What are the effects of gamma knife radiation to the cavernous sinus cavity & pituitary gland due to an invasive atch secreting tumor?

Possible scarring. Pituitary adenomas are nearly always benign tumors, but can be quite locally destructive or infiltrative, making them difficult to remove from all of the nooks and crannies of the region. Radiation provides a way of controlling the growth. Cavernous sinus isn't usually affected, although rare thrombosis can be seen.
You mean sphenoid? The procedure to remedy this by surgery is called transsphenoidal surgery. This is where surgey is used to go through the nose and throough the sphenoid sinus to get to the pituitary gland, and hence, to the adenoma. With todays advances lasers might be used to dissect this out with minimal trauma/damage to the rest of the pituitary and surrounding structures.