Inaugural Giant 18-cm Solid-Cystic Plasmacytoma of the Calvaria with Superior Sagittal Sinus Invasion: A Rare Mimicker of Aggressive Primary Bone Tumors

Abstract: Background: Giant calvarial plasmacytoma is an exceptional inaugural presentation of Multiple Myeloma (MM), often posing a diagnostic challenge due to its aggressive radiological appearance and atypical cystic components. Case Presentation: A 56-year-old male presented with a massive, rapidly progressive frontal mass (18 cm) evolving over 15 months. MRI revealed a voluminous solid-cystic lesion causing extensive bone destruction, invading the superior sagittal sinus (SSS) and the left orbit. Functional MRI (DWI/ADC) suggested high cellularity, while venous TOF-MRA confirmed the amputation of the anterior SSS. Systemic staging via PET-CT identified a hypermetabolic D6 vertebral lesion. Histopathology of the cranial mass confirmed a plasmacytic proliferation, establishing the diagnosis of Multiple Myeloma. Conclusion: This spectacular case emphasizes the need to include plasma cell dyscrasias in the differential diagnosis of giant cystic-solid skull tumors and highlights the critical role of advanced neuroimaging in evaluating dural and venous sinus involvement for surgical and oncological planning.