Would you consider definitive radiotherapy for IPMN in a patient who is not a surgical candidate?

This is an interesting question, but no I would not. It does seem like a good idea. However, the dose of and impact of definitive radiation in the setting is not defined because there are no published data. In general, treatment of any patient without invasive disease is problematic unless the pre-invasive disease is symptomatic. We don't know the dose to use and any complication such as bleeding could be more problematic than someone with proven cancer. One could consider developing a clinical trial, but it does not seem likely that local treatment will have a survival benefit for IPMN. It is not even clear that there is one with surgery.