Is there an elapsed time interval from prior radiation to the brainstem or optic chiasm for which you would not take into account the prior dose?  

Is there a point (ex > 15 or 20 years) when the dose can be "forgiven"? There are some manuscripts that give cumulative dose recommendations for shorter intervals between radiation treatments or discuss CNS toxicity with reirradiation but the intervals between treatments are generally not long, e.g. 2.5 years (