Case Report A New Case of Hybrid Epidermoid and Apocrine Cyst Fulvia Serra 1,* and Gürkan Kaya 1,2,* 1 Department of Pathology, University Hospital of Geneva, 1205 Geneva, Switzerland 2 Department of Dermatology, University Hospital of Geneva, 1205 Geneva, Switzerland * Correspondence: [email protected] (F.S.); [email protected] (G.K.) Abstract: We described a new case of a hybrid epidermoid and apocrine cyst, known to be a rare histopathological entity. The cyst was located in the axillary region and completely excised, without complication. The diagnosis was made at the histological analysis, where we found a cystic lesion in the dermis, lined with both epidermoid and apocrine epithelium. Keywords: cutaneous cysts; hidrocystoma; hybrid epidermoid; apocrine cyst 1. Introduction Hybrid cutaneous cysts are little known lesions that can be overlooked by the patholo- gist. Inside this small and curious family, the hybrid epidermoid and apocrine cyst is rare and its origin and clinical significance are unknown. Here, we present a new case found in the axillary region in a middle-aged woman with no other known comorbidity. We discuss the histopathological findings, the hypothesis of its origin and we underline the difficulties that can be encountered in reporting this peculiar entity. 2. Case Report A 45-year-old woman with a previous history of two cysts in the right axillary region Citation: Serra, F.; Kaya, G. A New (never excised), admitted to surgically remove one of the two, which measured 0.95 cm. To Case of Hybrid Epidermoid and our knowledge, she does not have any prior disease. The cyst was removed without any Apocrine Cyst. Dermatopathology 2021, complication and sent for analysis with the diagnosis of epidermoid cyst. The healing was 8, 442–445. https://doi.org/10.3390/ uneventful. The histopathological analysis reveal a cystic lesion in the dermis (Figure1). dermatopathology8030046 The cyst is lined in part by a squamous epithelium with an intact granular layer, sometimes atrophic, and in part by a distended apocrine epithelium. The passage between the two Received: 28 June 2021 different epithelia is abrupt. We estimate that the apocrine component represents 30% of Accepted: 19 August 2021 the epithelium. The cyst is filled with keratin lamellae. In proximity of the cyst’s wall, there Published: 1 September 2021 are some apocrine and some sebaceous glands. No further exam was performed. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Dermatopathology 2021, 8, 442–445. https://doi.org/10.3390/dermatopathology8030046 https://www.mdpi.com/journal/dermatopathology DermatopathologyDermatopathology 20212021,, 88, FOR PEER REVIEW 4432 FigureFigure 1.1. Histology.Histology. ((AA)) HEHE× ×10.10. The wholewhole cyst.cyst. ((BB)) HEHE MagnificationMagnification ××2020 (inset).(inset). Transition betweenbetween epidermoidepidermoid andand apocrine epithelium. (C). HE ×40 Magnification. The apocrine epithelium. apocrine epithelium. (C). HE ×40 Magnification. The apocrine epithelium. 3. Discussion In dermatopathology dermatopathology the the cutaneous cutaneous cysts cysts are are well well known known entities, entities, but have but have been beenclas- classifiedsified in different in different ways ways by bymany many authors. authors. In In the the literature, literature, a a simplified simplified classification classification of cutaneous cysts, according to their origin in the folliculosebaceous u unitnit and sweat glands, is proposed proposed by by Kaya Kaya and and Saurat Saurat [1]. [1 ].With With this this classification classification we wecan can easily easily see that see thatan epi- an epidermaldermal cyst cyst will will have have an aninfundibular infundibular origin, origin, a tricholemmal a tricholemmal cyst cyst an an isthmic isthmic origin, origin, that that a asteatocystoma steatocystoma arise arise from from a a sebaceous sebaceous duct, duct, an eccrine/apocrineeccrine/apocrine hidrocystoma hidrocystoma from thethe glandular epithelial wall, and so on. We findfind this classificationclassification to be a useful tool also to discuss the hybrid cysts: once identified identified the different epithelia, we can easily refer to their origin. A hybrid cyst is in fact simply the combination of different types of epithelia found in the folliculosebaceous unitunit andand sweatsweat glands.glands. TheThe conceptconcept isis introducedintroduced inin aa 19911991 articlearticle on follicular follicular hybrid hybrid cysts, cysts, by by Requena Requena et etal., al., that that defines defines the thehybrid hybrid cyst cystas an as entity an entity