SUN-094 - Delayed Endocrine Evaluation of an Aggressive Skull Base Mass Revealing A Giant Invasive Prolactinoma

Background:
Sellar and parasellar masses with extensive skull base invasion are often presumed malignant. Giant invasive prolactinomas can mimic unresectable skull base tumors radiographically, demonstrating clival destruction, cavernous sinus invasion, and intracranial extension. Delayed recognition may result in preventable neurologic morbidity.

Case Presentation:
A 66-year-old man presented with progressive diplopia and visual difficulty. Exam revealed impaired supraduction of the right eye consistent with superior rectus underaction, with otherwise normal visual acuity and optic nerves. Imaging showed a large destructive clival mass with skull base invasion, lytic bone destruction, and intracranial extension. Initial differential favored primary skull base malignancy. Endoscopic biopsy revealed a low-proliferative, Pit-1–positive neuroendocrine tumor with prolactin immunoreactivity (Ki-67 ~3%), consistent with a sparsely granulated lactotroph PitNET. Extensive invasion precluded surgery. Endocrine evaluation revealed markedly elevated serum prolactin (>1,000 ng/mL), confirming a giant invasive prolactinoma.

Conclusion:
Aggressive imaging does not exclude pituitary origin. Giant prolactinomas can produce extensive skull base destruction while remaining biologically distinct from primary malignancies and are often highly responsive to dopamine agonists. Early hormonal evaluation is inexpensive, widely available, and essential for all sellar or parasellar masses to prevent irreversible neurologic deficits. Sparsely granulated lactotroph PitNETs may behave more aggressively and respond less predictably to therapy, but timely medical management can achieve meaningful tumor reduction. This case underscores the importance of comprehensive pituitary hormonal testing to ensure accurate diagnosis and appropriate management, even when imaging suggests malignancy.

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