It's not uncommon for cancer to return, minus HPV related, due to field cancerization in tobacco, carcinogen related cancer, and could be metachrounous or synchronous cancer too, and one could even being chemo and radio resistant due to other factors. I had roaming cancer, and had surgery to remove tumors in level III, V last year, had surgery, and in less than a month cancer went into epidermal layer of the skin in level V, and needed further surgery, high dose radiation during surgery, pec flap, and more chemo and radiation. As Cheryl mentioned, chemo alone usually does not kill cancer, if it does, it usually returns, and happened to me at 8 months due to not being able to have further curative treatment at that time due to illness, but did resume radiation treatment after a year from my original chemo induction.

The throat irritation, coughing, can happen after radiation, and I still have it two years later, but speak with the doctors to see if it is caused by a tumor.

Good luck with everything.