Introduction: Identifying metastatic breast carcinoma (mBC) in malignant effusion cytology (MEC) specimens is critical, as this will determine the patient’s prognosis and therapeutic management. Overlapping cytomorphologic features of breast carcinoma (BC) with other neoplastic entities makes the use of sensitive and specific markers highly desirable. Recent studies have reported trichorhinophalangeal syndrome type 1 (TRPS1) as a sensitive and specific marker for primary BC and mBC. We aimed to investigate TRPS1 expression in MEC of mBC and its most common diagnostic mimickers. Materials and Methods: A retrospective search from the pathology archives identified 82 MEC. TRPS1 expression in mBC was analyzed, and the results were compared to those in metastatic carcinoma of Müllerian origin (mMC) and metastatic pulmonary adenocarcinoma (mPAC). TRPS1 immunoperoxidase was performed on cytospin or cell block preparations, and p < 0.05 was considered significant. Results: Nuclear expression for TRPS1 was evaluated and scored as positive (≥1% of tumor cells) or negative. Nuclear TRPS1 expression was seen in 100% (30/30) mBC, 72% (18/25) mMC, and 7% (2/27) mPAC. This resulted in sensitivity, specificity, positive predictive value, and negative predictive values of 100%, 61%, 60%, and 100%, respectively. Conclusion: TRPS1 is a sensitive marker for mBC and can be reliably performed on cytology specimens. TRPS1 expression was also identified in a significant proportion of mMC, creating a potential diagnostic pitfall. Therefore, caution should be exercised when evaluating MEC of mBC with TRPS1. Consequently, a combination of immunoperoxidase panels should be employed in this setting.

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