The radiation oncologist examined it visually and with his finger (Ow!) when I was at the med center for a neurology appointment about the buzzing sensation in my legs (which turns out to be Lhermitte's Syndrome, a fairly rare, temporary condition from radiation damage to the nerve sheathing on the spinal chord in my neck).
He said the sore is a shallow ulcer, not uncommon, probably caused by a food irritation. He said to watch it, but it's likely to heal within a couple weeks.
And the quick pulses of numbness from Lhermitte's are not a big deal, especially now that I've read about the condition.