What factors, if any, (such as microscopic ENE (<=2 mm) or larger nodal size (e.g., >4 cm)) would prompt you to treat the contralateral neck for oropharyngeal cancers otherwise eligible for unilateral radiotherapy?   

From my review of literature, the rate of contralateral neck failures with ipsilateral radiotherapy for lateralized tonsil T1-2 N2a disease is very low, as below. However, only a minority of studies comment on ECE status or the size of N2a nodes in the range 3 to <6 cm (bigger potentially more obstructive leading to greater risk of backflow). 

- Maskell et al 2019: 0/5 
- Chin et al 2017: 0/10 (among all ipsilateral patients: LN >3 cm 46%, ECE 77%)
- Kim et al 2017: 0/6 (among all ipsilateral patients: ECE 26%)
- O'Sullivan et al 2016: 0/11 
- Kennedy et al 2016: 0/8 
- Rackley et al 2016: 0/12 (among all ipsilateral patients: LN >3 cm 45%, ECE 27%)
- Ye et al 2015: 1/7
- Dan et al 2015: 0/14 (among neck dissection patients: ECE 52%)
- Lynch et al 2014: 0/31 (among all ipsilateral patients ECE 20%)
- Liu et al 2014: 0/14 
- Koo et al 2013: 0/2 
- Al-Mamgani et al 2013: 0/18 (among all patients: 18% ECE)
- Chronowski et al 2011: 0/21 
- Rusthoven et al 2009: 0/3 

Cumulative contralateral recurrence risk from above studies = 1/162