Ovarian cancer: density equalizing mapping of the global research architecture.

We employed the established NewQIS platform [ 15 , 16 ] to conduct this study. The NewQIS platform was developed in 2009 as a multidisciplinary project involving scientists from different backgrounds such as engineering, computer sciences and medicine and numerous studies were published so far using the platform [ 17 – 32 ]. It constitutes a novel tool that was designed for the objective, precise and reliable scientometric analyses of research productivity based on validated protocols. Benefits of the platform include the efficient and standardized investigation of the scientific progress chronologically and geographically, the visualization of the results in expressive global maps via density equalizing map projections (DEMP), as well as unique evaluation tools deciphering national and international scientific relations and gender distribution among authors. We used an index database of the Web of Science (WoS core collection, Thomson Scientific) and analyzed the total research productivity by quantification of ovarian cancer-specific publications. Parameters describing the articles’ recognition by the scientific community were assessed based on the number of related citations, i.e. h-indices and citation rates. The WoS was selected as data source because of its unique Citation Report function allowing the extraction of citation performance parameters [ 33 ]. We refrained from extracting data from other platforms such as Google Scholar or Scopus due to the lack of data congruence in these three databases hampering triangulating, comparing and integrating data related to ovarian cancer research since 1900 [ 34 ]. We conducted a “title” search for the time period of 1900 (01-01) to 2014 (31-12). The search term [“( ovarian OR ovary ) AND ( cancer OR neoplasm OR carcinoma )”] was used. The year 2015 was excluded to avoid incomplete data acquisition at the time the study was performed. We used the filter option “document type” to restrict our search to “original articles” as described previously [ 15 ]. Articles were saved in a plain text format using the download application provided by the WoS. All related metadata were collected in an interim database and, analyzed according to the following criteria: originating country, language, citations, cited references, authors, journal, year published and subject categories. The subject categories represent standard categories assigned to every publication by the Journal Citation Reports (provided by the Thompson Reuters/Institute of Scientific Information) during the publication process. We computed the country-specific modified h-index (HI) and the citation rate (CR, number of all citations per total ovarian cancer publication volume). In 2005, the HI was developed by Jorge Hirsch to assess the recognition of an author’s research performance in the scientific community [ 35 ]. In our study, this proxy measure was adapted to evaluate the productivity of single countries in ovarian cancer research and therefore termed “modified HI”. Also, a glossary was added in the Additional file 1 describing important terms used in this manuscript. DEMP visualize benchmarking processes by the creation of anamorphic world maps. After the transfer of the metadata to excel charts and parameter analysis, DEMP were calculated based on the algorithms of Gastner and Newman. Therefor, the territories of countries publishing ovarian cancer research were resized in proportion to the selected criteria (i.e. the total number country-specific articles) [ 36 ]. In order to quantify country-specific contributions to ovarian cancer research in regards to their economic resources and manpower, we evaluated research productivity in relation (1) to the gross domestic product (GDP) per capita, (2) to the total economic power index GDP per 1000 billion US-$ and (3) to the population size. Economic facts were obtained from the World Economic Outlook Database of the International Monetary Fund of 2014 [ 37 ]. Only countries with a minimum of 50 ovarian cancer publications were included. We also collected the absolute numbers of ovarian cancer incidence and the crude rate (defined as the new cancer cases diagnosed in a specific year per 100,000 persons at risk) of the 25 countries that have published more than 100 ovarian cancer items during the investigated time span. The data reflect the incidence of ovarian cancer in 2012 and were obtained from http://globocan.iarc.fr/Pages/summary_table_pop_sel.aspx . Based on these numbers we calculated the ratio of country-specific articles per each new ovarian cancer case. To determine research collaborations from a global viewpoint, affiliations of authors were analyzed and chart diagrams were computed as previously described [ 38 ]. We defined an article as “collaborative” if at least two authors, who work in different countries as stated in the affiliations, contributed to the work. Publications with shared authorship were counted one time only (independent of the number of authors from the same country defined in the affiliations) towards the complete count of joint publications this specific country is involved in. For example, when 10 publications were analysed of which eight were affiliated with the USA, five with the UK and three items were joint publications, these were counted as 3 out of 8 for the USA and 3 out of 5 for the UK. Also, we related the total count of collaborative items to the overall number of publications for each investigated country. For example, 2240 items were published by US-American authors in a joint effort with other countries. These were related to the overall scientific productivity of the USA represented by 9312 items (24%). 747 collaborative publications were identified for the UK; these accounted for 39% out of 1900 items. In Fig.  3 , vectors represent the productivity of collaborations for each pair of countries. These are proportional in line width and shade of grey to the number of collaborations.

