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 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6200913/).