Outside of a clinical trial, what is your preferred third-line systemic therapy option for a patient with relapsed/refractory ES-SCLC?   

That will depend on what the patient has received in the second-line setting. If the patient has good PS, the NCCN recommends one of the various options. You could try either topotecan or lurbinectedin, whichever was not used in the second-line setting.