Is it possible for teeth to decay inside out?

No: Decay starts on the outside and travels inside. However, the penetration may be small and then the destruction occurs like cutting the inside out of a pumpkin. It will seem like it is from inside -> out. Also, you can have internal resorption from inside. This is not decay, per se, but the result is the same.

Decay: Tooth decay starts from the outside and destroys the tooth getting deeper and bigger as it progresses. There is a process called internal resorption that does start on the inside and progress outward. This is not decay, butt an entirely different process.