Conversely, a film about prison does not necessarily have to be set in one. David Hayman's film Silent Scream (1990) concerns the suffering and mental anguish brought on by incarceration, yet this film is not predominantly set in prison. We're No Angels (1955), Breakout (1975), In the Name of the Father (1995), and Sleepers (1996) could all be seen as concerned with prison, yet a significant part of each film takes place outside the prison walls. Furthermore, Laurel and Hardy in The Hoose Gow (1929), Elvis Presley's Jailhouse Rock (1957), and Porridge (1978) -- all set within prison walls -- are merely star vehicles with prison as a backdrop.
Having considered these problems, I settled on the following definition of a prison movie: "a film which concerns civil imprisonment and which is mainly set within the walls of a prison or uses prison as a central theme." While not without its flaws (not least of which being the well-documented problem of delineating genre), this working definition allows us to begin discussing the prison movie as a film genre.