MON-419 Sellar Plasmacytoma: A Commonly Misdiagnosed Sellar Mass

Background:&nbsp;Plasmacytoma, defined as neoplastic proliferation of a clone of plasma cells, is a rare presentation of multiple myeloma (MM). Plasmacytomas very rarely present in the sellar/parasellar region, with a reported prevalence of < 0.1%.&nbsp; Case:&nbsp;51-year-old man with no past medical history presented with a 3-month history of severe headache, fatigue, and back pain. Brain&nbsp;MRI showed a&nbsp;large&nbsp;sellar mass 34 x 25 x 25 mm eroding the clivus and extending into the sphenoid sinus, with bilateral cavernous sinus invasion but no optic chiasm compression. Endocrine evaluation showed central hypogonadism and hypothyroidism: TSH 2.62 uIU/mL (0.45-4.5); free T4 0.8 ng/dL (0.82-1.77); total testosterone 229.3 ng/dL (264-916); LH 4.8 mIU/mL (1.7-8.6).&nbsp;Prolactin and AM&nbsp;cortisol&nbsp;levels were normal. Total protein and gamma gap were elevated. Patient endorsed 2 years of low libido and difficulty with erections. Physical exam showed no visual field deficit or cranial nerve palsies. Patient was presumptively diagnosed with an invasive non-functioning pituitary adenoma and underwent endoscopic endonasal trans-sphenoidal debulking of the mass. Intraoperatively, the mass was found to be fibrous and friable. Frozen pathology revealed a plasma cell neoplasm, and accordingly only a subtotal resection was performed. Final surgical pathology confirmed plasmacytoma with lambda light-chain restriction on immunostains. Post-op CT scan revealed multiple lytic lesions in the pelvis, spine, and bilateral hips. Oncologic evaluation confirmed the diagnosis of IgG lambda MM. Given evidence of systemic MM, patient was initiated on systemic chemotherapy within 20 days of surgery.&nbsp; Discussion:&nbsp;Sellar plasmacytoma is a diagnostic challenge, as preoperative imaging and clinical features may be indistinguishable from those of invasive pituitary adenoma. It often presents with mass effect symptoms such as headache, as seen in this case. However, this patient did not have visual disturbances or cranial nerve palsies (reported in 80% and 65% of cases, respectively). This patient's partial hypopituitarism is also&nbsp;atypical, as hypopituitarism is reported in 15% of cases.&nbsp;Misdiagnosis may potentially lead to inappropriate surgical approach and delay in therapy. Upon diagnosis, complete assessment for MM must be performed, as it is diagnosed concurrently with or during workup of plasmacytoma in 53% and 37% of cases, respectively. Conclusion: This case illustrates the importance of close attention to lab parameters such as gamma gap and total protein and to musculoskeletal symptoms in patients presenting with a sellar mass that were later found to be secondary to MM. Reference:&nbsp;(1) Lee J, Kulubya E, Pressman B, Mamelak A, Bannykh S, Zada G, and Cooper O. Sellar and&nbsp;clival plasmacytomas: case series of&nbsp;5 patients with&nbsp;systematic review of&nbsp;65 published cases. Pituitary.&nbsp;2017 Jun;20(3):381-392