Gross cystic disease fluid protein-15 reactivity in extramammary Paget's disease with and without associated internal malignancy.

Extramammary Paget's disease (EMP) is a rare eczematoid disorder occurring mainly in apocrine-gland-bearing regions. Its histogenesis is controversial. Several investigators have proposed that EMP is a heterologous entity, with some cases representing a de novo adenocarcinoma in situ arising in the epidermis and others being epidermotropic metastases or a direct extension of an associated internal malignancy. We have studied 26 cases of EMP with and without associated internal malignancy for their reactivity with a monoclonal antibody directed at gross cystic disease fluid protein-15 (GCDFP-15). The diagnosis of EMP has been previously established for all cases using histochemical and immunohistochemical stains. Six of our 26 cases had a concomitant underlying carcinoma (one transitional cell carcinoma of the bladder, four adenocarcinomas of the rectum, one adenosquamous carcinoma. Only one of these six cases showed reactivity with GCDFP-15. In contrast, 16 of 20 cases of EMP without associated internal malignancy were strongly reactive with GCDFP-15 (> 5% of tumor cells). Variable GCDFP-15 reactivity in cases of EMP with and without associated internal malignancy supports the concept that EMP is a heterologous disorder. Positive GCDFP-15 in a patient with EMP may indicate a low probability of associated internal malignancy and may provide valuable clinical information.