Collision tumor versus multiphenotypic differentiation: A case of carcinoma with features of colonic and lung primary tumors

Background: Collision tumors are rare tumors composed of two histologically distinct neoplasms coinciding at the same location. Collision tumors need to be distinguished from tumors originating from a progenitor cell with potential for multiphenotypic differentiation. Case Report: A clinically puzzling case of an intricate malignant pulmonary nodule in a patient with previous history of colorectal carcinoma is reported. A brief review of the clinical literature on collision tumor and tumor stem cells is presented. Conclusion: This case report emphasizes the importance of detailed histopathologic and immunohistochemical analyses, and clinical history in diagnosing a tumor composed of multiple malignant morphologies.