Have you tried gargling with a mixture of salt and baking soda. Fill an 8 oz glass with warm water ad 1/4 tsp salt and 1/4 tsp baking soda, and stir. It usually takes 4-6 gargles to finish the glass. I do this twice a day but you can do it more often. Make sure you get the back of your throat.
You might want to speak to your oncology team for advice on how to take care of this condition - I got the above from the oncology nurse practitioner in my team.