Apocrine Hidrocystoma of the Axilla

Hidrocystomas are benign cystic lesions arising from apocrine or eccrine sweat glands, thought to result from cystic dilation secondary to ductal obstruction or degeneration.¹ First described by Mehregan in 1964,² the apocrine variant classically presents in adults aged 30 to 70 as a solitary, slow-growing, dome-shaped, translucent or bluish nodule in the periorbital region.³ Histologically, AHCs are unilocular or multilocular cysts lined by a double epithelial cell layer: outer myoepithelial cells and inner tall columnar or cuboidal cells exhibiting characteristic “decapitation” secretion.^1-3 This secretion results from apical portions of the secretory epithelial cells pinching off into the lumen and appears microscopically as cytoplasmic blebs at the luminal surface. 

Although generally indolent, AHCs can, depending on their clinical presentation, mimic various cutaneous and adnexal lesions, including eccrine hidrocystomas, epidermal inclusion cysts, mucoceles, syringomas, hemangiomas, melanocytic nevi, basal cell carcinomas, and, in rare cases, malignant melanomas.¹ Therefore, histopathology is often useful for diagnosis and management. Primary treatment involves simple excision with narrow margins, though alternatives include electrodessication,⁴ topical anticholinergics, trichloroacetic acid injections, carbon dioxide⁵ and pulsed-dye laser vaporization,6 and botulinum toxin A.^7-9

Here, we report a rare case of a 74-year-old female patient with an AHC in the axilla, an uncommon site for this cystic lesion, and review the current literature on axillary presentations. 

Case Presentation

A 74-year-old female patient with a history of melanoma in situ (MIS) and multiple nonmelanoma skin cancers presented in February 2025 for elliptical excision of the MIS on her right chest. During the visit, lymph node examination revealed a palpable, firm, non-tender nodule in the right axilla. The patient reported that the lesion had been present for nearly 10 years, causing only mild discomfort with arm movement. Given concern for possible metastatic melanoma, an ultrasound was ordered. It revealed a 0.9- by 1.4-cm, oval, anechoic lesion just beneath the dermis with through transmission, thin walls, no associated vascularity, and partial compressibility—findings consistent with a simple cyst. No axillary lymphadenopathy was noted. 

Figure 1. Low-power view showing a well-circumscribed, unilocular dermal cyst in the lower right corner (Hematoxylin & eosin; 20x objective). 

The patient and her Mohs surgeon elected to excise the lesion during a scheduled Mohs procedure in May for basal cell carcinoma. On removal, it appeared cystic and expressed dark red gelatinous material. Histopathology showed a well-circumscribed, unilocular cyst in the dermis (Figure 1), lined by a double layer of epithelial cells without significant cytologic atypia or mitotic activity (Figure 2). The inner layer exhibited prominent apical “decapitation” secretion, characteristic of an apocrine hidrocystoma (Figure 3). 

Figure 2. Medium-power view showing a double layer of epithelial cells lining the cystic lesion (Hematoxylin & eosin; 40x objective).

Figure 3. High-power view demonstrating apical ‘decapitation’ secretion (Hematoxylin & eosin; 100x objective).

Discussion

We report a rare case of a 74-year-old female patient with an axillary apocrine hidrocystoma. Before histologic evaluation, the leading differential was an epidermal inclusion cyst (EIC), based on the lesion’s axillary location, skin-colored appearance lacking the typical bluish or translucent hue of hidrocystomas, decade-long stability, and characteristic sonographic features.¹⁰ Expression of dark red gelatinous material at excision supported the possibility of a ruptured or hemorrhagic EIC. Differential diagnosis of a reactive axillary lymph node with cystic degeneration was also considered, though the lesion’s well-circumscribed, avascular appearance made this less likely.¹¹

Given the patient’s history of MIS on the right chest, the most concerning differential was metastatic melanoma to an axillary lymph node, as cystic or necrotic nodal metastases can mimic benign cysts clinically and radiographically. A retrospective review found that a small but notable subset of lesions clinically labeled as “cysts” were ultimately diagnosed as melanomas on histopathology.¹² In addition, reports of rare melanoma variants, such as follicular and nevoid subtypes, presenting as nodular or cyst-like lesions with benign-appearing features reinforced the importance of histologic confirmation.¹³

Histopathology in this case was ultimately consistent with an AHC, a lesion that, despite the axillae’s dense concentration of apocrine glands, is, for unknown reasons, rarely encountered outside the face and neck.1 Extra-facial AHCs have been reported in the nipple,¹⁴ genitalia,^15,16 conjunctiva,¹⁷ and nail bed,¹⁸ but remain uncommon. A 2005 review of 167 AHC cases from the Japanese literature identified only 7 (4.3%) in the axilla.¹⁹ To our knowledge, three adult cases of axillary AHCs^20-22 (Table) and one pediatric case²³ of multiple congenital axillary AHCs have been reported in the English-language literature. 

