Collision tumor of malignant tumors of the skin: dermal squamomelanocytic tumor coexisting with basal cell carcinoma—a rare case

Collision tumors are neoplasms coexisting in the some anatomical area. The most common combination is melanocytic nevus with basal cell carcinoma. Melanocytic nevus with basal cell carcinoma constitutes the most common cutaneous combination. Co-existence of two malignant neoplasms is extremely rare. We describe the case of a 69-year-old man who was admitted to our hospital with a nodular mass on the back. We performed an excisional biopsy that revealed collision tumor, consisting of basal cell carcinoma along with mixed melanosquamous carcinoma. Subsequently, wide excision with sentinel node biopsy was performed. The sentinel node was negative. The patient did not receive any ongologic therapy.