Real-World Experience With TRBC1 Immunohistochemistry Across Cutaneous T-Cell Lymphoma Subtypes: A Large Cohort Study.

T-cell receptor β-chain constant region 1 (TRBC1) immunohistochemistry identifies clonal αβ T-cell populations on tissue sections, but its real-world performance across cutaneous T-cell lymphoma (CTCL) and related infiltrates is uncharacterized. The analytic cohort comprised 665 biopsies (566 patients) with paired T-cell receptor (TCR) clonality testing, classified clinicopathologically as mycosis fungoides (MF; MF-Patch, MF-Plaque, MF-Tumor, and MF-Folliculotropic); MF or Sézary syndrome; primary cutaneous small or medium T-cell lymphoproliferative disorders (LPDs); other CTCL-cutaneous LPDs; or reactive. At the primary <15%/>85% threshold, TRBC1 IHC achieved 85.8% sensitivity (337/393), 79.8% specificity (217/272), 86.0% positive and 79.5% negative predictive value, and 83.3% accuracy. Sensitivity was lowest in MF-Patch (84.2%). Three-reader agreement (Fleiss κ = 0.943) fell to κ = 0.776 in 176 reflexed biopsies, with disagreement concentrated on MF-Patch and CD30-positive LPDs. Monotypic TRBC1 predicted neoplasia, with odds rising with infiltrate density: MF-Patch (odds ratio, 5.41), MF-Plaque (10.40), MF/Sézary syndrome with MF-Tumor (15.19), and CTCL-cutaneous LPD (18.16). Polytypic TRBC1 was associated with reactive disease (odds ratio, 53.65), effectively excluded clonality (negative likelihood ratio, 0.18), and was uniformly observed in an independent 270-biopsy reactive cohort. For observer-independent validation, digital image analysis-derived TRBC1 quantification (QuPath) was applied to a stratified random subset of 250 biopsies representative of the cohort's tumor-burden distribution. The digital read-tracked molecular clonality (85.1% sensitivity, 80.1% specificity against TCR; area under the curve, 0.842) agreed with the dermatopathologist's manual read in 87.6% of cases (κ = 0.752), with the data-derived cutoff matching the prespecified <15%/>85% threshold value. Discordance was directional for both manual scoring and digital quantification: in MF-Patch, 25 of 38 (65.8%) and 12 of 17 (70.6%) cases were polytypic with monoclonal TCR (false-negative-dominant); in reactive biopsies, 34 of 45 (75.6%) and 19 of 20 (95%) were monotypic with polyclonal TCR (false-positive-dominant). These findings support a TRBC1-first approach, reserving reflex TCR testing for borderline expression or clinicopathologic discordance, preserving diagnostic accuracy while reducing molecular testing and reimbursement-based costs.