Generally, there is no serum CEA threshold to differentiate benign and malignant conditions. CEA, which was originally speculated to occur exclusively in the fetal and neoplastic epithelial cells of the large bowel [
9], is a glycoprotein found in several organs. In the skin, CEA is present exclusively in the sweat gland [
6,
10‐
12]. Postoperatively, we confirmed that the high CEA level was due to apocrine hidrocystoma.
Apocrine hidrocystomas are histologically speculated to be benign cystic proliferations of apocrine glands. They are characterized by a double epithelial lining with decapitation secretion and an outer myoepithelial layer [
6‐
8]. In the present case, immunohistochemical analysis revealed that the luminal cells of the apocrine hidrocystomas were CEA positive. Kariniemi
et al. [
13] reported that staining for CEA was found in 32 (64%), including two hidrocystomas, of 50 benign sweat gland tumors and CEA was occasionally found also in proliferating cells. Furthermore, Tokura
et al. [
14] demonstrated that all five cases of apocrine hidrocystomas examined were consistently positive for CEA in the luminal cells. In addition, no variations in staining pattern were observed among those cases. Although the reason for CEA production by these cells is unknown, it is possible that CEA may play a role in the innate immune defense and bind and trap microorganisms at the cell surface [
15]. Therefore, CEA in the apocrine hidrocystoma may likewise play a similar role.
Apocrine hidrocystomas were first described by Mehregan in 1964 [
7]. They appear as well-defined, dome-shaped, clear, and cystic nodules with smooth surfaces. Their color varies from flesh color to blue-black. They are generally located on the face, most often on the eyelid. However, rarely they can be found on the trunk or on the limbs, as in the present case [
5,
16‐
18]. Their diameter ranges between 1 and 15 mm, and the giant type is a rare finding in the general population [
5,
6]. A literature review identified exclusively eight cases of “a giant apocrine hidrocystoma,” including the present case [
17‐
23]. Of the eight reported cases, seven were males and one was a female, with ages at presentation ranging between 29 and 70 years (mean, 52 years). The lesions were located as follows: four occurred on the face [
19‐
22], two on the trunk [
17,
18], one on the head [
23], and one on the limb in the present case. Sizes ranged between 1.8 and 7 cm in diameter. In addition, three lesions, including the present case, exceeded 5 cm in diameter [
17,
18]. Furthermore, in the seven cases, excluding the present one, serum CEA levels were not measured and tissue staining for CEA was not described. The present case is the first report of an apocrine hidrocystoma associated with high serum CEA levels. It is speculated that a large amount of CEA would be produced in cases of a giant apocrine hidrocystoma. Honma
et al. reported a case of acquired idiopathic generalized anhidrosis with an elevated serum CEA, in which immunohistochemical analysis revealed CEA expression in eccrine sweat glands, and suggested that the increased serum CEA can be derived from sweat glands [
24]. However, the correlation between lesion size and serum CEA level is unclear. This is due to the fact that previous reports have not examined serum CEA levels in patients with an apocrine hidrocystoma. We believe that the number of increasing cases may ascertain the correlation.