Received: 19 July 2018
Accepted: 5 February 2019
First Online: 11 February 2019
Ethics approval and consent to participate
: The study protocol was approved by the ethics committee of Keio University School of Medicine (approval number: 20160010), and written informed consent was waived because of the retrospective design.
: Not applicable
: The authors declare that they have no competing interests.
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