The pituitary gland is anatomically divided into the anterior and posterior pituitary glands, also termed adenohypophysis and neurohypophysis, respectively. The neurohypophysis is formed by the pars nervosa and the infundibular stalk. The pars nervosa acts as a repository for oxytocin and vasopressin, accounting for most of the volume of the posterior pituitary. The infundibular stalk connects the hypophyseal and hypothalamic systems. The anterior and posterior pituitary lobes are divided by a thin boundary, termed the pars intermedia. Most adenohypophysis cases are caused by the pars distalis, which stores ACTH, TSH, FSH, GH, LH, and prolactin. The pars tuberalis is an extension of the pars distalis, encircling the infundibular stalk of the neurohypophysis.^9,10).

Embryologically, the neurohypophysis and adenohypophysis originate from the neuroectoderm and ectoderm, respectively. At the 4th week of gestation, the extracranial ectoderm of the primitive oral cavity is invaginated upward, forming the Rathke’s pouch. The neuroectoderm forms an infundibular process that extends downward in contact with the invaginating ectoderm. At weeks 6–8, the Rathke’s pouch pinches off, giving rise to the pars distalis, pars intermedia, and pars tuberalis. The pituitary formed resides within the mesoderm-derived sella turcica. The majority of pituitary adenomas, ectopic to the pituitary, originate from the remnant rest of the craniopharyngeal duct during the embryonic period11. Pituitary adenomas in the pituitary stalk not continuous to the gland are mostly considered to originate from the pars tuberalis^12,13).

Pituitary adenomas on CT are usually iso-dense to gray matter and moderately enhanced by contrast dye. On MRI, the pituitary shows a low signal on T1WI and a high signal on T2WI. MRI with gadolinium shows mild-to-moderate enhancement^11,14); however, it is sometimes difficult to distinguish the ectopic form of pituitary adenoma from other juxta-sellar lesions such as craniopharyngioma, germ cell tumors, or neuro-epithelial tumors, even with a long-term follow-up of imaging studies¹²⁾.

As for our circumstance, there are several reported cases that mimicked other kinds of intracranial tumors on imaging and were later proven otherwise by pathologic confirmation^15,16). It has been debated that weighing hormone levels earlier could avoid unnecessary biopsy and/or surgical interventions^11,17). However, deeming surgery unnecessary may just be a conclusion by hindsight⁵⁾. In a comprehensive review published in 2019, Shuman et al. reviewed 85 cases of EPAs from 64 eligible publications out of 116 publications searched to date. According to the review, EPAs tend to be more active. Approximately 85% of tumors (72 out of 85 cases) were secretory; 50 cases were clinically functional and 35 were non-functional. This is a significantly higher ratio than that of pituitary adenomas. More importantly, in contrast to most pituitary tumors being considered benign, six out of 85 EPAs were reported to be malignant⁵⁾. Furthermore, Mason et al. reported 10 cases of ACTH-secreting EPAs and found that only six cases showed corresponding MR with operative findings⁸⁾.

Considering that ectopic pituitary adenomas and pituitary adenomas in general have a slightly different pathophysiology, and that imaging studies can only provide a limited amount of information, the need for tissue confirmation is crucial for diagnosis. In our case of asymptomatic and ectopic pituitary adenoma, although follow-up observation was considered as an option, consequent growth to a symptomatic mass was one of the concerns. Furthermore, an initial imaging study suggested multiple differential diagnoses. Thus, preservation of the pituitary stalk, surgical biopsy, and removal were decided.

The transcranial approach is often considered when managing suprasellar lesions, in addition to trans-sellar, transdiaphragmatic, pterional, and trans-sphenoidal approaches^11,18-19). In our case, the trans-sphenoidal endonasal approach was selected.