Pituitary adenoma pathophysiology

On gross pathology, a well circumscribed suprasellar mass is a characteristic finding of pituitary adenoma. On microscopic histopathological analysis, a monomorphic expansion of usually one cell type with lack of reticulin network among neoplastic cells is a characteristic finding of pituitary adenoma.

Pituitary adenoma may be classified into 4 grades based on radioanatomical findings:^[1]

• Stage I involves microadenomas (<1 cm) without sella expansion
• Stage II involves macroadenomas (≥1 cm) and may extend above the sella
• Stage III involves macroadenomas with enlargement and invasion of the floor or suprasellar extension
• Stage IV involves macroadenomas that cause destruction of the sella

• Pituitary microadenomas are defined as adenomas less than 10 mm in size.^[2]
• Most frequently diagnosed as a result of investigating hormonal imbalance.
• They are confined to the sella and have no scope to produce mass effect related symptoms.

• 
  Histopathological image of pituitary adenoma with GH production. Acidophilic cell type. Hematoxylin & esoin stain.^[3]
• 
  Histopathological image of pituitary adenoma with GH production. Acidophilic cell type. Hematoxylin & esoin stain.^[3]

• Pituitary macroadenomas are the most common suprasellar mass in adults.^[4]
• They are defined as adenomas greater than 10 mm in size and are most frequently diagnosed due to compression of the surrounding structures, such as the optic chiasm.
• Larger adenomas can lead to hormonal imbalance due to mass effect rather than secretion.
• Hypopituitarism or moderately elevated prolactin are both seen, the latter due to stalk effect. Prolactin release (unlike other pituitary hormones) is tonically inhibited by prolactin inhibitory hormone (dopamine) and as such compression of the pituitary infundibulum can result in elevation of systemic prolactin levels due to interruption of normal inhibition.
• Macroadenomas are approximately twice as common as micoadenoma.

• 
  Histopathological image of nonfunctioning pituitary adenoma. Hematoxylin & eosin stain.^[5]

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