The first case report of an abscopal effect in a patient of retroperitoneal sarcomas (RPS) treated with proton therapy. A 67 year-old female with inoperable metastatic unclassified round cell RPS was treated with palliative proton radiotherapy only to the primary tumor.

Following completion of radiotherapy, the patient demonstrated complete regression of all un-irradiated metastases, and near complete response of the primary lesion without additional therapy. Abscopal effects are rare and incompletely understood, involving a balance of radiation's immunogenic and immunosuppressive effects.