Mammary Paget disease (PD) of the nipple was first described by Sir James Paget in 1874. Extramammary PD (EMPD) was first recognized and reported as a distinct clinical entity by Radcliffe Crocker in 1889. EMPD is morphologically and histologically identical to mammary PD of the nipple, the primary difference being the anatomic location. See the image below.
Crocker described a patient with erythematous patches on his penis and scrotum. Since he made this distinction, the term EMPD is more commonly used to describe the condition in women. This process targets the genital skin, perianal skin, and other cutaneous sites rich in apocrine glands. EMPD is not a common disorder, but it must be considered in the differential diagnosis of patients with chronic genital or perianal dermatitis.
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Pathophysiology and Etiology
EMPD arises as a primary cutaneous adenocarcinoma in most cases. The epidermis becomes infiltrated with neoplastic cells showing glandular differentiation. Tumor cells may originate from apocrine gland ducts or from keratinocytic stem cells.
Approximately 25% of the cases of EMPD (range, 9-32%) are associated with an underlying in situ or invasive neoplasm. In all patients, the neoplasm most likely to be associated with EMPD is an adnexal apocrine carcinoma. This associated neoplasm probably represents infiltration of the deeper adnexa by epidermal Paget cells. Other malignancies besides cutaneous adnexal carcinoma that may be associated with EMPD include carcinomas of the Bartholin glands, urethra, bladder, vagina, cervix, endometrium, and prostate.
The anatomic location of EMPD plays a role in predicting the risk of associated carcinoma. For instance, genital disease is associated with carcinoma in about 4-7% of patients. Perianal disease is associated with underlying colorectal carcinoma in 25-35% of cases.
Rare cases of EMPD associated with tumors arising in distant organs without direct epithelial connection to the affected epidermis have been reported. Roy et al reported PD in a retrorectal dermoid cyst.  No clear evidence supports a causal link between these distant tumors and cutaneous EMPD.
The cause of primary EMPD is unknown. However, a minority of cases do represent a direct extension of an underlying carcinoma along contiguous epithelium.
EMPD is a rare condition; there are only several hundred cases in the world literature. It most commonly appears in individuals aged 50-60 years. Women are more commonly affected by EMPD than men. The female-to-male ratio was 4.5:1 in one series of 55 patients and 3:1 in another series of 197 patients.  EMPD is most commonly reported in white patients, but it may occur in other races.
The course of EMPD may extend over a period of 10-15 years without evidence of cancer or metastases.  In a minority of patients, tumor cells infiltrate the dermis, adnexa, or lymph nodes. Both mortality and morbidity are increased in these patients because of the extensive surgical treatment or chemotherapy that they need.
The prognosis for EMPD heavily depends on early diagnosis with definitive surgical treatment. Full recovery is possible in patients with purely epidermal disease and negative margins after micrographic surgery. One study showed a mortality of 18% for patients without associated carcinoma and 46% for those with underlying carcinoma.
Perianal disease, dermal invasion, and lymph node metastasis are poor prognostic indicators. The recurrence rate of EMPD is 30%, even with margin control. The average time to recurrence is 2.5 years, with case reports of more than 10 years follow-up.
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