How should masticator space involvement be defined: anatomically or by AJCC's vague definition with regards to Nasopharynx Cancer?

That's a great question. I found the following article (linked below), which showed that the anatomic definition of masticator space independently predicts for increased locoregional recurrence and overall survival, and that since the mandibular nerve passes into the anatomic masticator space, that one should cover radiation fields that you would for T4, as the mandibular nerve can be a pathway for retrograde transit of cancer cells from the masticator space to the base of skull via the foramen ovale. From there, it could spread to the gasserian ganglion and to the cavernous sinus. I don't know what others do, but in this type of cancer, it probably is a good idea to get aggressive and not miss.

Tang et al., PMID 20697119