Nodular subcutaneous mass arising underneath a congenital nevus

A 14-year-old girl presented with a congenital nevus on the thigh, which had been present from birth and gradually increased in size. Recently, an indurated area underneath the nevus had developed. Physical examination revealed a homogeneous, brown, sharply demarcated hairy patch on the lateral left thigh (Figure 1a). Deep to this was a non-tender palpable mass without erythema or swelling of the surrounding tissue. Following the initial presentation, an ultrasound was performed; this demonstrated a well circumscribed, nodular hypoechoic, inhomogeneous mass (12.8–15.6 mm) in the deep dermis/hypodermis, with posterior enhancement (Figure 1b). Doppler ultrasonography revealed central and peripheral blood vessels of different calibers (Figure 2a, b). Elastography showed increased rigidity of the mass, with a strain ratio of 3.24, indicative of a potential malignant process (Figure 2c). The patient underwent en bloc excision of the tumor. Macroscopically, a firm, pigmented, well circumscribed and lobulated nodule was seen (Figure 3a, b). Histology revealed fascicles of spindle cells associated with blood vessels, stromal hemorrhage, nodular and diffuse lymphoplasmacytic infiltrate and hemosiderin deposits (Figure 3c). Immunohistochemistry showed diffuse CD68, desmin-positive and CD31-negative tumoral cells, S100 positivity in isolated dendritic cells as well as EMA-positive plasmocytes. Nodular subcutaneous mass arising underneath a congenital nevus Case for Diagnosis