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"lead-in" refers to the initial phase or period of the trial before the main study or treatment phase begins. In this case, the phase II study includes a "safety lead-in" phase for testing the combination of nivolumab with either bevacizumab or bevacizumab alone. During this safety lead-in phase, a small number of participants receive the treatment to assess its safety and identify any potential adverse effects. The primary goal of this phase is to ensure that the combination treatment does not pose unacceptable risks to the patients involved.

To determine if something is safe.