Our investigation supporting the feasibility of in situ PET monitoring in proton therapy is presented. We simulated by means of the FLUKA code the number and the spatial distribution of the main beta+ emitters created in PMMA targets by protons at typical therapeutic energies. The quantitative comparison with the activation induced by 12C ions of energies corresponding to the same range shows that the available signal at the same physical dose level should be up to twice as intense for protons than that actually successfully used for the control of carbon ion therapy at GSI Darmstadt. The spatial correlation between the activity and the dose profile for protons is poorer than for 12C nuclei. However, an important check of the particle range, dose localization and stability of the treatment during all the fractions seems to be possible.