Inclusion Criteria:  Must have histopathologic confirmation of the particular rare hematologic disease.  Diseases that will be considered as rare hematologic diseases for this study will include the following  Langerhans cell histiocytosis (LCH) Erdhiem Chester disease (ECD) Rosai-Dorfman disease (RDD) Miscellaneous histiocytic entities -indeterminate dendritic cell tumor, interdigitating dendritic cell sarcoma, follicular dendritic cell sarcoma, fibroblastic reticular cell tumor Unicentric Castleman disease Multicentric Castleman disease including TAFRO Follicular Dendritic Cell sarcoma (FDCS) Newly diagnosed treatment naïve patients as well as patients who received prior therapies (e.g. chemotherapy, targeted therapy, surgery, or radiation) will be included. -Tissue specimens collected within the past 5 yearse will be considered acceptable for study inclusion will include the following Collected as part of the evaluation for diagnostic confirmation Tissue specimen or extracted DNA (from blood sample) banked in IRB approved tissue repositories and obtained within five years prior to the date of informed consent. -Tissue samples are planned to be collectedfrom previously stored surgical specimens already being stored in pathology lab Consent to have germline testing performed in parallel to tumor testingg)Patients willing to receive treatmen  Exclusion Criteria:  Life expectancy of less than 6months Patient unwilling to have germline testing performed on peripheral blood or buccal mucosa