It depends on the massive damage rules used. That wizard who casts that 10d6 fireball at his 10th level fighter friend, who happens to have a 14 Con, would easily kill his friend if a more realistic Massive Damage threshold is used.
Average roll of that fireball is 30 hit points damage, which is 16 points higher than the fighter's Constitution. Base Fortitude DC would be 15 + 1 point for every five points of damage that exceeds the Damage threshold for a total of DC 18. Another rule is that the Fortitude DC is equal to the damage dealt.