Immunity Question: Toughness

[DamonGalos]

The 30pt version of Immunity seems to allow you to select a saving throw to be immune to, however I have noticed that toughness is never mentioned (it's always reflex, fortitude, will). Is this just implied it can be selected or because Toughness is handled a little differently (ie: a different power level cap than the other saves) its not available as a 30pt purchase and you need to be purchasing 100s of points of immunities to truly be immune to toughness effects (energy, physical, mental blasts that use the toughness save etc etc)