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.
Davos visits the tent of Stannis's daughter Shireen Baratheon to say goodbye. She is enjoying reading a book by Grand Maester Munkun about the Dance of the Dragons, a major civil war in the history of the Seven Kingdoms.