Superficial Acral Fibromyxoma/ Digital Fibromyxoma
Histopathology:
- Variably cellular lobulated neoplasm
- Random/haphazard distribution of bland spindle to stellate shaped fibroblastic cells, in a myxoid to loose fibro-collagenous stroma with numerous slender thin wall vessels.
- Mild cytologic atypia is seen in focal areas.
- Multinucleated stromal cells (green circle) and mast cells (blue arrow) are seen.
- Some areas show an increased amount of stromal collagen.
- Mitotic figures are very low. Atypical mitoses are not seen.
- Storiform or fascicular growth patterns are not seen in this case.
- The tumor border is circumscribed and smooth, without an irregular infiltrative pattern.
Practical Points of Pathoclinics:
- It is a benign fibroblastic neoplasm with myxoid changes, affecting patients with a wide age group with slight male preponderance.
- Most patients usually presented with a solitary slow-growing nodule on fingers or toes with dermal and subcutaneous location (subungual or periungual areas-nail bed region). These clinical findings are important for making a diagnosis. Non-acral sites are very rarely affected. It may extend to the fat, muscle and bone on rare occasions.
- Grossly, acral fibromyxoma is a small size (1 cm to 3 cm commonly) dome shaped/polypoid well-circumscribed uncapsulated nodule with a firm or gelatinous, white to-gray cut surface.
- More cellular examples of acral fibromyxoma overlap with cellular digital fibroma. Both are CD34 positive.Cellular digital fibroma is morecellular tumor with uniform spindled cells arranged in a short intersecting fascicle in a parallel or haphazard fashion in the upper reticular dermis. Prominent myxoid stroma and vascular pattern of acral fibromyxoma are not seen in cellular digital fibroma.
- Strong and diffuse expression of CD34, along with variable positivity for CD99, CD10 and EMA are useful to arrive at the diagnosis. The cells of acral fibromyxoma are negative for smooth muscle actin, desmin, S-100 protein, MUC4, claudin-1, HMB-45, and keratins.
- It has a consistent loss of Rb1 immunoexpression (Deletions of 13q12). Other tumors with loos of RB1 are spindle cell/ pleomorphic lipoma, mammary-type myofibroblastoma, and cellular angiofibroma.
- Any mild to moderately cellular myxoid-rich spindle cell neoplasm falls in the differential of acral fibromyxoma. The most useful diagnostic features are clinical presentation.
- Superficial acral fibromyxoma is best treated by complete surgical excision with negative margins. Overall outcome is excellent. Recurrence has been reported with incomplete excision. Malignant transformations are not reported.
| Differentiating points | Acral fibromyxoma | Dermatofibrosarcoma protuberans |
| Site | Classic locations are fingers or toes; subungual or periungual areas-nail bed region. | Uncommon at fingers or toes. Extension into subcutaneous tissue is common. |
| Histopathology | Variably cellular lobulated neoplasm, with even/random distribution of bland spindle cells to stellate-shaped fibroblastic cells in myxoid to fibro-collagenous stroma with numerous slender, thin wall vessels. Multinucleated stromal cells and mast cells are seen. Acral fibromyxoma can show nodular, lobular, or irregular/infiltrative growth pattern with superficial involvement of subcutis and fat. Strong and diffuse CD34 staining. | Relatively more cellular neoplasm, with monotonous storiform growth pattern. Infiltration into the subcutis and fat is common. In a small and superficial biopsy with myxoid-rich thin-walled blood vessel areas, along with strong and diffuse CD34 staining, mimic superficial acral fibromyxoma. In such instances, the location is important. |
| Molecular | Rb1 loss | t(17;22) with COL1A1–PDGFB fusion |
| Differentiating points | Acral fibromyxoma | Dermatofibroma (Benign fibrous histiocytoma) |
| Site | Classic locations are fingers or toes; subungual or periungual areas-nail bed region. | Distal extremities with cutaneous location |
| Histopathology | Variably cellular lobulated neoplasm, with even/random distribution of bland spindle cells to stellate shaped fibroblastic cells in myxoid to fibro-collagenous stroma with numerous slender thin wall vessels. Multinucleated stromal cells and mast cells are seen. Acral fibromyxoma can show nodular, lobular, or irregular/infiltrative growth pattern with superficial involvement of subcutis and fat. Strong and diffuse CD34 staining. | Circumscribed or but may be uncapsulated. Variably cellular to sclerotic lesion. Inflammatory cells, histiocytes, Touton giant cells, and lipidized siderophages are seen. Variable atypia and mitoses are seen. Prominent myxoid stroma is uncommon. Entrapment of dermal collagen is the typical finding. CD34 is negative. |
