“…Recently, federated learning (FL) has emerged as a privacy-preserving solution for this, which allows to learn from distributed data sources by aggregating the locally learned model parameters without exchanging the sensitive health data [8,12,14,16,23]. However, despite progress achieved, existing FL algorithms typically only allow the supervised training setting [3,13,15,24,26], which has limited the local clients (i.e., hospitals) without data annotations to join the FL process. Yet, in realistic scenarios, most hospitals usually cannot afford the intricate data labeling due to lack of budget or expertise [22].…”