IgG4-Related Disease in the Frontal Convexity Concomitant with Smoldering Multiple Myeloma: A Case Report and Review of the Literature Regarding Therapeutic Implications.

BACKGROUND: We report an extremely rare case of a frontal convexity tumor diagnosed as IgG4-related disease (IgG4-RD) with unique neuroradiological images.

CASE DESCRIPTION: A 64-year-old man with a history of monoclonal gammopathy of undetermined significance (MGUS) and conservative follow-up presented with left facial spasm. Computed tomography showed a high-density round tumor with perifocal edema in the right frontal convexity. Magnetic resonance imaging demonstrated unique findings including low intensity on T1-, T2-, fluid-attenuated inversion recovery, and diffusion-weighted images with slight gadolinium enhancement. The tumor, which was totally removed via right frontal craniotomy, was located in the subdural space, did not adhere to the dura, and was less vascular than a meningioma. Histological investigation demonstrated plasma cells that were strongly positive for IgG4 containing κ and λ light chains at a ratio of 1.5:1. The serum IgG4 level was elevated. Based on diagnostic criteria, the tumor was determined to be IgG4-RD. The patient was followed up for 3 years while receiving postoperative adjuvant steroid therapy. Steroid therapy was then discontinued, and over the next 4 years, neither recurrence of the tumor nor symptoms were observed.

CONCLUSION: Intracranial IgG4-RD with smoldering MGUS is extremely rare. We review the differential diagnosis of plasma cell granuloma and plasmacytoma, therapeutic implications, and clinical outcomes. Complete resection of conspicuous and solitary IgG4-RD in the frontal convexity is easy and may provide a cure with less aggressive adjuvant therapy.