What To Do For Anactintic Keratosis On The Upper Chest?

I have anactinic keratosis on the upper chest on the left just in the dip of my suraclavicular fossa which I need to treat with cream. However i am having difficulty in locating the small area to be treated with Effudix. Are you able to help me here please? I am a 58 year old woman, whose height is 5' 7" and my weight is about 60 kilos. I run and keep fit. I am on thyroxine having had papillary carcinoma of the thyroid. i have two other marks which the dermatologist has said i should treat. These i have easily identified.Susan Walker