Neutropenia shortly after RT and in the 2nd cycle after RT was associated with CDK4/6i dose reduction (p = 0.0007; OR 5.5; p = 0.0002, respectively), especially G3-G4 neutropenia (p < 0.0001; OR 25.2; p < 0.0001; OR 7.8). This evidence concerns the gene CDK4 and neutropenia.