Hidradenoma Papilliferum (Intrapapillary Hidradenoma)

Also called hidradenoma papilliferum, papillary hidradenoma, or intrapapillary hidradenoma, this is a benign adnexal adenoma with apocrine/mammary-like differentiation.

Definition and Origin

Hidradenoma papilliferum is an adenoma arising from anogenital mammary-like glands (MLAGs) - specialized glands located in the vulvar and perianal skin that share morphological and immunohistochemical features with breast ductal tissue. This explains its close histological resemblance to mammary intraductal papilloma, which is essentially its breast counterpart.

Robbins Pathologic Basis of Disease, p. 919
Fitzpatrick's Dermatology, p. 1865

Epidemiology and Clinical Features

Occurs almost exclusively in females, in the anogenital region (vulva, labia majora, interlabial folds, perianal skin)
Rare ectopic cases have been reported on the scalp
Presents as a firm, flesh-colored to red papule or nodule, typically < 1 cm in diameter
Covered by normal-appearing skin
May occasionally ulcerate, which can raise concern for carcinoma clinically, but is benign
Usually solitary

Pathogenesis / Molecular Features

Classified as an adenoma of MLAGs, morphologically and molecularly similar to breast ductal papillomas
Activating mutations in PIK3CA (PI3K pathway) and AKT1 identified - the same mutations found in mammary ductal papillomas, reinforcing the shared biology between these entities
Fitzpatrick's Dermatology, p. 1865

Histopathology

The histology is characteristic:

Architectural features:

Well-circumscribed, unencapsulated, solid-cystic dermal nodule - no connection to the overlying epidermis (rare exception)
Complex pattern of interconnected tubules and glands in a labyrinth-like arrangement
Prominent papillary projections extending into cyst-like cavities lined by "villi"
The tumour lies wholly within the dermis

Cytology:

Bilayered epithelium - the hallmark:
- Luminal layer: cuboidal to columnar secretory cells, with decapitation (apocrine) secretion
- Outer layer: flat myoepithelial cells
Oxyphilic (oncocytic) metaplasia is not uncommon
Mitotic figures may be seen; proliferative index is variable
Adjacent remnants of MLAGs in ~1/3 of cases

Papillary hidradenoma of the vulva - well-circumscribed dermal nodule with complex glandular architecture (Robbins Pathology)

Fig. 22.9 (Robbins) - Papillary hidradenoma of the vulva: well-circumscribed tumor with benign papillary projections covered by columnar secretory epithelium and underlying myoepithelial cells.

Hidradenoma papilliferum histology - A: low power showing well-circumscribed solid-cystic nodule with labyrinthine glands; B: high power showing bilayered epithelium with decapitation secretion and basal myoepithelial cells (Fitzpatrick's Dermatology)

Fig. 109-10 (Fitzpatrick's) - A: well-circumscribed solid-cystic nodule with interconnected glands in a labyrinth-like arrangement; B: bilayered projections with columnar/cuboidal cells showing decapitation secretion and basal myoepithelial layer.

Immunohistochemistry

Cell type	Markers
Luminal epithelial cells	CK5/6, AE1/AE3, GCDFP-15, Estrogen receptors
Myoepithelial cells	S100, smooth muscle actin (SMA), calponin, p63

The myoepithelial layer is a key diagnostic and prognostic marker - its loss signals malignant transformation.

Differential Diagnosis

Condition	Distinguishing feature
Epidermal/dermoid cyst	No papillary glandular architecture
Syringocystadenoma papilliferum	Connection to overlying epidermis; plasma cell-rich stroma; apocrine not MLAG origin
Intraductal papilloma of breast	Identical histology but intramammary location
Carcinoma (clinical DDx)	Ulceration mimics malignancy, but histology is benign; absence of true invasion

Fitzpatrick's Dermatology, p. 1865
Andrews' Diseases of the Skin, p. 769

Clinical Course, Prognosis, and Management

Benign in the vast majority of cases
Rare reports of ductal carcinoma in situ arising within hidradenoma papilliferum (manifested by increased cellularity, nuclear atypia, abnormal mitoses)
Adenocarcinoma (hidradenocarcinoma papilliferum) arising in this lesion is a controversial but recognized diagnosis
Loss of the myoepithelial cell layer suggests malignant transformation
Treatment: Excisional biopsy - both diagnostic and curative
Andrews' Diseases of the Skin, p. 769; Fitzpatrick's Dermatology, p. 1865

Summary Table

Feature	Detail
Nature	Benign adnexal adenoma
Origin	Anogenital mammary-like glands (MLAGs)
Location	Vulva/labia majora (almost exclusively female, anogenital)
Size	Usually < 1 cm
Architecture	Well-circumscribed, intradermal, labyrinthine glands with papillae
Epithelium	Bilayered: luminal columnar cells + myoepithelial layer
IHC	GCDFP-15+, ER+; myoepithelium: S100, p63, calponin+
Molecular	PIK3CA and AKT1 mutations
Breast analogue	Intraductal papilloma
Treatment	Excisional biopsy
Malignant risk	Rare (DCIS or adenocarcinoma in situ reported)

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