Plasma cells, derived from B-lymphocytes, are essential for immune defence through immunoglobulin production. Plasma cell neoplasms arise from abnormal monoclonal plasma cell proliferation and can present as either diffuse diseases like multiple myeloma (MM) or localized forms such as plasmacytomas [1]. Plasmacytomas represent an intermediate stage between monoclonal gammopathy of undetermined significance (MGUS) and MM, and may be classified as solitary bone plasmacytoma (SBP) or extramedullary plasmacytoma (EMP), based on their site of origin [1,2]. SBPs are localized bone lesions without systemic features of MM and typically show low marrow involvement (<10%) [3]. They most commonly affect the vertebrae and skull, with rare occurrences at the skull base [4]. Among these, clival involvement is exceedingly rare, with only a few reported cases in the literature [5]. Notably, SBPs have a higher risk of progression to MM (up to 67%) compared to EMPs, leading to poorer overall prognosis [6,7].

A 47-year-old male presented with a 3-month history of diplopia. He denied associated symptoms such as headache, vomiting, seizures, or limb weakness. Neurological examination revealed isolated left sixth cranial nerve palsy with visual field defects, while the remaining cranial nerves and systemic examinations were normal. MRI brain revealed a 4.5×3.3 cm T1 and T2 isointense soft tissue mass in the clival region with homogeneous contrast enhancement and osteolytic destruction, bilaterally encasing the internal carotid arteries without sellar invasion [Fig.1]. Whole-body PET scan identified a solitary metabolically active lesion at the clivus, with no other abnormalities. Given the location, a provisional diagnosis of chordoma was considered. The lesion was partially resected via a transnasal transsphenoidal approach, and biopsy was obtained. Histopathological analysis revealed sheets of mature plasma cells with eccentric nuclei, abundant eosinophilic cytoplasm, and distinct borders. Immunohistochemistry was positive for CD138 and MUM1, with a Ki-67 index <5% [Fig.2]. No evidence of amyloid or epithelial malignancy was seen, and the lesion was diagnosed as clival plasmacytoma. Further evaluation to exclude multiple myeloma including skeletal survey, serum and urine electrophoresis, CBC, serum calcium, and bone marrow biopsy yielded normal results. A diagnosis of solitary clival plasmacytoma was thus confirmed.

Solitary bone plasmacytoma (SBP) and multiple myeloma lie at opposite ends of the spectrum of plasma cell dyscrasias. SBP is a localized collection of monoclonal plasma cells, commonly affecting vertebrae and skull bones. Although vertebral plasmacytomas are frequently reported, intracranial plasmacytomas, especially those at the skull base, remain rare. The clivus is an uncommon site, with only a few isolated cases reported in literature, typically presenting with cranial nerve deficits due to its critical anatomical location.

Radiologically, clival plasmacytomas appear as osseous-destructive lesions on CT and demonstrate iso to hypointense signals on T1-weighted and iso to hyperintense signals on T2-weighted MRI, with uniform enhancement [12]. Histologically, these tumors are composed of sheets of plasma cells showing varying degrees of cytologic atypia. Hallmark features include eccentric nuclei with clock-face chromatin and expression of plasma cell markers like CD138 and CD38. Light chain restriction confirms clonality [13]. Diagnosis of SBP requires the exclusion of systemic disease. Criteria include a solitary lesion, normal skeletal survey, bone marrow with <5% plasma cells, serum monoclonal protein <2g/dL, no Bence-Jones proteinuria, and absence of anemia, hypercalcemia, or renal impairment [14,15]. Whole-body PET scan plays a crucial role in excluding multiple myeloma.

Solitary clival plasmacytoma is a rare entity and can mimic other skull base lesions like chordoma [23,24]. Radiotherapy offers excellent local control, while surgery plays a diagnostic and symptomatic role. Given the high risk of progression to multiple myeloma, long-term surveillance with imaging and laboratory tests is essential. Clinicians should consider clival plasmacytoma in the differential diagnosis of isolated diplopia, as early recognition and radiotherapy can achieve good outcomes while avoiding unnecessary surgical morbidity.

Contributors: HKG: manuscript editing, patient management; NPS: manuscript writing, patient management. BK: Histopathology and critical inputs into the manuscript. HKG will act as a study guarantor. All authors approved the final version of this manuscript and are responsible for all aspects of this study.

Funding: None; Competing interests: None stated.