Case report
We describe the case of a 37-year-old male kickboxer, who presented with a lump under the nail of his left great toe. The lesion had been rapidly growing for the previous 6 months and measured about 2.4×2.7 cm at presentation (Fig. 1A and B). The patient associated the onset of the lesion with kickboxing, but there had not been any trauma to the region. When he first noticed the mass, the patient did not experience any pain; however, he subsequently developed pain and as a result visited Kindai University Hospital (Osaka, Japan). Radiographic imaging showed an osteolytic lesion of the distal phalanx bone (Fig. 1C). Magnetic resonance imaging (MRI) revealed a T1 low, T2 high intensity lobular, cyst-like nodule under the nail (Fig. 1D and E). The MRI also showed a tumor infiltrating the distal phalanx bone. Surgical excision of the tumor with primary closure was performed, with the purpose performing a biopsy (Fig. 2A and B). The tumor was excised piece by piece with transverse skin incision. The nails, including the nail bed, were preserved. The tumor had partially invaded to the distal phalanx bone. The lesion had a firm consistency and was a light pink color (Fig. 2C). Histologically, spindle-shaped cells had proliferated in the myxocollagenous interstitium (Fig. 3A). No evidence of cellular atypia or mitotic figures was found. The immunohistochemical study revealed positivity for CD34 (sc-74499; dilution, 1:100) and negativity for S100 protein (sc-53438; dilution, 1:200), α-smooth muscle actin (sc-53142; dilution, 1:200) (all from Santa Cruz Biotechnology, Dallas, TX, USA), epithelial membrane antigen (M0613; dilution, 1:400; Dako Corporation, Carpinteria, CA, USA), and desmin (60226-1-Ig; dilution, 1:50; Cosmo Bio, Tokyo, Japan) in the neoplastic cells (Fig. 3B-F). The diagnosis was superficial acral fibromyxoma. At 24 months of follow-up, the patient had no symptoms or signs of recurrence. Radiography of the bone after treatment showed no erosive lesion.
Discussion
Superficial acral fibromyxoma is a very rare soft tissue tumor that was first described in 2001 by Fetsch et al (1). It is a slow-growing tumor (median duration of ~3 years) with a preponderance in men (male/female ratio of 2:1) and a mean age at presentation of 43 (range, 4–86) years (1,2). The most frequently affected sites are the subungual and periungual regions of the toes or digits (2). Previously, superficial acral fibromyxoma of the great toe had been described in 8 cases (Table I) (3–8). Superficial acral fibromyxoma rarely occurs on the volar surface of the digits (1). In the current case, the tumor occurred on the volar side of the great toe under the nail. It presented as a single, relatively rapid-growing, and generally painless mass. Previous trauma of the affected site has only been reported in a very small number of cases (1,9). Previous superficial acral fibromyxoma of the great toe has also been reported to be slow-growing (3–8). Only two cases have been described in which the patients experienced pain, and only one case involved a patient with a traumatic history (Table I). In the current case, the patient showed a painful rapid-growing tumor without a traumatic history. Despite the absence of trauma, it was suggested that chronic stimulation may have promoted tumor growth and lead to the rapid growth rate. In ~36% of superficial acral fibromyxoma cases, bone involvement may be present in the form of erosion (2), like in our case. Cases involving the great toe have shown no features of erosion or lytic bone lesions (Table I). We hypothesized that chronic stimulation may have promoted the invasion of the bone.
Simple resection is the basic treatment of superficial acral fibromyxoma (1,2). Although the recurrence rate is ~20%, no malignant transformation has been documented (1,2,9). All cases of superficial acral fibromyxoma of the toe that were treated with marginal resection showed no recurrence (Table I).
Macroscopically, superficial acral fibromyxoma is a non-capsulated, gelatinous or solid lesion with a size range of 0.6 to 5.0 cm. It is located in the dermis and can extend to the subcutaneous tissue. On histologic examination, it consists of a proliferation of fibroblast-like cells with a myxocollagenous matrix. Mild nuclear atypia and mitotic figures are infrequently seen. Most reported cases are immunohistochemically positive for CD34 and CD99 and negative for epithelial membrane antigen, although the staining for these markers is variable. No reactivity is observed for S100 (1,2,10). Superficial acral fibromyxomas of the great toe show varied immunohistochemical features (Table I). We finally reached a diagnosis with histological findings of hematoxylin and eosin staining and immunopositive findings of CD34.
The differential diagnosis of superficial acral fibromyxoma includes fibroma of the tendon sheath, myxoid neurofibroma, glomus tumor, giant cell tumor of the tendon sheath, sclerosing perineuroma, acral fibrokeratoma, cutaneous myxoma, myxoinflammatory acral fibroblastic sarcoma, fibrous histiocytoma, and dermatofibrosarcoma protuberans. In particular, superficial acral fibromyxoma should be considered in the differential diagnosis of glomus tumors when they develop under the nail. The histologic features seen on immunohistochemical analysis are used to distinguish superficial acral fibromyxoma from these other lesions (1,2,10).
There are some limitations in the current study. First, we could not detect CD99 in the specimen by immunohistochemistry. However, we reached a diagnosis with other histological observations including hematoxylin and eosin staining and immunopositive findings. Second, we did not conduct a biopsy before resecting the tumor. We should have treated the tumor carefully because the radiological findings were doubtful for malignancy.
In conclusion, we described a rapid-growing superficial acral fibromyxoma of a great toe with bone erosion. If a soft tissue tumor occurs under the nail, we should suspect superficial acral fibromyxoma and we also should keep in mind that such tumors can grow aggressively.