Paraneoplastic Pemphigus in a Patient With Relapsed Breast Cancer: An Uncommon Presentation.

Paraneoplastic pemphigus (PNP) is a rare and severe autoimmune mucocutaneous blistering disorder most commonly associated with hematologic malignancies. Its occurrence in solid tumors, particularly breast cancer, is exceedingly uncommon. PNP may precede, coincide with, or signal recurrence of an underlying neoplasm. We report the case of a 65-year-old woman presenting with a six-month history of recurrent, painless palatal blisters. Clinical examination revealed multiple intraoral vesicles and a subtly positive Nikolsky sign. Her medical history was notable for breast cancer treated 11 years prior, with oncology discharge in 2021. Biopsy with histopathology and direct immunofluorescence confirmed intraepithelial blistering with IgG deposition, consistent with PNP. Positron Emission Tomography - Computed Tomography (PET-CT) demonstrated widespread recurrence of breast cancer with nodal, hepatic, adrenal, and osseous involvement. The oral lesions were managed successfully with low-dose topical corticosteroids, while systemic treatment for metastatic breast cancer included chemotherapy and palliative radiotherapy. This case highlights an unusual presentation of PNP secondary to recurrent breast cancer more than a decade after initial treatment. The subtle but persistent mucosal lesions served as an early clinical clue to underlying malignancy progression. Prompt recognition and biopsy-based confirmation were essential to establishing the diagnosis and guiding oncologic management. PNP should be considered in the differential diagnosis of chronic, unexplained oral vesiculobullous lesions, even in patients with remote histories of solid tumors. Heightened clinical suspicion may facilitate earlier detection of malignancy recurrence.