Inflammatory myofibroblastic tumor (IMT), also called “inflammatory pseudotumor”, is a rare benign tumor composed of spindle cells with a variable infiltrate of inflammatory cells and fibrous tissue. There have been many reported cases of IMT in every organ system; however, IMT in the mediastinum is rare. We report a rare, proven case of spontaneous regression of IMT in the right cardiophrenic angle. The diagnosis was confirmed by histopathology, and the observation of serial ultrasonographic and computed tomography images over the course of three months revealed a picture consistent with spontaneous regression of the inflammatory pseudotumor.

Keywords: inflammatory pseudotumor • spontaneous neoplasm regression • mediastinum