Review ArticleTransformational Changes Between Non–Small Cell and Small Cell Lung Cancer—Biological and Clinical Relevance—A ReviewClamon, Gerald MD; Zeitler, William MD; An, Josiah MD; Hejleh, Taher A. MBBSAuthor Information Department of Internal Medicine, Division of Hematology, Oncology, Blood and Marrow Transplantation, University of Iowa Hospitals and Clinics, Iowa City, IA G.C. has a financial interest in Pfizer Inc. and has received prior research funding from Merck. The remaining authors declare no conflicts of interest. Reprints: Gerald Clamon, MD, Department of Internal Medicine, Division of Hematology, Oncology, Blood and Marrow Transplantation, University of Iowa Hospitals and Clinics, 200 Hawkins Drive, C32 GH, Iowa City, IA 52242. E-mail: firstname.lastname@example.org. American Journal of Clinical Oncology: September 2020 - Volume 43 - Issue 9 - p 670-675 doi: 10.1097/COC.0000000000000720 Buy Metrics Abstract During the course of therapy, patients with small cell lung cancer have been noted to develop transformation to non–small cell lung cancer and conversely, patients with non–small cell lung cancer have had transformation to small cell lung cancer or other non–small cell histologies. Transformation may occur after prior tyrosine kinase inhibitors, chemotherapy, immunotherapy or radiation therapy. These changes reflect on the overlapping biology of these cell types and the clinical need for re-biopsy at times of disease progression. The optimum therapy after transformation will depend upon prior therapies received, the functional capacity of the patient, and further research to define the best therapy options. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.