How do you simulate a patient with a large beard who needs a thermoplastic mask?

I just sim these patients as normal. I assume you're talking about a H&N Ca patient. I counsel these patients that radiation "shaves" their beard, eventually and sparsely/unattractively at that, so it's usually not a big deal to get someone to shave if you so desired it. Also, you can't predict if the beard will lead to much error a priori; you might find via daily IGRT that the patient sets up well enough to treat the target in which you're interested. E.g., a beard probably won't be a problem in WBRT. I'd predict a beard would lead to more systematic/preparation error versus random/setup error, although that's arguable. Sudden elimination of a substantial beard part-ways through treatment would almost certainly be more of a systematic ver...