Case Report: hidradenocarcinoma presenting with lymph node metastasis: a diagnostic challenge and the pivotal role of morphology

Hidradenocarcinoma (HAC) is a rare malignant sweat gland tumor. When presenting with lymph node metastasis and an occult primary lesion, diagnosis is often significantly delayed. We report a case of a 58-year-old man with right axillary lymphadenopathy in whom the primary tumor remained elusive despite extensive workup, including two lymph node biopsies, multiple immunohistochemical(IHC) panels, and a PET-CT scan. This led to an initial diagnosis of carcinoma of unknown primary (CUP) and the initiation of empirical chemotherapy. Ultimately, a crucial morphological clue-"features favoring cutaneous sweat gland origin"-was identified on a subsequent biopsy, which prompted a systematic dermatological examination. The subsequent dermatological workup yielded the primary lesion: an 8 × 10 cm hidradenocarcinoma on the back, which was deemed inoperable due to its advanced stage. This case highlights a common diagnostic blind spot for cutaneous origins in CUP and underscores the critical role of meticulous histomorphological analysis when immunohistochemistry is equivocal. We thus advocate for the mandatory inclusion of a comprehensive full-body skin examination in the standard diagnostic protocol for CUP to prevent delays in diagnosing such rare yet treatable skin cancers.