Obaidat and Ghazarian described bilateral axillary fullness and discomfort in a patient whose clinical examination and mammography were suggestive of accessory breast tissue; however, histopathology revealed multiple AHCs with apocrine hyperplasia.²⁰ Similarly, Shukla et al reported bilateral axillary AHCs with apocrine hyperplasia in a young woman, with ultrasound showing fibro-fatty glandular appearing tissue suggestive of accessory breast tissue.²¹ More recently, Choi et al described a patient with a 25-year history of progressive axillary tenderness, initially suspected to have hidradenitis suppurativa, but ultimately found to have bilateral AHCs with dense inflammatory infiltrates.²²

Our case adds to this limited body of literature on axillary AHCs. In contrast to prior reports of bilateral, symptomatic lesions in women aged 31 to 42, our patient was in her 70s and presented with a solitary, longstanding, largely asymptomatic lesion lacking inflammation or apocrine hyperplasia on histopathology. The variability in presentation, combined with the overall rarity of extra-facial AHCs, raises the possibility of underlying genetic or developmental differences by anatomic site. Notably, Goto et al recently reported a series of hidrocystoma-like tumors in extra-facial locations—the digits, clavicle, and lower leg—that harbored RET or ALK rearrangements, genetic alterations absent in the classical periorbital lesions used as controls.²⁴ These findings raise the possibility that distinct molecular pathways may underlie the site-specific distribution of AHCs, which warrants further investigation.

Conclusion

This case emphasizes the rarity of axillary AHCs and their potential to mimic more common cystic lesions. Although benign, their overlapping features with cystic melanoma variants reinforces the importance of histopathologic evaluation, particularly in patients with a melanoma history. In the future, comparative genomic analyses of facial and extra-facial AHCs may help clarify their pathogenesis and provide insight into site-specific molecular differences. n

1. Sarabi K, Khachemoune A. Hidrocystomas—a brief review. MedGenMed. 2006;8(3):57.

2. Mehregan AH. Apocrine cystadenoma: a clinicopathologic study with special reference to the pigmented variety. Arch Dermatol. 1964;90:274-279. doi:10.1001/archderm.1964.01600030024005

3. Koumaki D, Papadakis M, Lagoudaki E, et al. Apocrine and eccrine hidrocystomas: a clinicopathological study. Acta Dermatovenerol Alp Pannonica Adriat. 2021;30(2):53-56.

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13. Pampena R, Lai M, Lombardi M, et al. Clinical and dermoscopic features associated with difficult-to-recognize variants of cutaneous melanoma: a systematic review. JAMA Dermatol. 2020;156(4):430-439. doi:10.1001/jamadermatol.2019.4912

14. Tsai IJ, Chou CM, Huang SY, et al. Nipple apocrine hidrocystoma in a male adolescent: a rare lesion site. Asian J Surg. 2023;46(12):5872-5873. doi:10.1016/j.asjsur.2023.08.186

15. Bharti S, Bharti JN, Madhubala R, et al. Apocrine hidrocystoma: a rare benign cystic skin tumor at uncommon site. Int J Trichology. 2024;16(1-6):52-54. doi:10.4103/ijt.ijt_76_20

16. Matsuyama T, Yahagi E, Mabuchi T. Apocrine hidrocystoma on the genitalia of a 9-year-old girl. Pediatr Dermatol. 2018;35(4):e231-e232. doi:10.1111/pde.13485

17. Mirzania D, Jacobson A, McHugh J, et al. Conjunctival apocrine hidrocystoma: a case report and review of literature. Cornea. 2024;43(11):1431-1435. doi:10.1097/ICO.0000000000003598

18. Salah H, Duran J, Russell MA, et al. Apocrine hidrocystoma of the nail: a unique case. Am J Dermatopathol. 2024;46(7):433-435. doi:10.1097/DAD.0000000000002729

19. Anzai S, Goto M, Fujiwara S, et al. Apocrine hidrocystoma: a case report and analysis of 167 Japanese cases. Int J Dermatol. 2005;44(8):702-703. doi:10.1111/j.1365-4632.2005.02512.x

20. Obaidat NA, Ghazarian DM. Bilateral multiple axillary apocrine hidrocystomas associated with benign apocrine hyperplasia. J Clin Pathol. 2006;59(7):779. doi:10.1136/jcp.2005.033670

21. Shukla S, Patiri K, Pujani M, et al. Bilateral multiple axillary apocrine hidrocystomas accompanied by apocrine hyperplasia: a rare presentation. Indian J Pathol Microbiol. 2014;57(2):281-283. doi:10.4103/0377-4929.134712

22. Choi S, Lew BL, Kwon SH. A case of multiple apocrine hidrocystomas accompanied by inflammation on bilateral axillae. Ann Dermatol. 2024;36(1):62-63. doi:10.5021/ad.21.145

23. Housholder AL, Plotner AN, Sheth AP. Multiple axillary papules in an infant: diagnosis—multiple congenital apocrine hidrocystomas. Pediatr Dermatol. 2013;30(4):491-492. doi:10.1111/j.1525-1470.2012.01838.x

24. Goto K, Kervarrec T, Tallet A, et al. Hidrocystoma-like tumours with RET or ALK fusion: a study of four cases. Pathology. 2024;56(6):865-873. doi:10.1016/j.pathol.2024.04.008