Alison, it's not unusual that you have increased imbalance symptoms after radiation, and onset at the four-month mark is quite typical. This will get better over time. As for vestibular therapy, that's likely to help. But you can do your own vestibular therapy: walk on uneven ground, spin in a chair that can rotate, etc. Basically, the more you challenge your balance, the quicker your brain will adapt and learn increasingly to ignore your vestibular system and rely on your vision and proprioceptors for balance cues.
You can also help reduce imbalance symptoms by avoiding foods and beverages that increase inflammation: sugar, caffeine, alcohol, etc. Eat foods rich in omega-3's , such as salmon, trout and sardines. Omega-3's help reduce inflammation. And stay well hydrated and get lots of sleep (nothing is more important than that, as it increases blood flow to your brain and allows your body to heal).
All of these strategies helped me immensely. I hope they help you, too!
Best wishes,
TW