Febrile neutropenia in a patient with non-small cell lung cancer treated with nivolumab.

We describe a case of febrile neutropenia in a patient with metastatic non-small cell lung cancer, occurring after 36 cycles of nivolumab therapy. A bone marrow aspirate was performed which showed myeloid maturation arrest without a clear alternative cause, supporting an immune-mediated mechanism. The patient responded well to broad-spectrum antibiotics and granulocyte colony-stimulating factor (G-CSF), with successful rechallenge until other immune-related toxicities led to discontinuation. While uncommon, haematological immune-related adverse events are potentially life-threatening, with neutropenia reported in <1% of patients treated with immune checkpoint inhibitors. This case demonstrates three important points: (1) neutropenia may occur even late in the course of treatment with immunotherapy, highlighting the persistent need for vigilance; (2) bone marrow biopsy and the careful review of medications are essential to differentiate immune-mediated from chemotherapy or other drug-related cytopenias; and (3) management requires urgent recognition, antimicrobial cover, and G-CSF, and caution in the consideration of rechallenge.