  GeV and TeV gamma rays have been detected from the supernova remnant W28 and
its surroundings. Such emission correlates quite well with the position of
dense and massive molecular clouds and thus it is often interpreted as the
result of hadronic cosmic ray interactions in the dense gas. Constraints on the
cosmic ray diffusion coefficient in the region can be obtained, under the
assumption that the cosmic rays responsible for the gamma ray emission have
been accelerated in the past at the supernova remnant shock, and subsequently
escaped in the surrounding medium. In this scenario, gamma ray observations can
be explained only if the diffusion coefficient in the region surrounding the
supernova remnant is significantly suppressed with respect to the average
galactic one.
