Diffuse <scp>CD30</scp> ‐Positive Cutaneous Infiltrate in a Clinically Suspected Hidradenitis Suppurativa Lesion: Histopathologic and Diagnostic Challenges

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disorder affecting apocrine gland-bearing areas. We report a 38-year-old male with a lesion in the left axilla, initially clinically interpreted as HS and resistant to antibiotics. Surgical excision and biopsy revealed a dense dermal and subcutaneous infiltrate of large atypical lymphoid cells with strong and diffuse CD30 expression (~80%-90% of tumor cells), CD4 positivity, ALK-negativity, and loss of pan-T-cell markers, consistent with anaplastic large cell lymphoma (ALCL). While the lesion was initially considered primary cutaneous, regional lymph node involvement and FDG-avid sites on PET created diagnostic uncertainty regarding systemic disease. The patient received multi-agent systemic chemotherapy followed by autologous stem cell transplantation, achieving complete metabolic remission. This case highlights the histopathologic challenges of diagnosing CD30-positive lymphoid infiltrates in the context of chronic inflammatory skin disease, particularly when lesions are initially clinically misinterpreted as benign inflammatory conditions such as HS. Recognition of diffuse CD30 positivity, cytologic atypia, subcutaneous involvement, and loss of pan-T-cell markers is critical for distinguishing malignant from reactive lymphoid infiltrates. Early biopsy, immunophenotypic analysis, and clinicopathologic correlation are essential to guide appropriate management in atypical or treatment-resistant lesions. Rather than demonstrating a novel association with HS, this report emphasizes diagnostic and classification challenges at the interface between primary cutaneous and systemic ALCL, which are of direct relevance to cutaneous pathologists.