¹ Histopathologically, these tumors are paucicellular with thick, vertically oriented, interwoven collagen bundles; cells may be positive for CD34 but are negative for EMA.^1,13 Related to acquired digital fibrokeratomas are Koenen tumors, which share a similar histology but are distinguished by their clinical characteristics. For example, Koenen tumors tend to be multifocal and are strongly associated with tuberous sclerosis. These tumors also have a tendency to recur.¹

Conclusion

Our report of 3 typical cases of SAFM highlights the need to keep this increasingly recognized and well-defined clinicopathological entity in the differential for slow-growing tumors in acral locations, particularly those in the periungual and subungual regions.