MON-124 Relapse of Multiple Myeloma Presenting as a Sellar Mass

Abstract Disclosure: K. Winnicki: None. Z.A. Ahmed: None. G. Olson: None. T. Porter: None. Background: The differential of a sellar mass is broad, and prior clinical history is important to take into consideration when formulating a differential diagnosis. Case: A 53 year-old gentleman with a history of multiple myeloma in remission who had undergone an autologous stem cell transplant within the last two years presented for several days of intermittent diplopia. Given the acuity of symptoms, an MRI was performed and revealed a T1-T2 isointense lesion in the right half of the sella extending into the right cavernous sinus with relative hypoenhancement compared to the pituitary gland; leftward deviation of the infundibulum was also noted. Functional testing revealed an 8am cortisol of 11.5 ug/dL (3.7-19.4 ug/dL) with ACTH of 8.8 pg/mL (7.2-63.3 pg/mL), free T4 0.9 ng/dL (0.7-1.5 ng/dL) with TSH 1.898 uIU/mL (0.350-4.940 uIU/mL), LH 5.1 mIU/mL (.57-12.07 mIU/mL), FSH 15.8 mIU/mL (1-10 mIU/mL), random total testosterone 185 ng/dL (264-916 ng/dL), and prolactin 21 ng/mL (3-19 ng/mL). Two days later the patient was noted to have a mild right abduction palsy. Given symptoms and history of multiple myeloma, the patient underwent endoscopic endonasal subtotal resection of the sellar lesion. Upon incision of the dura, a firm, white fibrous material with a gelatinous portion in the sphenoid was noted. Pathology revealed cells with prominent nucleoli, enlarged nuclei, minimal perinuclear Hof region, and minimal cytoplasm; these cells were positive for CD138 and CD20 and negative for synaptophysin and cytokeratin 8/18, suggesting a plasmablastic morphology. Monoclonal free lambda light chains were noted on serum electrophoresis, and bone marrow biopsy revealed evidence of plasma cell neoplasm (most recent bone marrow biopsy eight months prior was without evidence of persistent disease). The clinical picture was most concerning for CNS relapse of the patient’s myeloma; he received five fractions of radiation therapy to the right cavernous sinus, and was started on venetoclax, pomalidomide, and dexamethasone with plans to add carfilzomib in the future. Outpatient endocrinology follow-up was advised. Even in patients with known myeloma, such tumors are rare, and plasmacytomas are far less common than pituitary adenomas. Compared to pituitary adenomas, plasmacytomas have a greater propensity for cavernous invasion and parasellar bone erosion, including the cranial nerve foramina, contributing to a higher rate of cranial nerve palsies; one series of 31 patients found that 65% of patients with plasmacytomas had diplopia at presentation (1). Conclusion: Plasma cell tumors should be considered in the differential for lesions involving the sella, particularly in the presence of cranial nerve palsies. (1) DiDomenico J, Ampie L, Choy W, Lamano JB, Oyon DE, Kesavabhotla K, Bloch O. Sellar plasmacytomas masquerading as pituitary adenomas: A systematic review. J Clin Neurosci. 2018 Apr;50:20-23. Presentation: Monday, July 14, 2025