Collision Lesions: Genuine Collision (Conflict) or not?

By definition, the term "collision lesion" refers to two or more tumors coinciding in the same anatomic position or visceral organ. Collision lesions coexisting on the same skin location are defined as collision skin lesions (CSLs). Although this term implies a conflict between the tumors, this is not the case. CSLs appear to be rare, but still pose a significant diagnostic problem in everyday clinical practice and clinicians should be aware of their existence. The aim of this study was to elucidate the problem of CSLs in clinical practice, with an emphasis on classification of CSLs according to position dependence, tumor histogenesis, etiology, and possible lesion combinations in CSLs, as well as diagnostic possibilities. According to our results, accurate clinical diagnosis could be only rarely reached, requiring lesion excision and pathohistological confirmation of CSLs. Considering the fact that tumors in CSLs can be partially or completely overlying or can even be positioned one within the other, the existence of two or more tumors is extremely difficult to detect.