Received: 7 January 2018
Accepted: 19 March 2018
First Online: 27 March 2018
Ethics approval and consent to participate
: This retrospective study was approved by the Ethics Committee at Yamaguchi University Graduate School of Medicine in Ube, Japan. The need for informed consent was waived because of the study’s retrospective design.
: Not applicable.
: The authors declare that they have no competing interests.
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