Track: Case Reports

Abstract:

Diffuse large B-cell lymphoma (DLBCL), leg type, classically presents with skin nodules on the lower extremities composed of large atypical lymphocytes manifesting MUM1 expression. However, distinguishing cutaneous T-cell lymphoma (CTCL) from cutaneous B-cell lymphoma (CBCL) can sometimes be challenging due to similarities in skin lesions, overlapping histopathologic patterns, and atypical immunophenotypic findings. We describe an unusual case of tumor-stage MF in a 79-year-old male with a 2-year history of several red-violet papules and nodules on the right lower leg. Skin biopsies revealed a nodular to diffuse dermal infiltrate of atypical large lymphocytes manifesting pleomorphic, large, irregular nuclei, raising suspicion for DLBCL or anaplastic large cell lymphoma (ALCL). Immunohistochemical analysis demonstrated positive tumor staining for MUM1 and CD3, but not CD20 or CD30. Monoclonality was detected by T-cell receptor gene rearrangement analysis, but IgH clonality testing was negative. Further inquiry revealed the patient to have had a long history of presumed eczematous patches on the abdomen. The integrated histomorphologic, immunophenotypic, and molecular picture was deemed compatible with CTCL, and in light of the clinical context, as favoring tumor stage mycosis fungoides. Staging PET-CT revealed hypermetabolic right-sided inguinal and external/common iliac lymphadenopathy, while peripheral blood flow cytometry was unremarkable. Brentuximab vedotin (BV) was initiated, and after 4 cycles, the patient’s lesions demonstrated significant response with flattening of existing lesions and no new lesions. This case highlights MUM1+/CD30- MF as a rarely reported condition which can resemble DLBCL, and the efficacy of BV in CTCL even in the absence of significant CD30 expression.

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Jason Chen