Cutaneous collision tumors. An analysis of 69 cases and review of the literature.

The association of two different neoplasms in the same biopsy specimen is not uncommon and has been reported by numerous authors. Some associations may result from involvement of related cell types, however, most are serendipitous. Our study attempted to investigate the nature and types of contiguous or "collision" tumors seen in our dermatopathology service. A retrospective evaluation of approximately 40,000 cutaneous biopsies was performed, yielding 69 examples of collision tumors. Excluded were biphasic lesions of the same cell type (e.g., basosquamous carcinomas or combined nevi). Results were as follows: The most common combinations included basal cell carcinoma and nevus (14), basal cell carcinoma and seborrheic keratosis (8), nevus and seborrheic keratosis (14), actinic keratosis and nevus (7), and basal cell carcinoma and neurofibroma (4). Adnexal tumors were found combined with other neoplasms in four patients and blue nevi were noted in three additional tumors. We believe that most of these represent the presence of two or more common lesions juxtaposed by coincidence. However, certain combinations are of similar cell lineages or contain a relatively uncommon tumor and may indicate a possible link between the different cell types.