Use of <scp>TRBC1</scp> Immunohistochemistry in the Diagnosis of Hypopigmented Mycosis Fungoides

Immunohistochemistry with T-cell receptor β-chain constant domain 1 (TRBC1) has been reported to be helpful in distinguishing between a clonal or reactive process. TRBC1 interpretation can be challenging in cases with a low proportion of neoplastic T-cells and high numbers of reactive T-cells. In this study we evaluated TRBC1 immunohistochemistry retrospectively on four cases with known T-cell receptor clonality results by polymerase chain reaction that were clinically categorized as hypopigmented mycosis fungoides. TRBC1 immunohistochemistry was defined as monotypic-negative (TRBC1 < 25%, i.e., < 1 in 4 positive lymphocytes) or monotypic-positive (TRBC1 > 75%, i.e., > 3 of 4 lymphocytes positive), or non-monotypic (TRBC1 > 25% and < 75%). Immunohistochemical results when assessing TRBC1 monotypic expression in the intraepidermal component in all four cases were concordant with T-cell receptor clonality assays. Assessing TRBC1 expression with particular focus on the intraepidermal component may be helpful as a useful data point when evaluating for hypopigmented mycosis fungoides.