Lymph Node Involvement by a Clear Cell Nodular Hidradenoma-Like Tumor of Uncertain Malignant Potential

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To the Editor:
Clear cell nodular hidradenoma (CCNH) is a subtype of nodular hidradenoma, which is a benign sweat gland neoplasm.1 CCNH derivation from apocrine or eccrine glands is disputed with some literature, suggesting that CCNH possesses either apocrine differentiation or eccrine differentiation or both eccrine and apocrine differentiation.2–6 Histomorphologically, CCNH is a tumor composed of clear and pale cells, squamoid cells, and mucinous cells, with cylindrical or cuboidal cells that line the tubules.7 Although CCNH is benign, several cases have been described involving the lymphatic system, and these cases have been labeled as benign metastasis or a tumor of uncertain malignant potential.8,9
Benign metastasis implies that normal tissues and benign tumors can propagate along vascular structures and move into lymph nodes and internal organs without changing the prognosis of the patient.9–13 These rare cases stand in opposition to the general idea that the metastatic spread of a tumor to the lymph nodes is a positive indication of malignancy.14 We report another very rare case of CCNH involving the lymphatic system in a 36-year-old woman who presented with a subcutaneous mass in her left groin.
A 36-year-old woman presented with a subcutaneous mass in her left groin. She had been treated 8 years ago for a CCNH in the same region. Microscopic examination revealed a subcutaneous, well-circumscribed, nodular tumor involving a lymph node, composed of sheets of epithelial cells with ductal differentiation and cystic change surrounded by lymphoid tissue. The tumor cells demonstrated abundant pale to clear cytoplasm and focal squamoid features, whereas ductal cells had eosinophilic cytoplasm (Fig. 1). Nuclei were round to oval and monomorphic with no atypia. An infiltrative growth pattern, spread to adjacent soft tissue, necrosis, nuclear pleomorphism, or mitoses were not identified. Immunohistochemistry demonstrated that the tumor cells were positive for p63, CK903, and CK7. Carcinoembryonic antigen (CEA) and Epithelial membrane antigen (EMA) highlighted ductal differentiation. Tumor cells were negative for CAIX, PAX-8, CD10, calponin, Thyroid transcription factor 1 (TTF-1), synaptophysin, CK20, CDX2, and D2-40. Ki67 (MIB-1/proliferative marker) showed a very low proliferative activity within the lesion.
Similar to the case series recently presented by Stefanato et al,9 our present case of a morphologically benign CCNH involving the lymphatic system is another example of a benign metastasizing tumor. A subset of benign tumors has also been described in the literature as being capable of involving distant sites. These include but are not limited to uterine leiomyoma,10 pleomorphic adenoma,11 cutaneous fibrous histiocytoma,12 and melanocytic tumors. The term “tumor of uncertain malignant potential” comes from examples of a similar phenomenon occurring in melanocytic tumors. These neoplasms are termed “melanocytic tumor of uncertain malignant potential—MELTUMP.”15
The malignant counterpart of CCNH is hidradenocarcinoma, a rare aggressive neoplasm with a high local recurrence rate and metastasizing potential.16 Recurrence is usually followed by metastasis to the lymph nodes, bone, or visceral organs. These tumors demonstrate cytologic atypia, nuclear pleomorphism, increased mitotic activity often with abnormal forms, a diffuse and infiltrative growth pattern, and variable necrosis, often with comedocarcinoma-like features. The prognosis is typically poor, and it is recommended that a wide and deep excision be performed.17 Although lymphatic spread was identified in our case, the diagnosis of CCNH-like tumor of uncertain malignant potential was made because there were no other morphologic features of malignancy, such as a lack of an infiltrative growth pattern, necrosis, nuclear pleomorphism, and ≥4 mitoses per 10 High-power field (HPF).18
Benign metastasis does not necessitate malignancy, a fact that can be observed in benign inclusions in lymph nodes.9 This phenomenon has been found in multiple anatomical sites, including the abdomen, the pelvis, and the neck.
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