For linac based fractionated radiosurgery (5 fx) to a brainstem metastasis, is there a dose constraint for normal brainstem outside of the target volume?   

More data are needed on brainstem tolerance for multi-fraction SRS. I use the AAPM TG101 suggested constraints of <0.5 cc >23 Gy and <0.035 cc >31 Gy. For metastases, one could certainly consider exceeding these constraints, as tumor recurrence would result in similar (if not worse) pot...