What dose constraint do you use for the proximal bronchial tree for when treating NSCLC to 60 Gy/8 fx?

The Canadian LUSTRE trial (which randomizes patients to 48/4 or 60/8 versus 60/15) accepts a dose limit of 64 Gy max, and 60 Gy to 5 cc to the PBT for a 60/8 fractionation schedule. Typically we aim to keep the dose relatively less heterogeneous within the target so as not to draw any significant hot spots near the PBT.

A very helpful reference based on UK consensus for normal tissue constraints when considering 3, 5, 8 # SABR

https://www.clinicaloncologyonline.net/article/S0936-6555(17)30434-X/pdf

Note that there will always be the dilemma - prioritize OAR vs. PTV? I think the issue is nuanced and comes down to the willingness to accept toxicity vs. willingness to accept reduced probability of tumour control.