I was at court the other day (I'm a lawyer), after just having some laser treatment which results in bleeding and bruising around my neck scars...I got back to my office and already people were calling my partners (notably not me) to see if I was ok, because I "didn't look good". Apparently the rumors were flying for a second time that I was dying...
I then, same day, went into the store in our building and the counter person said "whoa - battle scars". I smiled and said "cancer", and she was speechless, then wished me the best.
At this point, all I can do is shrug and let it slide off. The people that matter know the drill, and that's all that matters anyway.
For the most part people don't mean to be insensitive - it's kind of like asking the overweight woman when she's due to give birth. People are just dumb sometimes.