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Abstract

Background: Lymphomatoid papulosis (LyP) is a cutaneous CD30-positive lymphoproliferative disorder characterized by recurrent, self-regressing papulonodular eruptions. Despite its indolent behavior, approximately 20% of cases progress to systemic lymphoid malignancies, particularly when clonal T-cell receptor (TCR) gene rearrangements are present. TRBC1, a marker of the TCR β-chain constant region, has been validated for clonality assessment in flow cytometry and molecular assays. Immunohistochemistry (IHC) for TRBC1 is a cost-effective alternative, but its diagnostic utility in LyP is not well defined. Design: We retrospectively identified biopsy-proven LyP cases (2020–2025) from our institutional archives. Formalin-fixed paraffin-embedded tissue was reviewed, and IHC for CD3 and TRBC1 was performed. TRBC1 expression was quantified as the percentage of CD3+ T-cells. Clonality was defined as monoclonal if >85% or <15% of CD3+ T-cells expressed TRBC1, and polyclonal if expression was between 15–85%. A binomial test evaluated whether the proportion of monoclonal cases differed from a null expectation of 50%. Result: Twenty-seven LyP cases were included. TRBC1 IHC demonstrated monoclonal T-cell populations in 22 cases (81.5%) and polyclonal expression in 5 cases (18.5%). The monoclonality rate was significantly greater than expected by chance (p < 0.01). Conclusion: TRBC1 IHC is a valuable adjunct for detecting T-cell clonality in LyP. It provides a rapid, inexpensive, and widely available alternative to molecular testing. Incorporating TRBC1 IHC into routine diagnostic workflows may improve diagnostic confidence and aid in risk stratification for patients with LyP.