What are your criteria to determine if there is a local failure versus post treatment changes after SBRT for inoperable early stage NSCLC?  

A growing lesion with increasing FDG avidity at the site of previous SBRT is cancer unless proven otherwise. One must be careful though since some residual FDG uptake can last a year after radiation. PET is not terribly useful at the primary site until at least 6 months after SBRT.

I agree with Craig. Additionally, it's worth a biopsy when those conditions are met. In our early experience the biopsy was negative (and the patient didn't progress) in 50% of the patients. Through the subtle dark arts of academics I've published 2 case reports that were particularly interesting...

There is a set of CT-based high risk features that have been evaluated and appear to be good predictors of local recurrence. These include an enlarging opacity (particularly after 12 months), sequential enlargement, loss of a linear margin, cranio-caudial growth, among others. Here is the article.Th...

This is a good review article talikng about high risk CT findings like the change in maximum width, loss of a bronchogram, effusion and SUV values 5 and above in predicting recurrence.