A Rare Case of Oligosecretory Multiple Myeloma With Intracranial Plasmacytoma.

Multiple myeloma (MM) is a hematologic malignancy characterized by monoclonal plasma cell proliferation within the bone marrow. According to current International Myeloma Working Group (IMWG) criteria, true non-secretory multiple myeloma (NSMM) is defined by the absence of detectable monoclonal protein even on serum free light chain (FLC) assay, unlike oligosecretory multiple myeloma (OSMM), in which electrophoresis and immunofixation are negative but the FLC κ/λ ratio is pathological - making diagnosis particularly challenging by conventional techniques. The association of OSMM with an intracranial plasmacytoma is exceptional. We report the case of a 70-year-old hypertensive former smoker admitted for diffuse osteoarticular pain evolving over nine months, associated with gait disturbance and complete left oculomotor palsy. Laboratory workup revealed aregenerative anemia, thrombocytopenia, hyperleukocytosis with hyperlymphocytosis, and initially normal calcium (1.73 mmol/L), which secondarily worsened to 4.31 mmol/L during hospitalization. Serum/urine electrophoresis and immunofixation were negative, but FLC assay showed a pathological κ/λ ratio of 13.37, suggestive of OSMM. Bone marrow aspiration confirmed 75% dystrophic plasma cell infiltration. Whole-body magnetic resonance imaging (MRI) showed diffuse myelomatous infiltration with vertebral compression fractures, and orbito-cerebral MRI revealed an expansile clival lesion with bone lysis, invading the sella turcica, optic chiasm, carotid canal, and left cavernous sinus (360° carotid encasement), left petrous apex, and sphenoid sinus - for which chordoma, chondrosarcoma, bone metastasis, and lymphoma were excluded in favor of an intracranial plasmacytoma, based on convergent histological, marrow, and radiological findings. The patient was classified as International Staging System (ISS) stage I; cytogenetic testing could not be performed due to technical constraints and rapid clinical deterioration, limiting prognostic stratification (Revised-ISS (R-ISS) not calculable). Radiotherapy targeting the sphenoidal mass was initiated, combined with corticosteroid therapy and zoledronic acid for hypercalcemia; the neurological emergency and the patient's frailty led to favoring this local approach over standard systemic therapy. The clinical course was unfavorable, marked by septic shock due to a multidrug-resistant organism, leading to death before completion of radiotherapy. This case highlights the pivotal role of FLC assay in diagnosing OSMM when conventional techniques are negative, and the need for a multidisciplinary approach in cases of intracranial involvement. Biochemical (serial FLC) and radiological (brain MRI at three months, then every three to six months) follow-up remain recommended in the literature, as does high-performance liquid chromatography (HPLC), currently under validation for minimal residual disease detection - these points, however, remain general literature-based recommendations rather than conclusions drawn from our single observation.