HI there... paul was on the mark with everything. Diligence is the key here.

One question... did the doctor mention perineural involvement? This is when the tumor touches a nerve. I ask this because my tumor (very similar to yours) was 1.5 cm deep - which is how yours was described. - mine was a little longer but the depth when dealing with nerves is what matters. Perineural involvement usually indicates that follow up treatment is needed.

There are several factors...
-spread to nodes (which you did not have)
-extracapular extension (which is related to nodal involvement)
-and/or perineural involvement.

the premise is that cancer follows the path of least resistance... in this case your nerves.

So based on the depth of your tumor, and the fact that you are moderate to poorly differentiated... I would be very diligent in knowing what is normal in your mouth, and neck and make sure you go for follow up appointments and hightail it back to the drs. at the first sign of anything unusual.

welcome - best of luck and congrats on getting through it.

PS - you may need physio for your neck and shoulder - r/t the neck dissection.

take care.