For very large locally advanced head and neck primary malignancies (namely SCCa), how do you decide between adaptive re-planning of a VMAT/IMRT plan during chemo-RT as option instead of induction chemotherapy?

Like Dr. @Dr. First Last nicely noted, there is no survival benefit associated with induction outside of unique circumstances (such as NPC), so we would not routinely use it.

Where induction could be helpful over adaptive VMAT/IMRT CRT, in our practice, are in circumstances for locally advanced HPV+...

After 3-4 negative trials of induction chemotherapy (assuming not nasopharyngeal EBV+ in origin), in general, would start with RT/chemo.

Sometimes we don’t have a choice and the chemotherapist dictates induction.

But, if we have a choice, we should push for concurrent CRT and use a gestalt to decide when to replan - if PTV going outside the skin, high dose lines crossing into OARs, OARs shifting into the high dose area, large weight ...