SUN-432 A Case of Sellar Plasmacytoma Masquerading as Pituitary Carcinoma with Hypopituitarism

Background: Sellar plasmacytomas are rare tumors arising from clonal plasma cells localized to the sellar/parasellar region. They may mimic other pituitary pathology such as pituitary adenomas, chordomas, meningiomas or metastatic carcinomas in both clinical manifestations and radiographic evidence. Certain features such as predominance of cranial nerve palsies and absence of hypopituitarism appear to be more common with sellar plasmacytomas. We present an interesting case of a sellar plasmacytoma, initially mistaken for pituitary carcinoma, with rapid development of hypopituitarism and systemic multiple myeloma. Case: A 75 year old healthy male presented with sudden onset left eye diplopia and retro-orbital pain. Left eye ptosis and diplopia with left lateral and upward gaze were noted on exam. Initial MRI brain revealed a 17mm soft tissue mass involving pituitary and clivus, extending into the left superior orbital fissure, concerning for pituitary carcinoma. With the exception of low LH (1.2) and testosterone (25), baseline pituitary hormone labs, complete blood count and metabolic panel were within normal limits. Within 1 month, he acutely presented with fever, confusion, weakness, and bony pains. Repeat pituitary labs showed low ACTH < 5, AM cortisol 4.3, TSH 0.07, FT4 0.5. New findings of acute renal injury, hypercalcemia, anemia with elevated Mspike, Kappa light chain 39.7 (0.3-2.0mg/dl), kappa/lamda ratio 66.2 (0.3-1.6) and beta2microglobulin 4484 were found. Repeat MRI brain noted multifocal osseous metastatic disease with left sellar and cavernous sinus lesion contiguous with clival osseous lesion displacing pituitary gland, concerning for plasmacytoma. PET/CT showed focal lytic lesions in clivus and posterior left iliac bone. Bone marrow biopsy confirmed 70% myeloma involvement with kappa monotypic plasma cell population. He was started on hydrocortisone, levothyroxine, chemotherapy and XRT. Conclusion: The rarity of plasmacytomas, combined with their clinical presentation and imaging characteristics similar to other sellar tumors, can often result in misdiagnosis. Accuracy of diagnosis is critical, as sellar plasmacytomas and underlying multiple myeloma (present in 50% of cases) require XRT and systemic chemotherapy respectively rather than surgery. Although pituitary function is usually preserved in plasmacytomas, our case shows that acute anterior pituitary dysfunction can be an early presenting sign.