Abstract
Primary cutaneous mucinous carcinoma (PCMC) is an exceptionally rare adnexal malignancy of sweat gland origin, with an age-adjusted incidence of approximately 0.04 per 100,000 person-years. We report a rare occurrence of PCMC on the right nasolabial fold of a 55-year-old male. To the best of our knowledge, based on a review of existing literature, this represents the first reported case of PCMC specifically involving the nasolabial fold within the Indian subcontinent. The lesion initially presented as a slow-growing subcutaneous swelling, mimicking benign entities. Histopathological analysis following excisional biopsy revealed neoplastic nests suspended within expansive lakes of extracellular mucin, partitioned by thin fibrous septa. Immunohistochemical (IHC) profiling demonstrated that the tumor cells were strongly positive for CK7 and EMA and also showed diffuse expression of GATA3, estrogen receptor (ER), and progesterone receptor (PR). This specific immunophenotype - particularly the GATA3 positivity - presents a profound diagnostic trap, as it closely mimics the profile of metastatic mucinous carcinoma of the breast. To navigate this GATA3/ER/PR trap, a comprehensive systemic evaluation was mandated, including whole-body positron emission tomography-computed tomography (PET-CT) and mammography, which revealed no evidence of an extracutaneous primary malignancy. This confirmed the diagnosis of PCMC, an indolent but locally aggressive tumor. The patient was managed with wide local excision and remains recurrence-free at one-year follow-up. This case emphasizes that while GATA3, ER, and PR are traditionally associated with mammary origin, their expression in primary cutaneous adnexal tumors is a critical pitfall. Clinicians and pathologists must integrate targeted IHC with rigorous systemic imaging to differentiate PCMC from metastatic disease, ensuring appropriate surgical management and avoiding unnecessary systemic therapy.