Table 1.

Quality assessment of CCSs based on the Newcastle-Ottawa scoring system.

		Selection				Comparability∗ (maximum of 2 stars)	Exposure
Author	Year	Is the case definition adequate?	Representativeness of the cases	Selection of controls	Definition of controls	Comparability on the basis of the design or analysis	Ascertainment of exposure	Same method of ascertainment	Nonresponse rate
Kim et al. [9]	2008	∗	∗			∗∗	∗	∗	∗
Topal et al. [10]	2008	∗	∗			∗∗	∗	∗	∗
Kawamura et al. [11]	2009	∗	∗			∗∗	∗	∗	∗
Kim et al. [12]	2011	∗	∗			∗∗	∗	∗	∗
Siani et al. [13]	2012	∗	∗			∗∗	∗	∗	∗
Bo et al. [14]	2013	∗	∗			∗∗	∗	∗	∗
Guan et al. [15]	2013	∗	∗			∗	∗	∗	∗
Hong et al. [16]	2013	∗	∗			∗∗	∗	∗	∗
Kim et al. [17]	2013	∗	∗			∗∗	∗	∗	∗
Kim et al. [18]	2013	∗	∗			∗∗	∗	∗	∗
Lee et al. [19]	2013	∗	∗			∗∗	∗	∗	∗
Shim et al. [20]	2013	∗	∗			∗∗	∗	∗	∗
Lee et al. [21]	2014	∗	∗			∗∗	∗	∗	∗
Lee et al. [7]	2015	∗	∗			∗∗	∗	∗	∗
Ramagem et al. [22]	2015	∗	∗			∗∗	∗	∗	∗

CCSs, case-controlled studies. ^∗Select controls for patients' characteristics (age and gender) and clinical or pathological TNM classification.