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.
James Arness and Milburn Stone are the only two regulars to stay with the show for its entire 20-year, 635-episode duration on CBS. There was one brief exception in 1971. Milburn Stone suffered a heart attack, and Pat Hingle, portraying Dr. John Chapman, replaced Stone for 6 episodes while he recovered.