The most common "precancerous" skin lesion is an "actinic keratosis", which is a sun-damaged area of skin that usually appears as a red and scaly patch of skin. Because it is caused by sunlight, the most common location for actinic keratoses is the face, ears, scalp, arms, upper back, and upper chest. These lesions are labeled as being "precancerous" because without treatment, a small percentage of these lesions develop into squamous cell carcinoma, which can be a very invasive skin cancer. It is important to note that not all squamous cell carcinomas develop from actinic keratoses. Although the time of progression from actinic keratosis to squamous cell carcinoma is variable, it is advisable to treat all actinic keratoses. The most common treatment for actinic keratoses is cryotherapy, which is "freezing" the lesions with liquid nitrogen. This is a very quick and effective therapy with minimal pain and scarring. For a person with numerous lesions, treatment with specific creams can chemically treat these lesions. The two most common creams used to treat actinic keratoses are 5-fluorouracil and imiquimod. Both creams cause treated areas to become very red and inflamed during treatment but this subsides after the treatment period. Lesions are usually treated with the cream for approximately one to three months. It is advisable to monitor an actinic keratosis which does not resolve with therapy as this may indicate is has progressed to become cancerous.
The other commonly treated precancerous lesion is the "dysplastic nevus", also known as an atypical mole. These lesions are usually brown or black moles which are biopsied due to having suspicious features of melanoma, such as irregular borders or asymmetry or having multiple colors. While not cancerous, these lesions are also clearly different from harmless, benign moles. These atypical moles have the potential to develop into melanoma, which is why it is advisable to have any changing or growing mole checked as it may be atypical and changing into a cancerous lesion. Importantly, not all atypical moles are equal; some moles are more "atypical" than others. This means that, under the microscope, some moles are closer to being normal in appearance while others are closer to melanoma. If you are diagnosed with an atypical mole, discuss the diagnosis and treatment options with your dermatologist. Treatment for atypical moles usually entails excision of the lesion with a small margin of normal skin, to insure complete removal. However, since each mole has its own features, always talk with your dermatologist regarding the proper treatment for an individual mole.
I hope this sheds some light on the term "precancerous". Of course, the best treatment is to prevent cancerous growths from developing so always remember to check with a dermatologist for new and changing growth and moles on the skin.