DIAGNOSIS Tumor-like deposits of κ light chain surrounded by mature kappa chain-expressing plasma cells.
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. Non amyloid light chain deposits have been reported as a complication of plasma cell dyscrasia and particularly of multiple myeloma and the condition has been designated light chain deposition (LCD) disease (9). The kidney is usually the first and most severely affected organ but others may be involved, including the peripheral nervous system (4) and ocular globes (1). However, the blood-brain barrier likely prevents entrance of protein macromolecules into the central nervous system and the brain is not usually affected by systemic light chain disorders, excepted in sites missing a tight blood-brain barrier such as the choroids plexus and the periventricular areas where they have no effect on the brain (10).
Occurrence of light chain deposits in the brain is rare, with only two cases previously reported in the literature (3, 7). In one of these cases, it was suggested that the light chain deposition originated from a low-grade cerebral lymphoma diagnosed 3 months earlier (3), while in the other case a monoclonal B-cell proliferation of "neoplastic or other undefined nature" was found in close proximity to the deposits (7). The cell infiltrate observed in the present case was composed of monotypic κ producing plasma cells showing no cytologic atypia, associated with a few mature T lymphocytes and macrophages of the foreign body type. There was no evidence of an aggressive or systemic lymphoplasmacytic neoplasm.
The tumor-like MRI presentation of the light chain disease in this case is reminiscent of primary intracerebral amyloidoma presenting as a mass lesion (6, 8) and likely produced by B-cell clone. Nevertheless, most of such cases lack clinical evidence of an aggressive or systemic lymphoplasmacytic neoplasm and are characterized by a relatively indolent course (2, 5). Intracerebral light chain amyloidomas are almost invariably of λ type, as for the two previously reported cases of LCD (3, 7). We report the first case of intracerebral κ LCD presumably derived from local synthesis by mature plasma cells and mimicking CNS neoplasm on MRI.
Contributed by Anne Vital, MD, PhD; Emmanuel Ellie, MD; Hugues Loiseau, MD