Unexpected Diagnosis in a Cutaneous Tumoral Lesion: Primary Cutaneous Leg-Type B-Cell Lymphoma.

Primary cutaneous diffuse large B-cell lymphoma, leg type (PCDLBCL-LT), is an uncommon and aggressive subtype of cutaneous B-cell lymphoma, typically affecting elderly women and predominantly involving the lower extremities. Its diagnosis relies on immunohistochemical profiling and clinical presentation. We report a rare case of a 45-year-old male presenting initially with scalp and supraciliary plaques. The early diagnosis suggested tumor-stage mycosis fungoides, based on histopathology and immunophenotype (CD4+, CD20-). However, over time, the patient developed disseminated nodular lesions, with new biopsies revealing CD20+, MUM1+, BCL2+, Ki-67 70%, and MYC expression, consistent with PCDLBCL-LT. Despite multiple lines of systemic therapy, including R-CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone combined with rituximab), methotrexate, gemcitabine, platinum-based chemotherapy, and autologous stem cell transplant, the disease progressed, ultimately leading to the patient's death. This case highlights an atypical clinical presentation of PCDLBCL-LT in a young male, initially mimicking T-cell lymphoma. The diagnostic evolution underscores the importance of repeated biopsies and immunohistochemical reevaluation in persistent or atypical cutaneous lymphoproliferative disorders. It also raises awareness of CD20-negative variants and potential coexistence with T-cell lymphomas such as mycosis fungoides.