A 40-year-old man presented with a nodule on the extensor surface of his left forearm. He couldn't remember when it formed. Initially, it showed no symptoms but with time, became painful. Physical examination revealed a slightly bluish to skin colored, hard mobile nodule on the left forearm measuring 1×1 cm in size (Fig. 1A). The initial clinical suspicion was epidermal cyst, trichilemmal cyst, or lipoma; thus, ultrasonography (US) was done. US results indicated a well-defined oval isoechoic mass in the subcutaneous fat layer, with moderate hyperemia (Fig. 1B). A complicated epidermoid cyst, or other hypervascular mass, such as vascularized leiomyoma or (least likely) a sarcoma (e.g., malignant fibrous histiocytoma) was suspected. Excisional biopsy was performed. Microscopic examination showed a deep, well-defined tumor with basophilic lobules, encapsulated by thick connective tissue in the dermis and subcutaneous layer. A trabecular arrangement of basophilic cells was present (Fig. 1C). Two types of cells (small, dark, basaloid cells with hyperchromatic nuclei, and cells with large, pale, ovoid nuclei) were seen (Fig. 1D). The diagnosis was confirmed as eccrine spiradenoma (ES). The tumor was completely excised.

ES is a rare benign soft tissue tumor first described by Kersting and Helwig (1956). It may occur at any age, but typically affects young adults aged 15~35 years, with no sexual predilection1. It presents as a solitary mass localized in the skin and subcutaneous tissue, smaller than 1 cm in size. Paroxysmal pain and tenderness may often present2. It tends to arise on the upper part of the body; head, neck, and trunk, especially ventral portion1. Rarely, it may be located on the upper and lower extremities, especially dorsal portion3. The summarization of the previously reported cases on extremities was described in Table 1. ES lesions are mostly solitary, but rarely multiple, linear, blaschkoid, or grouped. The pathogenesis is thought to be related to differentiation, mainly of the secretory portion of the secretory coil of eccrine sweat glands24. The presence of myoepithelial cells and phosphorylase, demonstrate the eccrine origin of the tumor. However, several evidences suggest its origin is apocrine rather than eccrine4. It is sometimes associated with trichoepithelioma and cylindroma, tumors of apocrine or folliculosebaceous-apocrine origin. US study revealed a well-demarcated mass with lobulated contours, which didn't have any relation with the epidermis and didn't extend into the neighboring muscular tissues. It was localized in the superficial part of the subcutaneous tissue35. When the mass shows well-defined hypoechogenicity, hypervascularity, no cystic portion, no hypoechoic scar, and when it is deeply located, an ES can be highly considered5. The essential histologic features include sharply demarcated nodules of basaloid cells in the dermis or in subcutaneous tissue (blue balls)5. Basaloid cells are composed of two types which might not be so apparent: one is paler, with more cytoplasm than in the darker cells1. Malignant change is rare2. However, in cases with incomplete surgical removal, a high risk of local recurrence has been reported25. Early and complete surgical excision is the diagnostic tool and curative treatment option for ES2.