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.
First mentioned in 1996's A Game of Thrones, Stannis is the son of Steffon Baratheon from the kingdom of Westeros. He subsequently appeared in Martin's A Clash of Kings (1998) and A Storm of Swords (2000) and A Dance with Dragons (2011) Stannis is portrayed by Stephen Dillane in the HBO television adaptation.