Early Pseudoprogression Mimicking Pneumonitis After Tarlatamab Therapy: A Case Suggestive of Immune Cell‐Associated Respiratory Syndrome ( ICARS )

Tarlatamab, a DLL3‐targeting bispecific T‐cell engager, is an emerging option for previously treated small‐cell lung cancer (SCLC). Pseudoprogression with tarlatamab is poorly defined and can mimic drug‐induced pneumonitis. A 77‐year‐old man with extensive‐stage SCLC developed fever and hypoxia by Day 3 after the first 1‐mg dose, with rapid tumour enlargement and diffuse left‐lung infiltrates. High‐dose methylprednisolone and tocilizumab produced prompt clinical and radiographic improvement; follow‐up imaging showed resolution of infiltrates and tumour regression below baseline, consistent with pseudoprogression. This case suggests a reversible, lung‐predominant immune reaction with acute peritumoral infiltration likely driven by T cells. We propose the provisional term ‘Immune Cell‐Associated Respiratory Syndrome (ICARS)’. We speculate that baseline carcinomatous lymphangitis may predispose to ICARS. Recognising ICARS may prevent misdiagnosis and avoid premature discontinuation of effective therapy.