Is there any advantage to proton beam therapy in a locally recurrent anal carcinoma, which has already been treated with concurrent chemo RT as well as APR for relapse?  

Even with a local recurrence, patients’ life expectancy can be relatively long, which often introduces the question of repeating radiation treatment. Although this option must be balanced with the potential toxicities and consequences of re-treatment, the possible morbidity from tumor progress...

@Christopher H. Crane's answer cannot be improved upon...very well reasoned and stated, and I agree completely. As he mentions, for the appropriately selected case, re-irradiation with permanent low dose rate implants (Cs-131, Au-198) is an outstanding option from every angle...radiobiological, dose...