Diffusion-weighted MRI in a case of nonsuppressing rebound thymic hyperplasia on chemical-shift MRI.

Rebound thymic hyperplasia (RTH) generally retains the quadrilateral or triangular shape typical of the normal thymus. Nevertheless, RTH may rarely appear as a focal mass, a condition that mimics tumors if reveals soft tissue attenuation at computed tomography (CT). In such cases, chemical-shift magnetic resonance imaging (MRI) can demonstrate microscopic fat within tissue, that would be indistinct at CT, by showing signal suppression on opposed-phase images relative to in-phase images, although two cases of nonsuppressing thymus have been described in early adulthood, both of them with minimally fat amount at histology, but none with RTH. Albeit diffusion-weighted MRI could be helpful in cases of nonsuppressing RTH at chemical-shift MRI, considering its capability in differentiating benign from malignant lesions, currently, no data are available on its use in such cases. Hereby, we report a case of atypical RTH at CT with no signal suppression on chemical-shift MRI, arose after corticosteroid treatment for mixed cryoglobulinemia in a 60-year-old woman. Diffusion-weighted MRI demonstrated unrestricted diffusion, as generally seen for benign tissue with no cellular atypia, and thus was helpful to differentiate RTH from malignant mediastinal tumors, especially from B cell lymphoma related to type II cryoglobulinemia.