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.
A Red priestess from Asshai, Melisandre worships the Lord of Light. Her visions have told her that Stannis is the true king and as his advisor, she has encouraged him to pursue the throne at all costs.