Received: 9 November 2018
Accepted: 22 July 2019
First Online: 1 August 2019
Ethics approval and consent to participate
: This study is in accordance with the Declaration of Helsinki and has been approved by the Institutional Review Board at the West China Hospital. The data was retrieved after our application was approved by the SEER database.
: Not applicable.
: The authors declare that they have no competing interests.