It can be a combination of factors, In addition to the lymphodemia. it can also be a fungal infection, like thrush or bacterial infection or combination, which can make the tongue swell. Some medications cause tongue, mouth swelling too, besides the cancer treatment. Other factors may also be inhibiting eating like mucocitis, ulcers, throat inflammation, and treating these with proper oral care, rinsing, avoiding spicy, acidic, hot, and sharp textured foods, antifungls, antibiotics, antivirals or ani-inflammatory agents, may improve eating, but see your doctor before self diagnosing.
As far as the toothbrush Satyanarayana asked about, some use soft baby toothbrushes, and theese brushes too can be cut down, use q-tips instead of a tooth brush, there are sponge brushes on sticks, usually yellow or green, that can be used, cut if needed, cotton gauze on fingers, and also the WaterPik on low setting can be used to cleanse the mouth, salt/baking soda, or saline solution added to resevior to rinse, but I returned mine, so maybe someone else has more experience how to use, and heard of a tongue scraper attachment, I think.
Good luck.