Transition between solar keratosis and basal cell carcinoma

Transition from solar keratosis (SK) to squamous cell carcinoma (SCC) is well known and vastly documented. The possible relation between SK and basal cell carcinoma (BCC) is rarely mentioned in the dermatopathological literature. In order to identify the characteristics of the relation between SK and BCC, 40 slides of the head and neck regions in which both SK and BCC had been diagnosed, were retrieved from a collection in the Institute of Pathology of Beilinson Medical Center, seen between 1984 and 1994. Gradual and continuous transition between SK and BCC was found in 15 (37.5%) of these 40 slides. In order to estimate the prevalence of this phenomenon, 73 additional slides, which had been diagnosed as BCC of the head and neck, were re-examined. Atypia of the spinous layer, as an initial marker for the development of SK, was sought in each slide. Revision revealed spinous layer atypia in 26 (35.6%) slides, in addition to the previously diagnosed BCC. In seven (9.6%) the transition between atypical spinous cells and BCC was gradual and continuous. A gradual and continuous transition between SK and BCC can be explained by the presence of pluripotent stem cells in the epidermis. Stem cells, following malignant transformation, may differentiate in different directions, resulting in both SK and BCC.