Global Scientific Outputs of Palliative Care Publications: A Holistic Approach with Bibliometric Analysis

Objective: This study aims to summarize the scientific outputs of the palliative care studies conducted between 1975 and 2019 through bibliometric analyses, identify the top effective journals, top-cited studies, the collaboration between countries, and trend topics. Materials and Methods: We performed a bibliometric analysis on palliative care. “Palliative care”, “terminal care”, “end-of-life care” and “hospice care” terms were used as the keywords in the title section of articles in Web of Science database. Spearman correlation coefficient was performed for the correlation analysis between the number of publications produced by the countries and economic indicators. Results: There were 22361 publications about palliative care. Of these publications, 10309 were articles. The top three countries that produced more than 100 articles were the USA, the United Kingdom, and Canada. The top three journals that had the highest number of publications were Journal of Palliative Medicine, Journal of Pain and Symptom Management, and Palliative Medicine. There was a positive, highly significant relationship between the number of publications and Gross domestic product (r = 0.709, p <0.001). Conclusion: This study is believed to be a beneficial guide about the global outputs of palliative care for the clinicians and scientists and present new research ideas to the researchers.


INTRODUCTION
Palliative care is defined as the supportive care provided to patients and their families who are facing a fatal condition in which the therapeutic approaches are insufficient [1][2][3][4]. The purpose of palliative care is to provide the patient with relief until the end-stage using pharmacological and nonpharmacological methods for preventing pain, dyspnea, and fatigue symptoms caused by the disease. The purpose of palliative care that starts with the diagnosis time and supportive care provided to the family in the mourning process is to relieve the pain of patients and families by making a comprehensive assessment of the physical, psychological and moral symptoms [1][2][3][4]. Just like pain affects the quality of life; anorexia, dysphagia, dyspnea, xerostomia, and weight loss are the most important symptoms that affect the course of the disease and have top priority in the treatment [5]. The main diseases that require palliative care include motor neuron diseases and progressive neurologic diseases (Alzheimer, ALS), advanced organ failure (heart, lung, kidney, liver, brain), cancers nonresponsive to the treatment, HIV/AIDS, and genetic/congenital, progressive diseases in children [5][6].
Bibliometrics is the statistical analysis of scientific publications, primarily articles [7][8][9]. Citation analysis as part of the bibliometric analysis is a method used for the relative effect of an article, author, or journal. Literature review becomes more and more difficult for researchers with the increasing number of publications [10][11][12]. Thanks to bibliometric analyses, researchers have access to a lot of summary information in a short time by obtaining the top-cited effective studies, related journals to be followed, the effectiveness of the countries, collaboration between the countries, and active authors and institutions [13][14].
The need for palliative care centers as well as the importance of these centers has been increasing due to factors such as an increase in cancers and longer length of life and an increase in chronic diseases [4]. However, the literature includes no bibliometric studies that provide a summary of the studies on palliative care. This study aims to summarize the scientific outputs of the studies conducted between 1975 and 2019 through bibliometric analyses, identify the top effective journals, top-cited studies, the collaboration between countries, and trend topics.

MATERIALS and METHODS
We performed a bibliometric analysis of the published literature between 1975 and 2019 on palliative care publications using Web of Science (WoS; Thomson Reuters, New York, NY, USA) database. "Palliative care", "terminal care", "end-oflife care" and "hospice care" terms were used as the keywords in the title section of articles (title: ("palliative care") or title: ("terminal care") or title: ("end-of-life care") or title: ("hospice care") refined by: document types: (article) timespan: 1975-2019. Indexes: SCI-Expanded, SSCI, A&HCI, CPCI-S, CPCI-SSH, BKCI-S, BKCI-SSH, ESCI.) The articles were downloaded from the WoS database (access date: 03.01.2020) and were analyzed bibliometrically. A web site (http://lert.co.nz/map/) was utilized for drawing the world map. Bibliometric network visualization was performed using the VOSviewer (Version 1.6.13) package programming [15]. Statistical analysis was performed using SPSS (Version 22.0, SPSS Inc., Chicago, IL, ABD) package programming. The normality of the data distribution was assessed using the Shapiro-Wilk test. Spearman correlation coefficient analysis was performed for the correlation analysis between the number of publications produced by the countries and economic indicators of GDP (gross domestic product) and GDP PPP (purchasing power parity). Estimations about the number of publications in the following years were done using the linear regression analysis. Statistical significance was taken p<0.05.

