A 61-year-old man with instability of gait and right hand clumsiness.

60 year-old man, without relevant medical history, noted a slight and progressive instability of gait for one month and right hand clumsiness. Brain MRI showed a cerebellar lesion, posterior to the middle cerebellar peduncle. This lesion was heterogeneous and hyperintense on FLAIR sequences, isointense on T1-weighted images, and showed gadolinium enhancement. Hematological and biological serum analyses were normal as were plasma and urine immunoelectrophoresis. CSF analysis including protein electrophoresis was unremarkable. CT scans of the abdomen, chest and pelvis were normal as were cervical echography and bone scintigraphy. A yellowish and firm lesion was surgically resected. The patient's recovery was good, with normal total body PET scan and bone marrow biopsy. Pathological study evidenced kappa light chain deposits and kappa-immunopositive mature plasma-cells in the vicinity. The deposits failed to show any birefringence in polarized light microscopy after Congo red staining, and electron microscopy revealed their granular ultrastructure. Light chains are well known for their amyloidogenic properties, but in a few cases, they are non amyloidogenic and may cause tissue deposits histologically similar to amyloid but Congo red-negative and non fibrillary at ultrastructural examination. Occurrence of light chain deposits in the brain is rare and the tumor-like MRI presentation is reminiscent of primary intracerebral amyloidoma presenting as a mass lesion. This is the first report of intracerebral kappa light chain deposits which presumably derived from local synthesis by mature plasma cells.