Primary Cutaneous Apocrine Carcinoma of the Scalp Arising in Syringocystadenoma Papilliferum: Report of a Rare Case.

Primary cutaneous apocrine carcinoma (PCAC) is an exceptionally rare malignant adnexal neoplasm arising from apocrine sweat glands. Scalp involvement is particularly uncommon, especially when occurring in association with syringocystadenoma papilliferum (SCAP), making diagnosis and management challenging. We report the case of a 56-year-old woman with a congenital left occipital scalp lesion that remained stable for decades before demonstrating progressive enlargement over the preceding two years. Clinical examination revealed a solitary exophytic scalp mass without palpable cervical lymphadenopathy. Contrast-enhanced computed tomography (CT) demonstrated a superficial scalp lesion without calvarial or intracranial invasion. Histopathological examination following wide local excision established the diagnosis of primary cutaneous apocrine carcinoma arising in a background of SCAP with an incidental superficial basal cell carcinoma. Although immunohistochemistry was not performed, comprehensive clinicoradiological evaluation, including CT of the neck, chest, abdomen, and pelvis, mammography, breast ultrasonography with core biopsy confirming fibroadenoma, and whole-body bone scintigraphy, excluded an extracutaneous primary malignancy and metastatic disease. Surgical margins were negative, and the patient remains disease-free on follow-up. This case highlights the importance of considering malignant transformation in longstanding scalp lesions demonstrating recent clinical change, the role of comprehensive staging investigations in excluding metastatic disease, and the value of complete surgical excision with long-term surveillance in managing this rare malignancy.