Imaging Studies
MRI, CT, ultrasonography, and plain x-ray films are not diagnostic for spiradenoma, but they can be used to locate metastases in the case of malignant spiradenoma.
Spiradenoma should be considered in the differential diagnosis of a well‐circumscribed hypervascularized, hypoechoic subcutaneous tumor on ultrasonography. [33] It is usually located in the subcutaneous fat layer and has a well-defined margin. Appearance is typically hypoechoic, lobulated, and can be tractlike with a heterogeneous echo texture. Other features reported include internal hypoechoic foci and posterior acoustic enhancement with peripheral blood flow. [34]
Malignant spiradenomas appear on MRI as multiple dispersive foci with clear circumferences in the skin and subcutis. They demonstrate low-signal intensity on T1-weighted images and high-signal intensity on short-tau inversion recovery images. [35]
Procedures
Skin biopsy is necessary to establish the diagnosis of spiradenoma. Surgical excision is the treatment of choice.
Fine-needle cytology of a spiradenoma of the breast can be performed to aid in diagnosis. [36]
Histologic Findings
Spiradenomas consist of one or more large, sharply contrasted, basophilic nodules in the dermis. [15] The nodules of spiradenoma are unattached to the epidermis and sometimes extend into the subcutis. The nodules consist of groups of cells in cords, islands, and/or sheets. A trabecular arrangement of cells can be present. [37]
See the image below.
Spiradenoma. Vascular channels within a basaloid tumor. Courtesy of Sarahkayb (own work), via Wikimedia Commons.
The two types of cells in the nodules of spiradenoma are small, dark, basaloid cells with hyperchromatic nuclei and cells with large, pale, vesicular, and ovoid nuclei. Pale cells tend to be at the center of the lesions. [15]
Ductlike structures and lymphocytes can also be present at the center of the lesions. [38] Strands of cells are positive for cytokeratin, and the lumina are positive for carcinoembryonic antigen. [38] In contrast to these findings, the nodules of spiradenoma in infants display a less distinct two-cell-type pattern and ductule formation is rare. [39]
The fibrous capsule surrounding spiradenomas usually contains nerves, blood vessels, and ducts. The nerves often invest the tumor or penetrate interlobular septae of the tumor along with the small vessels. [15] One study describes a case of a painful spiradenoma that showed a disorganized nerve fiber encasing the spiradenoma nodules, which seemed to correlate with the intense pain the patient felt. [40] Close examination revealed profiles of cystoid spaces resulting from stromal invagination into the parenchyma.
Malignant spiradenoma is characterized by an increased mitotic rate, necrosis, nuclear atypia, pleomorphism, hyperchromasia, loss of nested and trabecular growth patterns, and an absence of the classic dual-cell population. [41] Two distinctive histologic patterns have been described in malignant spiradenoma, both with focal squamous differentiation . [41] The first is an abrupt transition between a benign spiradenoma and a high-grade carcinoma component with marked pleomorphism. The second is a gradual transition between benign spiradenoma and malignant spiradenoma. In the latter case, the malignant component is defined by an increased nuclear-to-cytoplasmic ratio, hyperchromasia, and marked mitotic activity. For both histological subtypes, areas of benign and malignant spiradenoma coexist. [41] In some cases, malignant spiradenoma demonstrates the typical features of a spiradenoma, with areas of adenocarcinoma, squamous cell carcinoma, and sarcoma. [42]
Electron microscopy
Electron microscopy of spiradenomas shows cellular sheets separated into lobules by strands of amorphous and fibrillar material. Translucent polygonal or round cells with mitochondria, small vesicles, and sometimes glycogen granules are the dominant population of cells in spiradenomas. Sparse, dark cells are also often noted. Lumina may be present, with microvilli on the lining cells. [43]
Immunohistochemistry
Although spiradenomas were historically thought to be of eccrine lineage, more recent studies based on immunohistochemistry suggest this is not the case. Early studies showed spiradenomas stain with antibodies to the eccrine marker IKH-4 [44] and are positive for S-100 protein. [38] Spiradenomas also stain with antibodies to epithelial mucin, which is a marker for eccrine and apocrine glands. [45] Spiradenomas and cylindromas also demonstrate cytokeratin staining patterns similar to eccrine and apocrine glands, including expression of keratins 7, 8, and 18. [38]
However, tubular cells in both spiradenomas and cylindromas express human milk fat globulin and lysozyme, two markers associated with apocrine rather than eccrine differentiation. [38]
In addition to these secretory gland markers, spiradenomas and cylindromas express the follicular stem cell marker CD200, differentiating them from tumors of eccrine lineage, which are CD200-negative. [1] Spiradenomas also co-express cytokeratin and smooth muscle actin, suggesting differentiation toward myoepithelial cells. [46] Thus, spiradenoma and cylindroma are thought to represent the least differentiated follicular tumors and have their basis in the hair follicle bulge rather than the eccrine sweat gland.
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Spiradenoma. Vascular channels within a basaloid tumor. Courtesy of Sarahkayb (own work), via Wikimedia Commons.
