Abstract
BACKGROUND: Tumor collision is a rare event, with an estimated incidence of 0.0017%. Seborrheic keratosis, melanocytic nevi, and basal cell carcinoma (BCC) are by far the most common entities involved in collisions. Most authors consider collision to be an incidental event. I planned a retrospective study comparing BCC/apocrine-sebaceous-follicular unit (ASFu) neoplasm collisions with squamous cell carcinoma (SCC)/ASFu neoplasm collisions. MATERIALS AND METHODS: Files from 2005 to 2017 from Dr. José Molina Orosa Hospital were assessed; in the review, cases of collisions between BCCs or SSCs and ASFu tumors, including cysts, were identified. RESULTS: Out of 3247 BCC cases, 12 biopsies were retrieved. Of 825 biopsies, none belonged to the SCC group. The ASFu tumors that collided with a BCC were as follows: four hidrocystomas, three infundibular cysts, two steatocystomas, two trichilemmomas, one spiradenoma, and one clear-cell hidradenoma (one patient had two cysts associated with a BCC). These cases correspond to seven female patients and five male patients aged between 26 and 91 years old. A quarter of these patients were immunosuppressed. Most ASFu neoplasms were found to be located beneath the BCC (8/12). DISCUSSION: To the best of my knowledge, this report describes three new collisions of BCCs with ASFu neoplasms (infundibular cysts, steatocystomas, and a spiradenoma). My results also suggest that immunosuppression could be a factor that predisposes a patient to these collisions. I review current hypotheses in an effort to explain these collisions and contribute some new theories.