aris- arisinging from from one one component component of the of thefolliculosebaceous folliculosebaceous unit unit that that can can combine combine with with others others to toform form a large a large variety variety of follicular of follicular hybrid hybrid cysts, cysts, that that show show a distinctive a distinctive two two-component-component dif- differentiation,ferentiation, with with an an abrupt abrupt transition transition between between them them [2]. [2]. It It is is in in this this peculiar peculiar family thatthat we findfind thethe hybridhybrid epidermoid epidermoid and and apocrine apocrine cyst, cyst, a combinationa combination of epidermoidof epidermoid epithelium epithe- withlium awith hidrocystoma. a hidrocystoma. The epidermoid cyst is a well-knownwell-known and common entity that is usually found on the face, neck, trunk and perinealperineal area. The cyst is linedlined byby aa squamoussquamous cellcell epitheliumepithelium thatthat can be normal, atrophicatrophic oror hyperplastic,hyperplastic, but but always always with with a a well-defined well-defined and and intact intact granular granu- layer.lar layer. It is It filled is filled with with keratin keratin lamellae. lamellae. The apocrine hidrocystoma is uncommon, it is rarely found in the axillary region (it usuallyusually arisesarises inin the the head head and and neck neck area) area) and and is generallyis generally a standalone a standalone lesion. lesion. Histologically, Histologi- itcally, appears it appears as a cystic as a cystic lesion, lesion, lined lined by a by double a double layer layer of epithelial of epithelial cells, cells, the the outer outer layer layer of flattenedof flattened myoepithelial myoepithelial cells cells and and the the inner inner layer layer of cuboidof cuboid columnar columnar cells, cells, with with round round or ovalor oval nuclei nuclei and and decapitation decapitation secretion secretion usually usually present. present. Cyst Cyst distention distention can cause can cause flattening flat- andtening atrophy and atrophy of the epithelium, of the epithelium, therefore therefore sometimes sometimes it can be it difficult can be todifficult distinguish to distinguish between anbetween apocrine an apocrine and an eccrine and an hidrocystoma.eccrine hidrocystoma. Since the Since apocrine the apocrine and eccrine and eccrine origins origins come fromcome twofrom ductal two ductal systems systems that are that thought are thought to be identical, to be identical, some authors some haveauthors proposed have pro- the alternativeposed the alternative term of “ductal term of hidrocystoma”, “ductal hidrocystoma”, to reflect the to reflect possible the dual possible histogenesis dual histogen- [3]. esis [To3]. our knowledge, there are only few documented cases of hybrid epidermoid and apocrineTo our cyst knowledge, [4,5]. In a casethere report, are only Andersen few docum et al.,ented describe cases fourof hybrid cysts inepidermoid four different and patients:apocrine cyst one [4,5 on the]. In eyelid,a case report, one on Andersen the lip and et twoal., describe on the nipple,four cysts measured in four betweendifferent 0.5patients: and 0.6one cm on [the4]. eyelid, The histopathological one on the lip and analysis two on shows the nipple, the cysts measured lined withbetween foci 0.5 of Dermatopathology 2021, 8 444 cuboidal eosinophilic cells and decapitation secretion, alternated with a stratified squamous epithelium with an intact granular layer immediately adjacent. The luminas are filled with keratin lamellae. Regarding the location, in a previous case report, Bourlond et al. [6] described two cases of erosive adenomatosis of the nipple, suggesting that an apocrine differentiation in a nipple’s cyst is not that uncommon as thought to be. Despite this argument, the more common location of the hybrid cysts with follicular and apocrine differentiation seems to for now to be the eyelid, like can sometimes happen in Moll gland cyst [3] and as described by Milman et al. [5]. It is noteworthy that our case is the first hybrid epidermoid and apocrine cyst to be found in the axillary region. We do not find this to be a surprising location for such a lesion, being aware of the abundance of apocrine glands in this area. That said, we have to consider that maybe the hybrid epidermoid and apocrine cyst is underdiagnosed: especially when the apocrine component is present in less than 50% of the epithelium, it can be missed depending on the embedding and cutting level. As for the pathogenesis, it is still unknown. It is possible that this peculiar entity arises from