  The escape process of particles accelerated at supernova remnant (SNR) shocks
is one of the poorly understood aspects of the shock acceleration theory. Here
we adopt a phenomenological approach to study the particle escape and its
impact on the gamma-ray spectrum resulting from hadronic collisions both inside
and outside of a middle-aged SNR. Under the assumption that in the spatial
region immediately outside of the remnant the diffusion coefficient is
suppressed with respect to the average Galactic one, we show that a significant
fraction of particles are still located inside the SNR long time after their
nominal release from the acceleration region. This fact results into a
gamma-ray spectrum that resembles a broken power law, similar to those observed
in several middle-aged SNRs. Above the break, the spectral steepening is
determined by the diffusion coefficient outside of the SNR and by the time
dependence of maximum energy. Consequently, the comparison between the model
prediction and actual data will contribute to determining these two quantities,
the former being particularly relevant within the predictions of the gamma-ray
emission from the halo of escaping particles around SNRs which could be
detected with future Cherenkov telescope facilities. We also calculate the
spectrum of run-away particles injected into the Galaxy by an individual
remnant. Assuming that the acceleration stops before the SNR enters the
snowplow phase, we show that the released spectrum can be a featureless power
law only if the accelerated spectrum has a slope alpha > 4.
