Article History
Received: 19 December 2017
Accepted: 26 February 2018
First Online: 2 March 2018
Ethics approval and consent to participate
: This study was approved by the Ethics Committee of Chinese Academy of Medical Sciences & Peking Union Medical College Hospital. Following an explanation of the purpose of the study, written consent was obtained from participants.
: Not applicable.
: The authors declare that they have no competing interests.
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