Primary Cutaneous B-Cell Lymphoma Imitating Pyoderma Gangrenosum: A Rare and Complex Diagnostic Challenge

<b>Background:</b> Primary cutaneous B-cell lymphomas (CBCLs) are a rare and heterogeneous group of lymphomas, among which the anaplastic variant of diffuse large B-cell lymphoma (A-DLBCL) represents an exceptionally rare entity. Although they typically present as painless and non-ulcerated skin lesions, rare variants may exhibit atypical clinical features. Pyoderma gangrenosum (PG) is a rare inflammatory ulcerative disease that may overlap clinically with other ulcerative dermatoses and pose diagnostic challenges due to the absence of standardized differential algorithms. <b>Methods:</b> An 85-year-old male presented with multiple rapidly progressive, painful, ulcerative lesions, initially misdiagnosed as PG and treated with oral cyclosporine with no clinical response. Skin biopsy specimens underwent detailed histopathological and immunohistochemical evaluation. <b>Results:</b> The analyses revealed dense infiltration of atypical large lymphoid cells, with CD20, CD45, and CD30 positivity, and a Ki-67 proliferation index of approximately 90%, consistent with primary cutaneous A-DLBCL. Owing to the delayed correct diagnosis, the patient's condition deteriorated rapidly, leading to his death before appropriate therapy could be initiated. <b>Conclusions:</b> The case documents an exceptionally rare cutaneous presentation of A-DLBCL, expanding the extremely limited literature on this enigmatic entity. Furthermore, it underscores the fundamental role of early skin biopsy in the differential diagnosis of non-specific ulcerative lesions, which is critical for ensuring appropriate treatment administration within the therapeutic window in cases of malignancy.