Primary Cutaneous CD4+ Small/Medium T-Cell Lymphoproliferative Disorder or Primary Cutaneous Marginal Zone B-Cell Lymphoma? Two Distinct Entities With Overlapping Histopathological Features.

Primary cutaneous CD4+ small/medium T-cell lymphoproliferative disorder and primary cutaneous marginal zone B-cell lymphoma are 2 distinct entities with several overlapping features which can result in diagnostic uncertainty. Clinically, they both follow an indolent course and present with solitary or multiple papules or nodules. Histologically, they are characterized by polymorphous dermal infiltrates rich in mixed populations of B cells and T cells, often in similar proportions. The histological hallmark of primary cutaneous CD4+ small/medium T-cell lymphoproliferative disorder is the presence of follicular T-helper cells within the infiltrate and has historically been used as a marker for differentiating between the 2 conditions. However, there is now mounting evidence that follicular T-helper cells are also seen in primary cutaneous marginal zone B-cell lymphoma and nodal marginal zone lymphoma. The 2 cases presented herein caused diagnostic uncertainty because they displayed appreciable features of both conditions. We discuss the potential mechanisms behind these overlapping histopathological features and hypothesize a model that explores the idea of a collective organoid response to an antigenic stimulus.