The Non-Langerhans Cell Histiocytoses (Rare Histiocytoses) - Clinical Aspects and Therapeutic Approaches

Rare histiocytoses, also called non-Langerhans cell histiocytoses, include all proliferative disorders of histiocytes, macrophages and dendritic cells that are not classified as Langerhans cell histiocytosis (LCH) and do not belong to the hemophagocytic lymphohistiocytosis (HLH) group of diseases. Thus, the term includes numerous benign or malignant, localized or systemic, adult or pediatric diseases. The classification of the histiocytic disorders has been revised several times. Here, we follow the classification recently published by Jean Francois Emile and an international expert panel, defining subgroups of histiocytoses described as L-Group, C-Group, M-Group, R-Group, and H-Group, which stands for LCH-like, cutaneous or mucocutaneous, malignant, Rosai-Dorfman-Disease like and HLH like. Some of the diseases have an excellent prognosis after resection or even disappear spontanously, others progress rapidly, requiring intensive systemic therapies. The malignant non-Langerhans cell histiocytoses in general have a poor prognosis, here, complex chemotherapy protocols are usually applied, with inconsistant results. An interesting perspective in non-malignant rare histiocytoses might be small molecular inhibitors, in particular BRAF inhibitors, since BRAF mutations have been found in some subtypes of non-Langerhans cell histiocytoses. By prospective and retrospective collection of experiences in a new registry (the "International Rare Histiocytic Disorders Registry", IRHDR), knowledge about these rare diseases might hopefully be improved.