Roger, as long as the DCC system running the layout is supported by ProtoThrottle then this would work as others have said. But you would have a limited range of functions as the MTH DCC decoder doesn't support some of the CVs and functionality needed to get the best out of ProtoThrottle.
In particular, momentum/coasting and separate braking functions are important for the most realistic PT operation. Typically the latest decoders from Loksound, Soundtraxx and TCS are more adaptable and can match the physical inputs with decoder/loco behaviour that makes PT more realistic.
ProtoThrottle has increased my enjoyment of the hobby significantly and I highly recommend it! 