In 115 years, a total of 23,378 original articles were published in the WoS. The publication activities increased continuously throughout the decades: Until the 1950s we identified up to 10 articles each year; this number increased to more than 100 publications/year from 1979 onwards and doubled after 1984. In the next decade, the productivity increased to more than 500 annual items and doubled again after 2008. In 2014, 1540 articles were published (Fig.  1 a). The number of participating authors per publication increased from 2.5 authors in 1972 to 8.18 authors per ovarian cancer-related article in 2014. Fig. 1 Publication output. a Number of published items per year. b Density equalizing map of the global ovarian cancer research activity between 1900 and 2014. Colors and territorial sizes indicate numbers of ovarian cancer publications per country Publication output. a Number of published items per year. b Density equalizing map of the global ovarian cancer research activity between 1900 and 2014. Colors and territorial sizes indicate numbers of ovarian cancer publications per country A total of 99 countries participated in the publication of all articles. The majority of publication volume originated from a small number of countries: The United States of America (USA) was the most productive with 9312 ovarian cancer-specific articles. It was followed by the United Kingdom (UK, 1900 articles), China (1813 articles), Germany (1717 articles), Japan (1673 articles) and Italy (1672 articles). Hence, DEMP analysis demonstrated a distorted world map with the main focus on North America and Western Europe and a prominent China and Japan (Fig.  1 b). Asian, South American and African countries occupied only minimal areas on the cartogram.

The citation count of yearly published articles showed a course similar to the annual publication activity: After a very modest increase until 1974 the citations increased steadily with peaks in 1979, 1989, 1994, 1996, 2004. After 2005, we documented a steep decline in citation numbers until 2014 with the exception of a small plateau in 2008 (Fig.  2 a). Fig. 2 Citation analysis. a Number of citations between 1950 and 2014. b Density equalizing map of the number of citations. Colors and territorial sizes indicate numbers of citations per country Citation analysis. a Number of citations between 1950 and 2014. b Density equalizing map of the number of citations. Colors and territorial sizes indicate numbers of citations per country Country-specific citation analysis indicated a leading position of the USA with 354,891 citations (41.2% of all citations). It was followed by the UK (71,562 citations), Canada (55,964 citations), Italy (49,422 citations), Japan (35,995 citations), and Germany (34,278 citations) (Fig.  2 b). In contrast to publication activities, China dropped from third to position 10 when citations were quantified. The USA dominated the country-specific HI analysis (HI of 207), and was followed by the UK (HI = 122), Canada (HI = 99), Italy (HI = 97), Germany (HI = 84), and Japan (HI = 81) (Table  1 ). Regarding the citation rate (CR) of countries with a minimum of 30 articles published on ovarian cancer, we identified Canada (CR = 43.52) in the leading position. Then Finland (CR = 39.17), Hungary (CR = 38.81), the USA (CR = 38.11) and the UK (CR = 37.66) were followed by the Western European