When considering omission of elective nodal irradiation in the post-op head and neck bilateral dissected pN0 setting, what is considered an "adequately" dissected neck?  

Contreras et al., Phase II results suggests that elimination of PORT to the pN0 adequately dissected neck may be safe for some head and neck primary subsites. Further studies have demonstrated safety and improved quality of life in larger sample sizes for cancers of the oral cavity and p16+ oropharynx.

Given the variations in surgical technique for neck dissection, are there particular H&N subsites, high risk features, # of nodes total/per level that would make PORT omission more cautionary? Would this approach be considered for proton radiation which would further reduce low dose radiation to adjacent nodal basins?