Table 2. Methodological Quality Assessment of Included studies According to the categorization of lower-level evidence.

Study	Comparable cohort	Clear treatment strategy selection and detailed methodologies	Multivariate analysis or matching analysis	Covariates included	Bias	Evidence level
Okada 2001	Yes	Yes	No	–	Probably confounded	IIC
Yamato 2008	Yes	Yes	Multivariate analysis	Most relevant factors	Possibly confounded	IIB
Zhong 2012	Yes	Yes	Multivariate analysis	Most relevant factors	Possibly confounded	IIB
Tsutani 2013	Yes	Yes	Both	All known factors	Probably not confounded	IIA
Okada 2014	Yes	Yes	Multivariate analysis	Most relevant factors	Possibly confounded	IIB
Jiang 2014	Yes	Yes	Multivariate analysis	Most relevant factors	Possibly confounded	IIB
Ogawa 2015	Yes	Yes	No	–	Probably confounded	IIC
Khullar 2015	Yes	No	Multivariate analysis	Most relevant factors	Probably confounded	IIC
Kodama 2016	Yes	Yes	Both	Most relevant factors	Possibly confounded	IIB
Nishio 2016	Yes	Yes	Both	Most relevant factors	Possibly confounded	IIB
Koike 2016	Yes	Yes	Both	All known factors	Probably not confounded	IIA
Hattori 2017	Yes	Yes	No	–	Probably confounded	IIC