How long after high-dose IV methothrexate for chemo-refractory CNS lymphoma do you wait before giving brain radiation?

As was taught by my mentor- "As long as possible!". Of course, the prognosis is extremely poor if the patient is chemo-refractory to IV MTX and longer-term risks of WBRT may be less relevant than present-day symptoms requiring palliation. While I do recommend WBRT, as what is visualized on MRI is ju...