Table 4.

Top 18 Keywords in Terms of Frequency and Centrality of COPD and Lung Cancer Research

Rank.	Keyword	Frequency	Year	Centrality	Keyword	Frequency	Year	Centrality
1	Copd	560	1991	0.03	Asthma	75	1994	0.31
2	Lung cancer	440	1992	0.09	Adenocarcinoma	20	1997	0.22
3	Obstructive pulmonary disease	377	1999	0.07	Heart failure	12	2011	0.21
4	Risk	351	1992	0.15	Bronchogenic carcinoma	12	1991	0.2
5	Mortality	270	1994	0.13	Care	27	1998	0.19
6	Disease	180	1991	0.18	Cancer	135	1992	0.19
7	Expression	157	1996	0.05	Disease	180	1991	0.18
8	Prevalence	156	1997	0.01	risk	351	1992	0.15
9	Association	145	1999	0.05	Population	67	1999	0.14
10	Survival	141	2000	0.05	Alveolar macrophage	5	1996	0.13
11	Smoking	140	1997	0.08	Death	48	1996	0.13
12	Cancer	135	1992	0.19	Mortality	270	1994	0.13
13	Inflammation	134	2007	0.02	Air flow obstruction	47	1997	0.11
14	Emphysema	117	1997	0.05	Smoker	82	1992	0.1
15	Diagnosis	111	2002	0.04	Breast cancer	25	1995	0.09
16	Impact	110	2005	0.04	Air pollution	33	1997	0.09
17	Management	103	2006	0.01	Gene expression	34	2006	0.09
18	Risk factor	100	1997	0.04	Biomarker	42	1999	0.09