What is the role of radiation therapy in recurrent Rosai-Dorfman disease after surgical resection, which manifests as many cutaneous lesions on the arms and buttocks?

I agree with Dr. @Dr. First Last. When I was in training, we reviewed the cases at MD Anderson. Radiation was only used for 9 patients (infrequently), and local control was achieved in 4 patients. So very small numbers, with uncertain benefits. Median RT dose 36 Gy. In the setting of multifocal disease, I would imagine systemic therapy is the most useful. I suppose focal RT for a very problematic lesion could be considered, but approached with tempered enthusiasm.

Rosai-Dorfman disease, in addition to the histiocytic and dendritic cell neoplasms described in the 4th edition (2017) of the WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues (including such entities as histiocytic sarcoma and Langerhans cell histiocytosis) are relatively rare, infrequently referred for consideration of radiation therapy, with very little in the literature to guide the appropriate use of RT when pursued. Histiocytes are part of the mononuclear phagocyte system and are tissue macrophages but the term "histiocytosis" has historically been used somewhat loosely.

A reasonable RT literature exists for Langerhans cell histiocytosis. Low doses of RT (10-20 Gy) seem to be efficacious, particularly for osseous sites of disease.

Rosai-Dorfman disease (RDD) is a non-Langerhans cell histiocytosis and does not seem to respond to such low doses as favorably. Asymptomatic patients with RDD can be observed and spontaneous remissions are possible. Surgery would be appropriate for localized, resectable presentations (e.g., an isolated cutaneous tumor) or to debulk disease in sensitive locations. Systemic therapy is generally pursued for multifocal symptomatic disease. Options include steroids, sirolimus, immunomodulators such as lenalidomide or thalidomide, MTX, and other chemotherapeutics, along with several other options. 

Radiation therapy is not often employed for RDD but could be considered for patients with unresectable localized disease, symptomatic disease that is not responding to other modalities, or other similar scenarios. I would probably prescribe ~30 Gy in most cases and assess response. Doses of 30-50 Gy have been reported in the literature.

In the case described, if the disease was multifocal and responding to MTX, I would probably hold off on any RT.