| Differentiating points | Acral fibromyxoma | Low-Grade Fibromyxoid Sarcoma |
| Site | Classic locations are fingers or toes; subungual or periungual areas-nail bed region. | Younger age group, with slow growing deep situated mass at proximal extremities and trunk region |
| Histopathology | Variably cellular lobulated neoplasm, with even distributed bland spindle cells to stellate shaped fibroblastic cells in myxoid to fibro-collagenous stroma with numerous slender thin wall vessels. Multinucleated stromal cells and mast cells are seen. Strong and diffuse CD34 staining. MUC4 negative. | Very bland looking fibroblastic spindle cells, in a fascicular and whorling pattern, with myxoid collagenous stroma and prominent thin-walled vasculature. Strong diffuse MUC4 cytoplasmic expression |
| Molecular | Rb1 loss | t(7;16) with FUS–CREB3L2 |
| Differentiating points | Acral fibromyxoma | Neurofibroma |
| Site | Classic locations are fingers or toes; subungual or periungual areas-nail bed region. | Solitary/multiple superficial located nodules |
| Histopathology | Variably cellular lobulated neoplasm, with even distributed bland spindle cells to stellate shaped fibroblastic cells in myxoid to fibro-collagenous stroma with numerous slender, thin wall vessels. Multinucleated stromal cells and mast cells are seen. Strong and diffuse CD34 staining. S100 protein is negative. | Wavy spindle nuclei with pointed ends of schwann cells in a loose fascicular or storiform pattern. Shredded-carrot like collagen, Wagner-Meissner corpuscles, Pacinian corpuscles and mast cells are commonly seen. Loose fascicular or storiform patterns are seen. Strong S100 protein staining highlights the schwann cells Strong CD34 staining. |
| Molecular | Rb1 loss | Neurofibromatosis type 1 |
| Differentiating points | Acral fibromyxoma | Superficial angiomyxoma |
| Site | Classic locations are fingers or toes; subungual or periungual areas-nail bed region. | Head and neck, and trunk are the common sites. |
| Histopathology | Variably cellular lobulated neoplasm, with even distributed bland spindle cells to stellate shaped fibroblastic cells in myxoid to fibro-collagenous stroma with numerous slender thin wall vessels. Multinucleated stromal cells and mast cells are seen. Strong and diffuse CD34 staining. Variable positivity for CD99 and EMA | Lobulated growth pattern, abundant myxoid stroma, slit-like blood vessles, and neutrophilic infiltrate around blood vessels. Positive for CD34, but EMA and CD99 are negative. |
| Differentiating points | Acral fibromyxoma | Perineurioma |
| Site | Classic locations are fingers or toes; subungual or periungual areas-nail bed region. | Can occurs on the fingers and palms of young adults. Subcutaneous and dermal location. |
| Histopathology | Variably cellular lobulated neoplasm, with even distributed bland spindle cells to stellate shaped fibroblastic cells in myxoid to fibro-collagenous stroma with numerous slender thin wall vessels. Multinucleated stromal cells and mast cells are seen. Strong and diffuse CD34 staining. Variable EMA positive. Claudin-1 is negative. | Well circumscribed tumor with myxoid and collagenous stroma. Cells are arranged in storiform, onion-skin/whorled patterns, and lamellar growth patterns. Claudin-1 positive. EMA and CD34 are also positive. |
| Differentiating points | Acral fibromyxoma | Soft tissue angiofibroma |
| Site | Classic locations are fingers or toes; subungual or periungual areas-nail bed region. | Large size mass. Acral sites are uncommonly involved. |
| Histopathology | Variably cellular lobulated neoplasm, with even distributed bland spindle cells to stellate shaped fibroblastic cells in myxoid to fibro-collagenous stroma with numerous slender thin wall vessels. Multinucleated stromal cells and mast cells are seen. Strong and diffuse CD34 staining. Variable positivity for EMA. | Many overlapping histologic and immunohistochemical features with acral fibromyxoma. CD34 positive Variable positivity for EMA. |
| Molecular | Rb1 loss | AHRR-NCOA2 gene fusion. |
References:
- Crepaldi BE, Soares RD, Silveira FD, Taira RI, Hirakawa CK, Matsumoto MH. Superficial Acral Fibromyxoma: Literature Review. Rev Bras Ortop (Sao Paulo). 2019 Sep;54(5):491-496. doi: 10.1016/j.rbo.2017.10.011. Epub 2019 Oct 29. PMID: 31736517; PMCID: PMC6856000.
- Ashby-Richardson H, Rogers GS, Stadecker MJ. Superficial acral fibromyxoma: an overview. Arch Pathol Lab Med. 2011 Aug;135(8):1064-6. doi: 10.5858/2009-0684-RSR1. PMID: 21810002.