Collision Tumor of Basal Cell Carcinoma and Primary Extramammary Paget Disease: A Case Report and Literature Review.

We present the case of a 75-year-old woman with a vulvar collision tumor consisting of basal cell carcinoma (BCC) and primary extramammary Paget disease (EMPD). The patient presented for evaluation of a pigmented nodule on the right labium majus identified 1 year ago. The nodule was excised, and histopathology revealed nests of basaloid cells with nuclear palisading surrounded by myxoid stroma, consistent with nodular BCC. In addition, scattered intraepidermal cells with clear amphophilic cytoplasm at the periphery of the specimen raised suspicion for EMPD coexistence. Further visual inspection revealed bilateral hypopigmented patches on the labia majora. Gynecological and urological examination showed no abnormal findings. The hypopigmented lesion was excised with a 1-cm margin. Histopathology showed an intraepidermal pagetoid spread of tumor cells with atypical nuclei and abundant amphophilic cytoplasm. A diagnosis of a collision tumor comprising BCC and primary EMPD was made. A BCC-EMPD collision is rarely reported. This case highlights that coexisting EMPD can be overlooked, particularly when evaluating vulvar BCC, underscoring the diagnostic challenge posed by basaloid proliferation and pagetoid spread. Furthermore, our review of the literature elucidated the unique clinicopathological features of BCC-EMPD collision and discussed its differentiation from secondary EMPD with Pinkus-like changes and BCC with pagetoid spread.