I dont understand how a cancer that has been surigally removed with clear margins can appear in your neck nodes a couple of months later,if the margins are clear how did the cancer escape to the neck,im sorry just confused how this awful thing works

nov 2013 ssc of buccal mucosa stage 2
jan 2014 neck dissection because of cancer in lymphnodes
feb 2014 6 weeks of radiation
march 2015 tumour buccal mucossa stage 4
june 2015 lower jaw removed
nov 2015tumour in left node on neck
jan 2016 radiation and chemo to commence
Nov 2015 left deck dissesction,1 posative node