David,
I was to told about 2cm density can be effectively handled by radiation alone, but different cancers react differently and I don't know if this is correct for HNSCC.

I understand it has to do with the way the core of a solid tumor sustains a state of protective hypoxia which the treatment plan must be able to overcome.

I don't know the effective density that concurrent chemo/rad therapy would add, but I thought for large tumors they first use induction chemo to shrink the tumor and then they use chemo-rad and/or surgery.

I think with large solid tumors in structures like BOT they also have to consider the void that might be left after sterilization by RT.