ABSTRACT

It is often stated that basal cell carcinomas are more common than squamous cell carcinomas, but this is partly a matter of definition. If premalignant lesions (mainly Bowen's disease and actinic keratosis) are included in the definition, the incidence of squamous cell carcinoma might be much higher than that of basal cell carcinoma 1 . The main argument to do so is the fact that the distinction between actinic keratosis and squamous cell carcinoma is difficult. Furthermore, in the majority of squamous cell carcinomas, adjacent areas of actinic keratosis are found. Both share the etiology of excessive sun exposure and both lesions are identical under the microscope. It is therefore plausible to regard actinic keratosis as a precursor of squamous cell carcinoma. In fact, more than 60% of squamous cell carcinomas will develop out of actinic keratosis 1 . However, in spite of the similarities between actinic keratosis and squamous cell carcinoma, it is not justifiable to group them together. First and most importantly,

most actinic keratosis will never develop into squamous cell carcinoma 2 . The incidence that this will happen is estimated to be between 0.1 and 10% 3 . Second, the practical implications would be enormous. Most actinic keratoses are treated by non-invasive, locally destructive methods (coagulation, freezing, simple scraping off), whereas proven squamous cell carcinomas require resection, cryosurgery, or radiation therapy. If pathologists were to express doubt over the malignant potential of actinic keratosis, it is likely that this would result in overtreatment.