Collision of Basal Cell Carcinoma with Apocrine-Sebaceous-Follicular Unit Neoplasms

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.

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.

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).

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.