Firm Round Papule on Foot

Poromas are classified as benign glandular adnexal tumors historically thought to be derived from the terminal duct of the sweat gland. However, cases with apocrine, sebaceous, and follicular differentiation have been reported.1 The first report of a tumor of eccrine...

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Poromas are classified as benign glandular adnexal tumors historically thought to be derived from the terminal duct of the sweat gland. However, cases with apocrine, sebaceous, and follicular differentiation have been reported.¹ The first report of a tumor of eccrine origin was in 1956 when Pinkus et al elucidated poroma and its terminal ductal differentiation.² Early reports used the term acrospiroma when describing this tumor.

Some experts view poromas as a broad category of neoplasms encompassing nodular hidradenomas, clear cell hidradenomas, hidradenoma simplex, dermal duct tumors, and hidradenomas. After several years, a poroma is at risk of progressing to the malignant form, a porocarcinoma.¹ An estimated 18% of poromas transform to porocarcinomas with a mean progression to malignancy of 8.5 years.³

Tumors developing from eccrine and apocrine glands represent approximately 1% of primary cutaneous lesions. Because poromas comprise approximately 10% of eccrine and apocrine gland tumors, they are considered relatively rare. Age at onset is typically middle-aged and older without preference for race or sex.³ Although it is commonly believed that poromas arise most frequently found on hairless acral surfaces, a report of 353 patients found 30% occurred on the face, 15% on the feet, 14% on the trunk, 10% on the scalp, and 5% on the hands.⁴

Risk factors for poroma development include long-term radiation exposure; subsequent poroma occurrence in patients receiving electron beam therapy for mycosis fungoides and in areas of chronic radiation dermatitis have been reported. Risk for poroma development is also heightened in certain skin diseases including hypohidrotic ectodermal dysplasia, Bowen disease, and secondarily within a nevus sebaceous.³

The exact etiology of poromas remains unclear.³ However, the tumorigenesis of poromas is thought to be associated with highly recurrent gene fusions of YAP1-MAML2 and YAP1-NUTM1.⁵

A poroma classically presents as a skin-colored, pink, red, white, or even blue dome-shaped papule, plaque, or nodule. The lesion is typically solitary, slow growing, and asymptomatic; however, some patients may report pain and itching. The surface texture of a poroma can be smooth, verrucous, or even ulcerative.^6,7 Although rare, patients may present with multiple, widespread poromas at diagnosis, a phenomenon termed poromatosis.³

Other skin conditions to rule out in the diagnosis of poromas include basal cell carcinoma, seborrheic keratosis, hidradenoma, pyogenic granuloma, cylindroma, acrochordon, and verruca vulgaris. In contrast to poroma, seborrheic keratosis is unlikely to show ductal differentiation on histologic examination. Hidradenomas have larger epithelial cells with pale cytoplasm compared to the compact cuboidal cells of poromas.³

To aid in the diagnosis, dermoscopy of the vascular pattern is recommended. Prominent vascular patterns in a poroma include polymorphic, glomerular, linear-irregular, leaf and flower-like, and looped or hairpin forms.⁶ However, it is important to note that the polymorphic vascular pattern is also seen in melanoma and should prompt further evaluation. Conversely, a finding of the leaf and flower-like pattern is thought to be specific to the poroma.³ An alternative method for differentiating poromas involves analyzing the focus of the vascular pattern as the vascular pattern for a poroma is in less focus than the arborizing vessels of basal cell carcinoma.⁸   

Histopathologic examination of a poroma demonstrates small, monomorphic cuboidal epithelial cells distributed in symmetrical, sharply circumscribed anastomosing bands. Intercellular bridges exist in continuity with the epidermis, and lack peripheral nuclear palisading. The dermal papillae are swollen, filled with dilated blood vessels, and encompassed by a fibrinoid edematous halo.⁷

A poroma may be intraepidermal, wholly juxtaepidermal, or completely intradermal. In the epidermis, there is a sharp differentiation between poroid cells and adjacent keratinocytes. The cuboidal cells have a compact eosinophilic cytoplasm, monomorphous ovoid nuclei, and inconspicuous nucleoli. Distinguishing a benign poroma from a malignant neoplasm histologically is made more difficult as poromas may possess histologic characteristics that are more typical of malignancy. Examples include foci of necrosis en masse, variable number of mitotic figures, and a highly vascularized stroma. Poroid cells in a poroma may display ductal formation, but the degree of ductal differentiation can vary.³

As poromas are classified as benign adnexal neoplasms, treatment is optional. Simple excision, shave, or electrosurgical destruction are options for superficial lesions. Deeper lesions are preferentially treated with simple excisions. Additional treatment options include carbon dioxide laser-assisted removal or topical treatment with 1% atropine solution for superficial lesions. Unlike porocarcinomas, recurrence of poromas is rare.^3,9

The patient in this case was diagnosed clinically with a poroma. He desired removal so shave biopsy was performed, which confirmed the diagnosis of a poroma.

Sarah Friske BBA, is a medical student at Baylor College of Medicine in Houston, Texas. Tara Braun MD, is a dermatologist at Elite Dermatology in Houston, Texas.

References

1. Ahmed Jan N, Masood S. Poroma. In: StatPearls. StatPearls Publishing; July 4, 2022.
2. Goldman P, Pinkus H, Rogin JR. Eccrine poroma; tumors exhibiting features of the epidermal sweat duct unit. AMA Arch Derm. 1956;74(5):511-521.
3. Sawaya JL, Khachemoune A. Poroma: a review of eccrine, apocrine, and malignant forms. Int J Dermatol. 2014;53(9):1053-1061. doi:10.1111/ijd.12448
4. Abenoza P, Ackerman AB. Neoplasms with eccrine differentiation. In: Ackerman AB, ed. Ackerman’s Histologic Diagnosis of Neoplastic Skin Diseases, a Method by Pattern Analysis. Vol 1. Lea & Febiger; 1990:113-185.
5. Sekine S, Kiyono T, Ryo E, et al. Recurrent YAP1-MAML2 and YAP1-NUTM1 fusions in poroma and porocarcinoma. J Clin Invest. 2019;129(9):3827-3832. doi:10.1172/JCI126185
6. Shalom A, Schein O, Landi C, et al. Dermoscopic findings in biopsy-proven poromas. Dermatolog Surg 2012;38(7 Pt 1):1091-1096. doi:10.1111/j.1524-4725.2012.02407.x
7. Ferrari A, Buccini P, Silipo V, et al. Eccrine poroma: a clinical-dermoscopic study of seven cases. Acta Derm Venereol. 2009;89(2):160-164. doi:10.2340/00015555-0608
8. Nicolino R, Zalaudek I, Ferrara G, et al. Dermoscopy of eccrine poroma. Dermatology. 2007;215(2):160-163. doi:10.1159/000104270
9. Mahajan RS, Parikh AA, Chhajlani NP, Bilimoria FE. Eccrine poroma on the face: an atypical presentation. Indian J Dermatol. 2014;59(1):88-90. doi:10.4103/0019-5154.123515