Did the Efudex work? An Efudex experiment

I'm trying an "Efudex experiment" in which I reapply Efudex to a few small areas that were treated a few months ago to see if they react. My dermatologist agreed that if the first treatment really removed most of the sun-damaged cells, then the skin should not "light up" in response to a second treatment. So a week ago I started applying Efudex twice a day to my left cheek and to a small area (about two square inches) on my scalp.

What happened? The area on my scalp only had a small reaction. But my cheek lit up just as fast and intensely as during the original Efudex treatment. I can also feel it starting to burn like the first time.

What does this imply? Clearly, whatever property of the skin on my cheek made it react to the Efudex was not changed by the first Efudex treatment. In other words, if the reaction is due to sun damaged cells, then there are still a lot of sun-damaged cells there.

What to do next?  I sent an email to my dermatologist with this picture to see what she advises. I proposed to keep applying the Efudex, but go longer than last time -- perhaps 3-4 weeks instead of just two.