Discussion

Sweat gland malignant lesions are a rare entity in clinical practice. The reported incidence in the literature is around 0.005% of all tumor specimens resected surgically.¹ These malignant lesions can arise from 2 anatomically different types of sweat glands in the skin: apocrine or eccrine. The main difference between apocrine and eccrine sweat glands is that the secretions of the apocrine sweat glands are viscid, whereas the secretions of eccrine sweat glands are watery. Furthermore, apocrine sweat glands are always connected to hair follicles while eccrine sweat glands are not. Apocrine sweat glands are predominantly present in the axillary and perianal region, whereas eccrine sweat glands are present all over the skin. The breast (mammary gland) is also considered a modified apocrine sweat gland. Sweat gland carcinoma arises from skin appendages in the dermis.² Exposure to ultraviolet rays has been suggested as an etiological factor.³ Immune suppression has also been considered an etiological factor for development of eccrine carcinoma.⁴

Sweat gland carcinomas occur primarily in adults, with peak incidence in the fifth and sixth decades of life.^{5 - 7} The majority occur in the genital skin and perineum (34.5%), followed by the trunk (26.4%), head and neck (18.3%), and lower extremities (13.9%).^{5, 6, 8, 9} The reported incidence is the same in men and women.¹⁰ Tumors usually present as a solitary nodule or a plaque on the skin, and a history of rapid growth of the lesion suggests malignancy. These lesions have the tendency to infiltrate locally and also have regional nodal or sometimes distant metastasis.¹¹ Malignant sweat gland carcinoma of the breast is a rare site for a sweat gland malignancy, making it a very rare entity with few cases reported in the literature.

Histological evaluation, along with immunohistochemistry, is important in distinguishing these lesions from breast parenchymal malignant lesions. Microscopically they have the appearance of an adenocarcinoma with well-developed glandular lumina, showing characteristic evidence of “decapitation secretion.”¹² Tumor cells are PAS (periodic acid-Schiff)-positive due to glycogen granules; they are also diastase resistant.⁵ Other features include a glandular lumen, which may be narrow or slightly dilated with the cells of glandular and papillary structures being large, with a strongly eosinophilic cytoplasm containing hemosiderin.¹² Sudan stain for lipids may be either negative or positive with mucin often present in and around the lumen of the glandular structures.¹²

Because malignant sweat gland carcinoma of the breast is rare, no standard management guidelines exist. Surgery is the preferred upfront treatment for nonmetastatic operable lesions. For primary apocrine sweat gland carcinoma, wide local excision with regional lymph node dissection is considered for clinically node-positive disease.³ For clinically node-negative disease, prophylactic lymph node dissection remains controversial.³ Some studies have shown that prophylactic lymph node dissection has no influence on survival in these patients.^{12, 13} However, several case reports have found that sentinel lymph node biopsy is useful in these patients.^{14, 15} The local recurrence rate is high after surgery,¹⁶ and the role of postoperative adjuvant treatment remains unclear.³ Because weat gland carcinoma is generally considered resistant to chemotherapy, adjuvant chemotherapy is not routinely recommended.^{17, 18} However, in metastatic setting, chemotherapy has shown favorable responses.^{19, 20} Radiation therapy has a role in adjuvant setting as it reduces the risk of relapse.¹⁸ It has been suggested that adjuvant radiation therapy should be considered if the disease has 1 or more risk factors such as T size of 5 cm or more, positive margins, a moderate to poorly differentiated tumor, or LVSI.¹⁸ Prognostic factors for sweat gland carcinoma are difficult to identify owing to the small number of reported cases. The likely prognostic factors include tumor size, histological type, lymph node involvement, and distant metastasis.¹¹ A 10-year disease-free survival rate of 56% in the absence of lymph node metastasis is observed, which falls to 9% if nodes are involved.⁵ In the largest retrospective cohort study, the median overall survival and 5-year disease-specific survival rates were 51.5 months and 88%, respectively.²¹