The diagnosis of "atypical mole" is a term that is becoming all too common to describe what is seen underneath the microscope when moles are removed that appear suspicious on clinical exam. The diagnosis is considered benign to allay any concerns for morbidity associated with this diagnosis, however what does it really imply?
When a pathologist is examining a mole microscopically he is looking at a number of variables: the overall symmetry of the lesion based on the aggregates of pigment cells throughout the layers of the skin and then the overall appearance of the nucleus which is the command center of life and the home of DNA. The structure of the mole is an assessment of the "architectural atypia." The degree to which there is loss of healthy features such as a decrease in size of the cells as you go deeper into a mole, the symmetry of the overall lesion, will dictate the rendering of mild, moderate or severe architectural atypia. Likewise, if a nucleus is enlarged with prominent features, or mitotic figures, indentations then the assessment of mild, moderate, or severe atypia is given.
As a general rule, the dermatologic surgeon will use the diagnosis of mild, moderate or severe atypia as one factor to consider, along with the degree of concern for melanoma based on what the lesion looked like on initial exam, to determine if re-excision is necessary. Again, the diagnosis is considered benign, however one can never be too cautious when dealing with an atypical mole.
Because these lesions are removed, or at least partially removed, with the biopsy method to sample the tissue, we do not have definitive data available to follow these lesions over time with 100% certainty as to specific risk of melanoma if untreated. Studies have been done with mixed results.
The outcome of not removing an "atypical mole" depends on the degree of atypia seen. One would theoretically have a greater risk of developing a melanoma if the mole were deemed "severely atypical" than one would if the final diagnosis was "mild atypia."
The best practice with this diagnosis is to know your dermatologist and have a great deal of trust over time with his/her experience with atypical moles and skin cancer. This is an area of much debate and controversy on both the clinical dermatologist side and the dermatopathologist side.