Received: 7 May 2019
Accepted: 29 May 2019
First Online: 7 June 2019
Ethics approval and consent to participate
: This retrospective study was approved by the Akita University Hospital Institutional Review Board (No. 1914, approved on February 15, 2018). All methods were performed in accordance with the guidelines and regulations of the ethics board. For this type of study, formal consent was not required.
: Not applicable.
: The authors declare that they have no competing interests.