To treat a pituitary tumor with radiation, doctors may use a variety of techniques depending on the size and location of the tumor. Conventional radiation covers a wide area in and around the tumor and is usually given daily for several weeks.
In the show, Stannis Baratheon dies under the sword of Jamie Lannister, wielded by a woman to fulfill a promise to Catelyn Stark, to avenge the death of Renly Baratheon. You can state he died of internal complications.
Stannis is the second-born son of Steffon Baratheon and Cassana Baratheon, the younger brother of the late King Robert Baratheon and older brother of Renly Baratheon. Steffon was the head of House Baratheon and Lord Paramount of the Stormlands.