Most brown marks on the skin are benign and harmless and result from freckles, birthmarks, the mask of pregnancy, allergic reactions, moles, seborrheic keratosis or reactions to an insect bite, according to WebMD. However, some skin cancers also present as brown or discolored spots.
As WebMD points out, actinic keratosis and actinic lentigines are brown spots on the skin that are sometimes a cause for concern. Actinic keratosis is a precancerous condition that most often occurs on the face, neck or arms and should be checked out by a doctor. Actinic lentigines are more commonly known as brown spots or liver spots. While they are not related to cancer, any change in a brown spot could be an indication of melanoma.