CAMTA1 is a useful immunohistochemical marker for diagnosing epithelioid haemangioendothelioma

Aims The diagnosis of epithelioid haemangioendothelioma (EHE) is usually straightforward, based on characteristic histological features. However, it is sometimes difficult to differentiate EHE from a variety of other tumours with epithelioid morphology. The WW domain‐containing transcription regulator 1–calmodulin‐binding transcription activator 1 (WWTR1–CAMTA1) fusion gene, resulting in the overexpression of CAMTA1, is demonstrated in approximately 90% of EHEs, and the yes‐associated protein 1–transcription factor E3 (YAP1–TFE3) fusion gene, associated with the strong and diffuse nuclear expression of TFE3, is present in another small subset of EHEs. The aim of our study was to examine CAMTA1 expression in EHEs and a variety of other tumours to evaluate its diagnostic utility, and to analyse TFE3 expression status in EHEs. Methods and results Immunohistochemistry was performed on 16 EHEs, including five cases with CAMTA1 rearrangement and 276 non‐EHE tumours. Fourteen of 16 EHEs and only one case of ductal carcinoma of the breast were positive for CAMTA1 and its expression was focal and weak in the latter (sensitivity 87.5%, specificity 99.6%). TFE3 expression was expressed focally and weakly in three (19%) EHEs (two with the CAMTA1 rearrangement). Conclusions Nuclear CAMTA1 expression is sensitive and highly specific for EHE and can be applied to diagnostic immunohistochemistry in epithelioid tumours.