For a R0 resected large inverted papilloma of the maxillary sinus with invasion into multiple surrounding structures but only a small malignant portion, would you cover the entire papilloma extent or only the malignant portion post-op?  

Interesting question. It is very hard to accurately map out where the malignant portion is. I would treat the entire postop cavity to 60-66Gy.

If there is a very limited focus of carcinoma, I would consider observing. However, if I were to irradiate, I would cover the entirety of the preoperative location of the inverting papilloma.  No one can tell exactly where the carcinoma is anatomically in the surgical specimen.