Poroma

Poromas are rare tumors that in two large review studies represented 0.0058% and 0.134% of all skin tumors; dermal duct tumors are the rarest form of the poromas, representing only 3.3% of these tumors in 3 studies examining 675 poroid neoplasms.[2] They usually occur in the elderly population (mean age 65.1–66.6 in different studies) as small (<2 centimeters), solitary dome-shaped papules, plaques, or nodules, that are skin-colored, pink, red, white, or blue and range from smooth to wart-like, ulcerative,[4] or pyogenic granuloma-like lesions.[10] They may be located on the palms of the hands, soles of the feet, trunk, face, neck, or other cutaneous surfaces[4] such as the areola,[11] nipple,[12] or other areas of the breast,[13] on the scrotum,[14] or on the vulva.[15] Rarely, individuals present with multiple poromas either in one or widespread areas; these cases are termed poromatosis.[3]

Poromas present more commonly in: pregnancy;[3] patients treated with electron therapy for mycosis fungoides; sites of chronic radiation dermatitis caused by long-term radiation exposure;[4] patients who received chemotherapy with or with radiation therapy (these patients have often presented with poromatosis);[16] individuals with underlying skin conditions such as hypohidrotic ectodermal dysplasia and Bowen’s disease (i.e. a form of squamous cell carcinoma that is localized to the outermost layer of the skin); and skin plaques of individuals with the congenital disease, nevus sebaceous.[3]

Poromas are usually slow growing and asymptomatic but some individuals report that their lesions are itchy,[4] mildly tender, or painful.[9] An existing poroma that develops spontaneous bleeding, ulceration, sudden itching, pain, or rapid growth over a short period of time may indicate that it has become a cancerous porocarcinoma.[4] These cancers may metastasize to local lymph nodes, nearby or distal skin sites, bones, bladder, breast, retroperitoneum, ovary, liver, lung,[17] brain, or stomach. However, the progression of a poroma to a porocarcinoma is very rare,[18] i.e. less than 1% of cases.[9]

Dermoscopy and histopathology of eccrine poroma: (A) Clinical presentation of a nonpigmented eccrine poroma sensu stricto as a pink nodule located on the foot (B,C) Dermoscopy shows milky red areas (light blue arrow), milky red globules (dark blue triangle), and dotted vessels (black arrow) (original magnifications ×20 and ×40, respectively). (D,E) Histopathologically, the neoplasm consists of poroid and cuticular cells and tubular structures that are continuous with the epidermis (H&E stain, original magnifications ×10 and ×40, respectively).[19]

Microscopic histopathological examinations of the tumor tissues of all poroma variants stained with hematoxylin and eosin dyes reveal: a) basophilic "poroid cells" (i.e. small, cuboid-shaped cells with oval nuclei which resemble cells in the peripheral layer of the distal portion of eccrine sweat gland ducts[20]) that may form cords and broad columns extending downward from the epidermis; b) larger cuticular cells (i.e. squamous epithelial-like eosinophilic cells that resemble the luminal cells lining eccrine sweat gland ducts[20]); and in some cases c) clear cells (i.e. cells with small nuclei surrounded by pale cytoplasm).[3] Poroma tumor tissues may appear highly vascularized and/or have areas of necrosis, i.e. dead or dying cells. Hidroacanthoma simplex variants are mainly composed of poroid cells, few cuticular cells, and no clear cells and are confined to the epidermis; dermal ductal variants are mostly confined to the superficial dermis and are composed of small solid and cystic nodular aggregates of poroid, cuticular, and clear cells; poroid hidradenoma variants have large aggregates of solid and cystic components and extend deeper into the dermis or even subcutis; and eccrine poroma variants are composed of all three cell types but are primarily located in the epidermis and superficial dermis. Poromas may have 2 or more of these variants in the same tumor tissue and the variants typically have histopathology findings that are not clearly distinguishable from each other.[3]

• 
  Hidroacanthoma simplex
• 
  Eccrine poroma

The great diversity, rarity, and complex terminology of poromas make their diagnosis challenging. They have been misdiagnosed as other types of skin tumors including porocarcinomas, basal cell carcinomas, pyogenic granulomas, skin tags, plantar warts (i.e. warts on the palms or soles), fibromas, hemangiomas, pigmented moles, seborrheic keratosis, trichilemmomas, melanomas, Kaposi sarcomas, and other adnexal tumors. Poromas are typically diagnosed based on their clinical presentation; microscopic histopathology showing tumor tissues consisting of poroid, cuticular, and/or clear cells; and, in unclear cases, the presence of tumor cells that express a YAP1::NUTM1, YAP::MAML2, and/or WWTR1::NUTM1 fusion gene.[3][7] However, these fusion genes also occur in porocarcinomas and cannot be used to differentiate poromas from their malignant counterpart. Poromas and porocarcinomas are distinguished based on their clinical findings and histopathology, e.g. a higher Ki67 labeling index and aberrant expression of p53, RB, and p16 proteins are more frequent in porocarcinoma (see Diagnosis of porcarcinomas).[7] Dermatoscopy, particularly when revealing a "leaf- and flower-like pattern" in a skin tumor, has been used as a strong indicator that the lesion is a poroma, but confirmation of this diagnosis ultimately relies on histopathological analyses.[3]

There is no clear-cut evidence that diagnosing a poroma's variant type carries any clinical or therapeutic significance.[3]

• Porocarcinoma
• Hidradenoma