An asymptomatic hyperkeratotic tumor on the left great toe

A man in his 40s, employed as a soccer coach with prolonged periods of standing and activity, presented with a 4-year history of an asymptomatic skin tumor on the plantar first digit of the left foot. He had no relevant medical or family history and no known inciting factors, such as acute trauma or recent travel. Physical examination demonstrated a solitary hyperkeratotic skin tumor with a central crater on the plantar surface of the interphalangeal joints of the left great toe (Figure 1). An excisional biopsy was taken for histopathological examination. Histopathological examination revealed a hypocellular proliferation of bland fibroblastic cells in the lower dermis within a fibromyxoid stroma. Focal cartilaginous differentiation was identified but lacked a lobular arrangement (Figures 2, 3). Punctate foci of calcification were present, without evidence of osteoid or bone formation. A pseudocystic cavity was also observed. Immunohistochemistry demonstrated fibroblastic cells positive for SMA, and chondrocytes positive for S100, heterogeneously positive for SOX9, and focally positive for ERG. There was no significant expression of cytokeratin, EMA, MUC4, CD34, SOX10, or SATB2. Your diagnosis? … Diagnosis: Myxochondroid metaplasia of the plantar foot Myxochondroid metaplasia of the plantar foot (MMPF), first described by Shon and Folpe in 2013,1 is a rare reactive pseudoneoplastic lesion with histologic features resembling those of nuchal fibrocartilaginous pseudotumor and the equine digital cushion.2, 3 It typically arises on the plantar aspect of the foot, likely in response to chronic mechanical stress, and follows a benign clinical course. These lesions may occur at any age, show no sex predilection, and usually present as soft tissue masses associated with variable degrees of pain. Histologically, it shows a partially circumscribed, subepidermal proliferation of bland fibroblastic cells embedded in a fibromyxoid or chondromyxoid stroma. The cellularity is variable, with basophilic zones mimicking immature cartilage and areas resembling fibrocartilage with lacunae formation. Additional features include wiry collagen fibers, cystic degeneration, fat necrosis, hemosiderin deposition, and capillary proliferation. Immunohistochemistry typically reveals variable S100 and ERG positivity, especially in basophilic and cartilaginous areas. There is no osteoid and woven bone formation. Reported cases to date have been managed with surgical excision, with no documented recurrences, supporting its benign behavior and excellent prognosis.1-3 The differential diagnosis includes several acral soft-tissue tumors. Soft tissue chondroma is a benign cartilaginous neoplasm characterized by well-formed lobules of mature hyaline cartilage, often with focal calcification and osteoclast-like giant cells.4 Acral fibrochondromyxoid tumor presents as a well-circumscribed, multilobulated or unilobulated neoplasm composed of epithelioid cells within a chondromyxoid stroma, featuring central vascular cores, minimal cytologic atypia, and intervening thin fibrous septa.5 It consistently expresses CD34 and ERG, and is only rarely positive for S100. Fibro-osseous pseudotumor of the digits typically occurs in the proximal phalanx of the fingers in young women, and less commonly involves the toes.6 Histologically, it shows a combination of cartilaginous tissue, nodular fasciitis-like areas, and woven bone formation.6 The presence of bone production and a prominent myofibroblastic component helps differentiate it from MMPF. Superficial acral fibromyxoma or digital fibromyxoma are characterized by CD34-positive spindle to stellate cells, but neither shows cartilaginous differentiation.7, 8 Calcifying aponeurotic fibroma arises in the palms or soles of children and young males. It is poorly circumscribed and composed of infiltrative fibroblasts with variably calcified chondroid islands.9 In summary, myxochondroid metaplasia of the plantar foot is a rare but distinct benign reactive process that may clinically and histologically mimic other soft tissue neoplasms. Awareness of its characteristic acral location and key histologic features – including bland fibroblastic proliferation in a fibromyxoid to chondromyxoid stroma and focal, non-lobulated cartilaginous differentiation with minimal atypia – is critical for accurate diagnosis. None.