Development of Publications
The distribution of the publications according to years is demonstrated in Figure 1, which also demonstrates the regression analysis results about the estimations about the number of publications between 2020 and 2024. Regression analysis results showed that there would be 998 articles to be published in 2020 (CI%: 865-1131) and 1087 articles in 2024 (CI%: 898-1275) ( Figure 1). There was a statistically significant increase trend in the publications (p<0.05).

Active Countries
The distributions of the world countries that produced the highest number of publications are demonstrated in Figure 2 Figure 3 demonstrates the international collaboration network map of the 66 countries that produced at least 5 articles among 133 countries that had articles about palliative care. Cluster analysis results indicated 7 different collaborations in 7 different colors. Table 1

Active Organizations
The top 10 active universities that had publications about this issue were King's College London (241),   Table 2 demonstrates the bibliometric analysis of the top 15 articles that received most citations according to the total number of citations in the articles published between 1975 and 2019. The last column of Table 2 demonstrates the average number of citations per year.

Citation Analysis
The total number of citations per article was calculated by dividing the total number of citations received by the journals, and the network visualization map is demonstrated in Figure 4.

Co-citation Analysis
There were a total of 153676 publications cited in the references sections of all the articles that were analyzed. The top 5 articles that received the highest number of co-citations in the references section were the studies conducted by Temel [16][17][18][19][20].

Trend Topics
There were 9619 different keywords used in the 10309 articles. These words included 88 keywords that were used in at least 40 different articles. The cluster analysis between these keywords is given in Figure 5.a. Trend visualization network map is given in Figure 5.b. The most frequently used keywords about this issue are given in Table 3. "

Correlation Analysis
There was a positive, highly significant relationship between the number of publications and GDP (r = 0.709, p <0.001 respectively). A positive, moderate level relationship was found between the number of publications and GDP PPP (r = 0.642, p<0.001).

DISCUSSION
The findings showed that there was a significant increase trend in the articles about palliative care. The number of articles low at the beginning started to increase after the year 2006 with more than 200 articles published, and there have been more than 800 articles published every year since 2015. Regression analysis results also indicated that the increase trend would continue. An analysis of the publication distributions of the world countries showed that the developed countries were effective in publication productivity. Several studies in literature reported that the economic growth of the countries was related to their article productivity [9][10][11]. In their bibliometric study on Palliative Care, Liu et al. (2018) also reported that there was a highly significant relationship between the economic power of the countries and their publication productivity [21]. An analysis of the authorship collaboration of the countries showed that geographical regional neighborhood was the most important factor in collaborations. The topproductive countries such as the United Kingdom, Canada, and Australia were in the same cluster, which shows that collaborations and co-research are important factors for scientific outputs.
An assessment of the articles analyzed according to the number of citations showed that the study that received the highest number of citations in total and per year was the study entitled "Early palliative care for patients with metastatic nonsmall-cell lung cancer" written by Temel, JS. et al. and published in the New England Journal of   on depression, anxiety, and adjustment disorder prevalence in cancer patients. The study was also ranked fourth according to the total number of citations [26].
As to the number of co-citations in all the articles that were analyzed, the studies conducted by were the top-cited studies [16][17][18][19][20]. Researchers interested in this topic could be recommended to read the above-mentioned publications first.  [21]. The present study is more comprehensive than these studies mentioned. Firstly, it encompasses a wider interval in terms of the years when the articles were published. This study is the most comprehensive one in the literature that analyzed A limitation of this study is that it did not utilize databases such as PubMed, Google Scholar, and Scopus because the WoS database indexes articles published in the journals with higher impact in comparison to other databases. In addition, since the WoS index enables no access to the publications before 1975, we could not analyze the publications before 1975.
As a result, there was a statistically significant increase trend in the number of publications about palliative care. The most productive countries were the USA, United Kingdom, Canada, and Australia. Publication productivity was affected by the economic growth of the countries. This article provides a comprehensive bibliometric summary of the 10309 articles on palliative care published between 1975 and 2019. This study revealed the effective journals that produced publications about this issue, effective articles that received more citations, and trend topics. This article is believed to be a beneficial guide about the global outputs of palliative care for the clinicians and scientists and present new research ideas to the researchers.

CONFLICT of INTEREST
Authors declares that they have no conflict of interest.