What dose volume constraints do you use with repeat stereotactic radiosurgery for targets adjacent to brainstem?

What cumulative and/or second SRS brainstem dose constraint(s) do you use when considering reirradiation of a lesion adjacent to the brainstem. Would this change if it was a persistent arteriovenous malformation adjacent to brainstem after prior radiosurgery versus a recurrent malignant lesion adjacent to the brainstem?