Abstract

Introduction: Granulomatous dermatitis (GD) may present with variable clinical and histopathologic features and has a broad variety of etiologies including infectious, non-infectious, hypersensitivity, and neoplastic (with the latter differential particularly including cutaneous T-cell lymphoma). Case Presentation: A 68-year-old male presented with red, mildly pruritic plaques, located on the back, extremities, and face. There was a high clinical suspicion for cutaneous lymphoma (CTCL). Histopathologic examination demonstrates well-formed, dermal sarcoidal granulomas with a prominent admixed lymphoid infiltrate. The lymphocytes are composed primarily of T-cells (CD3 positive) with an unremarkable CD4 to CD8 ratio, and retained expression of CD2, CD5, and CD7. Grocott's methenamine silver (GMS), Acid-fast bacillus (AFB), and Periodic acid–Schiff (PAS) stains are negative. Due to the consideration of lymphoma, clonality studies by next Generation Sequencing are performed and show the same clonal T-cell population in all three sites evaluated. During the patient's clinical workup for granulomatous dermatitis, Rubella capsid antigen was performed and was positive. Discussion: The diagnosis of CTCL (mycosis fungoides) can sometimes be challenging and additional studies such as T-cell gene rearrangement can be helpful. Clonal T-cell populations can also be identified in reactive conditions and may represent a diagnostic pitfall for CTCL. As in our case, the patient’s lesions from different areas demonstrated the same clonal T-cell population. However, given the histopathologic and immunohistochemical features of the lesions, particularly lack of lymphocyte atypia, normal T cell antigen expression, and the presence of a Rubella antigen, favors the diagnosis of systemic granulomatosis secondary to Rubella, rather than CTCL.