Case Report: Rare collision tumors: ACTH-secreting pituitary neuroendocrine tumor and pituicytoma – histopathological and ultrastructural analysis

Collision tumors are rare conditions characterized by the coexistence of two histologically distinct neoplasms in the same region without histological admixture or an intermediate cell population zone. Our report represents one of the few studies on a collision tumor composed of an ACTH-secreting pituitary neuroendocrine tumor (PitNET) and a sellar pituicytoma. Here, we describe the clinical presentation, radiological findings, immunohistochemical/histopathological analysis, and ultrastructural examination of a 21-year-old woman with two different intracranial primary tumors at adjacent sites. Magnetic resonance imaging of the sellar region revealed a focal, 4 mm, nodular lesion in the left lateral recess that was hypointense on T2 and compatible with a micro-PitNET. Transnasal transsphenoidal resection was performed. Histopathological analysis revealed patterns consistent with a corticotroph PitNET, with diffuse cytoplasmic ACTH immunostaining and a Ki-67 index of 4%. Furthermore, the specimens included scattered adenohypophyseal and neural tissue. The latter was characterized by increased neoplastic proliferation consistent with a pituicytoma, confirmed by nuclear positivity for TTF-1 and a Ki-67 index of 3%. Ultrastructural analysis confirmed the coexistence of two morphologically distinct lesions. Postoperatively, the patient developed diabetes insipidus and remained in clinical and biochemical remission with no remaining tumor at nine months post-surgery. Pituitary collision tumors are sporadically reported and rare. Specifically, pituicytomas associated with pituitary hyperfunction are notably infrequent, and those linked to Cushing's disease are extremely uncommon. This case highlights the exceptional rarity of sellar collision tumors and underscores the importance of reporting such cases to improve recognition, refine diagnostic strategies, and expand current understanding of complex pituitary tumor biology.