How would you approach a patient receiving lung SBRT whose tumor appears to grow partway through their treatment course?

It kind of depends. Which is always the perfect non-answer answer.

1. Degree of change. If just a little bit, press forward. If it's a lot (which I have not seen for a 3-5 fraction course), then I would pause and replan.
2. If you think it is really substantial growth, you can give more dose, so practical...