Spindle cell squamous cell carcinoma arising in Bowen's disease: Case report and review of the published work

A 79‐year‐old Japanese woman presented with an ulcerated, brown‐red nodule in the center of a sharply demarcated, tan‐brown plaque situated on the left side of her right breast. Histologically, the plaque demonstrated an acanthosis with an intraepidermal epithelioma of Borst‐Jadassohn. Small oval nests of bland‐appearing basophilic cells in the periphery gradually enlarged into nests of various sizes and irregular shapes, composed of densely cohesive, atypical basophilic cells above the central nodule. The atypical keratinocytes shifted to atypical spindle cells beneath the acanthotic epidermis, penetrating deep into the subcutaneous tissue. In addition to vimentin and p63, the spindle cells were positive for several cytokeratin (CK) markers, including AE1/AE3, 34βE12 and CK5/6, which showed more intense signals closer to the epidermis. Basophilic cells in the clonal nests were positive for p63, AE1/AE3, 34βE12 and CK5/6. The MIB‐1 index was estimated at approximately 40–50% in both the bland‐appearing and the atypical basophilic cells. We describe the first case of spindle cell squamous cell carcinoma arising in an intraepidermal epithelioma expressed by clonal Bowen's disease, which was difficult to differentiate from clonal seborrheic keratosis.