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CDK4/6 Inhibitor Neutropenia — SCE Medical Oncology MCQ

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ModerateBreast CancerCDK4/6 Inhibitor NeutropeniaSCE Medical Oncology

A 65-year-old woman with metastatic ER+/HER2- breast cancer on palbociclib + letrozole develops grade 4 neutropenia (ANC 0.3 × 10⁹/L) with fever. What is the key difference between CDK4/6 inhibitor-induced neutropenia and chemotherapy-induced neutropenia?

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Correct answer: B — CDK4/6 inhibitor neutropenia causes bone marrow arrest (G1 cell-cycle block) without stem cell depletion, conferring lower infection risk than chemotherapy-induced neutropenia

CDK4/6 inhibitor-induced neutropenia has a fundamentally different mechanism from chemotherapy-induced myelosuppression. CDK4/6 inhibitors cause reversible G1 cell-cycle arrest of neutrophil precursors without destroying haematopoietic stem cells. This results in rapid neutrophil recovery on drug hold and significantly lower rates of febrile neutropenia (~1-3% vs 15-25% with chemotherapy). G-CSF is generally NOT recommended for CDK4/6 inhibitor neutropenia. Dose reduction and dose delays are the standard management.

Reference: ESMO 2024 Metastatic Breast Cancer Guidelines; Diéras et al Breast Cancer Res Treat 2019