Trichorhinophalangeal Syndrome Type 1 Is a Highly Sensitive and Specific Marker for Diagnosing Triple-Negative Breast Carcinomas on Cytologic Samples

center dot Context.-Definitive diagnosis of metastatic triple-negative breast carcinoma (TNBC) is challenging on cytologic samples. Recent studies demonstrated that trichorhino-phalangeal syndrome type 1 (TRPS1) is a highly sensitive and specific marker for diagnosing breast carcinomas, including TNBC, on surgical specimens. Objective.-To evaluate TRPS1 expression in TNBCs on cytologic samples and a large series of nonbreast tumors on tissue microarray sections.Design.-Immunohistochemical (IHC) analysis of TRPS1 and GATA-binding protein 3 (GATA3) was performed on 35 TNBC cases on surgical specimens, and 29 consecutive TNBC cases on cytologic specimens. IHC analysis of TRPS1 expression was also performed on 1079 nonbreast tumors on tissue microarray sections. Results.-Of the surgical specimens, 35 of 35 TNBC cases (100%) were positive for TRPS1, all with diffuse positivity, whereas 27 of 35 (77%) were positive for GATA3, with diffuse positivity in 7 cases (26%). Of the cytologic samples, 27 of 29 TNBC cases (93%) were positive for TRPS1, with diffuse positivity in 20 cases (74%), whereas 12 of 29 (41%) were positive for GATA3, with diffuse positivity in 2 cases (17%). Of the nonbreast malignant tumors, TRPS1 expres-sion was seen in 9.4% (3 of 32) of melanomas, 10.7% (3 of 28) of small cell carcinomas of the bladder, and 9.7% (4 of 41) of ovarian serous carcinomas.Conclusions.-Our data confirm that TRPS1 is a highly sensitive and specific marker for diagnosing TNBC cases on surgical specimens as reported in the literature. In addition, these data demonstrate that TRPS1 is a much more sensitive marker than GATA3 in detecting metastatic TNBC cases on cytologic samples. Therefore, inclusion of TRPS1 in the diagnostic IHC panel is recommended when a metastatic TNBC is suspected.(Arch Pathol Lab Med. 2024;148:e1-e8; doi: 10.5858/ arpa.2022-0411-OA)