SAT616 Differentiating Resistant Prolactinoma From Macroprolactinemia

Disclosure: S. Lacount: None. P. Belany: None. Introduction: Prolactinomas are the most common secretory tumor of the pituitary gland. Hyperprolactinemia can present as hypogonadism, infertility, galactorrhea, or it may remain asymptomatic. In asymptomatic hyperprolactinemia, macroprolactinemia and other secondary causes should be considered and ruled out. Prolactinomas treated with dopamine agonists typically respond well, with decreasing tumor size and prolactin level reduction. We present a case of a prolactinoma not responding biochemically or anatomically to maximum doses of dopamine agonists, concerning for resistant prolactinoma versus macroprolactinemia. Clinical Case: A 62 y.o. male with PMH of HTN, HLD and CAD who presented with fatigue and headaches in 2009. Initial work-up showed an elevated prolactin level to 178 ng/mL (reference range 2.0-18.0 ng/mL). MRI pituitary showed a 1.8 x 1.9 x 1.5 cm mass but without optic chiasmal compression. Patient refused medications and opted to pursue endoscopic endonasal approach (EEA), which was completed in March 2009. Pathology and hormonal immunostains confirmed prolactinoma. Due to persistently elevated prolactin levels following EEA, cabergoline was started. Follow-up MRI Pituitary in November 2012 showed stable heterogeneous residual tissue without tumor recurrence. Despite escalating cabergoline doses (up to 8 mg weekly), his prolactin levels remained slightly elevated (50 ng/mL) after surgery with unchanged symptoms from presentation. Due to the association of cabergoline and cardiac valve disease development, his dose was decreased back down to 0.5 mg twice weekly. Given his persistent asymptomatic hyperprolactinemia that was nonresponsive to high-dose cabergoline, there was consideration for macroprolactinemia causing a falsely high level. Screening polyethylene glycol (PEG) precipitation test in November 2022 was normal. MRI pituitary in December 2022 showed a progressive sellar lesion with increased size and cavernous sinus invasion near the carotid artery. Patient was referred to neurosurgery in January 2023 for further evaluation of potential resistant prolactinoma. Due to the MRI findings and persistently elevated prolactin levels, he will undergo repeat EEA in May 2023 for treatment of resistant prolactinoma. Conclusion: Dopamine agonists are very effective in treating prolactinomas. In asymptomatic hyperprolactinemia, PEG precipitation is an inexpensive test to screen for macroprolactinemia. Once macroprolactinemia and other secondary causes are ruled out, resistant prolactinoma can be considered. Resistant prolactinoma is defined by failure to achieve normoprolactinemia and at least 50% reduction in tumor size on maximum-tolerated therapy. Risk factors for resistant prolactinoma include larger tumor volume and male gender. Treatment options include different or increased dopamine agonist doses, radiotherapy and resection. Presentation: Saturday, June 17, 2023