PU.1 is a useful nuclear marker to distinguish between histiocytosis and histiocyte‐rich tumours

Abstract

Aims The aim was to test the expression of PU.1 on different types of histiocytoses and to test the utility of PU.1 in confirming or excluding a histiocytic origin in tumour samples with suspicion of histiocytosis. Methods and Results We analysed 66 biopsies of nonmalignant histiocytoses represented by Langerhans‐cell histiocytosis (n = 13), Erdheim–Chester disease (ECD) (n = 19), Rosai–Dorfman disease (RDD) (n = 14), mixed ECD‐RDD (n = 3), ALK‐positive histiocytosis (n = 6), and juvenile xanthogranuloma (n = 11). All cases were positive for PU.1 in reactive and neoplastic histiocytes. In addition, 39 cases of tumours with high‐grade cytological atypia were referred to our center as suspicion of malignant histiocytosis/histiocytic sarcoma and only 18 were confirmed. Indeed, more than half of these tumours (21/39) were either undifferentiated malignant tumours with a stroma rich in histiocytes, diffuse large B‐cell lymphoma, or high‐grade dedifferentiated liposarcoma. PU.1 was useful to distinguish between the negativity of large atypical nuclei and the positivity of stromal reactive histiocytes. Conclusion PU.1 is expressed by all types of histiocytosis. It distinguishes histiocytosis from histiocyte‐rich tumours with an easy interpretation due to its sharp nuclear staining. Its negativity in lesional/tumour cells in histiocyte‐like lesions is useful to eliminate a histiocytosis.