The issue with sun exposure after radiation is really not whether or not you burn, the issue is that the sun is also additional radiation on an area that had already been radiated. Additional radiation exposure to a pre-radiated area simply increases your likelihood of an additional cancer-in this case a melanoma and it does that whether or not you have any sign of burning.

The safest way to protect yourself is to use a high SPF sunscreeen and /or protective clothing. Although some skin exposure to sun is good because it stimulates Vit. D production and Vit. D deficiencies have also been shown to be strongly related to cancer (though all the research I've read on that related it to other kinds of cancer, not oral). But you can put high spf suncreen on your face and neck, where you were radiated, and let your arms get more sun--which is what I usually do in the summer (though I also wear a lower spf on my arms and legs if I'm going to be out for a while).

Why put yourself at higher risk than you have to be for an additional (and deadly if not caught early) cancer?

Nelie