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 Baratheon is Azor Ahai come again, the warrior of fire. In him the prophecies are fulfilled. The red comet blazed across the sky to herald his coming, and he bears Lightbringer, the red sword of heroes.