countries Belgium (CR = 36.92), Sweden (CR = 36.61), the Netherlands (CR = 34.03), Norway (CR = 31.8), Italy (CR = 29.56) and France (CR = 23.79). China dropped to a CR of 11.05 (Table  2 ). Table 1 Modified h-indices Rank Country h-index 1 United States 207 2 United Kingdom 122 3 Canada 99 4 Italy 97 5 Germany 84 6 Japan 81 7 Netherlands 78 8 Australia 74 9 France 70 10 Sweden 68 11 Belgium 58 12 China 57 13 Finland 55 14 Denmark 54 15 Norway 54 16 Israel 52 17 Austria 51 18 Spain 49 19 Switzerland 48 20 South Korea 45 21 Greece 42 22 Poland 40 23 Taiwan 37 24 Turkey 23 25 India 22 26 Ireland 22 27 South Africa 20 28 Hungary 20 29 Portugal 20 30 Czech Republic 19 31 Russia 18 32 Thailand 18 33 Singapore 18 34 Iceland 18 35 Brazil 16 36 New Zealand 15 37 Mexico 14 38 Slovakia 14 39 Slovenia 13 40 Chile 13 41 Croatia 12 42 Egypt 12 43 Malaysia 12 44 Iran 10 45 Saudi Arabia 10 46 Argentina 10 47 Belarus 10 48 Serbia 9 49 Romania 8 50 Pakistan 8 51 Lithuania 8 52 Bulgaria 7 53 Latvia 6 54 Ukraine 4 55 Tunisia 3 The table summarizes the h-indices related to research on ovarian cancer and published by the countries investigated Table 2 Ovarian cancer-specific citation rates Rank Country Citation rate 1 Canada 43.52 2 Finland 39.17 3 Hungary 38.81 4 United States 38.11 5 United Kingdom 37.66 6 Belgium 36.92 7 Sweden 36.61 8 Netherlands 34.03 9 Norway 31.80 10 Australia 30.55 11 Italy 29.56 12 Portugal 29.44 13 Switzerland 29.43 14 Spain 28.76 15 Denmark 28.53 16 Ireland 28.51 17 Mexico 27.16 18 Israel 25.96 19 Greece 24.91 20 France 23.79 21 Austria 22.58 22 Slovenia 22.47 23 South Africa 22.35 24 Japan 21.52 25 New Zealand 20.66 26 Germany 19.96 27 Poland 19.33 28 Taiwan 18.17 29 Thailand 17.85 30 Czech Republic 17.16 31 Slovakia 16.69 32 South Korea 15.30 33 Egypt 13.81 34 Singapore 13.03 35 India 12.65 36 Malaysia 12.31 37 Brazil 11.90 38 China 11.05 39 Romania 10.50 40 Saudi Arabia 9.30 41 Croatia 8.82 42 Turkey 8.59 43 Russia 7.55 44 Iran 4.34 45 Serbia 4.14 The table summarizes the citation rates related to ovarian cancer research and published by investigated countries with a minimum of 30 publications Modified h-indices The table summarizes the h-indices related to research on ovarian cancer and published by the countries investigated Ovarian cancer-specific citation rates The table summarizes the citation rates related to ovarian cancer research and published by investigated countries with a minimum of 30 publications When the country-specific publications were related to the gross domestic product (GDP) per capita, the USA was ranked first with an average of 169.9 ovarian cancer-related publications per GDP per capita in 1000 US-$ (Q1). The USA was followed by China as the first middle-income country in the ranking (Q1: 140.5), the UK (Q1: 50.4), Italy (Q1: 48.5) and Japan (Q1: 44.3) (Table  3 ). Table 3 Socio-economic analysis of ovarian cancer research of the most active countries in ovarian cancer research. Source for GDP (Currency in 1000 Billion US Dollars) and GDP per capita (Currency in 1000 US Dollars) in 2014 was the World Economic Outlook Database of the International Monetary Fund of 2014. (Threshold: 50 ovarian cancer-specific publications) Country Rank Number of articles GDP (in 1000 Bill. US$) Articles/GDP (1000 Bill. US$) Rank ratio (Articles/GDP in economic group) GDP per capita (in US$) Articles/GDP per capita (in 1000 US$) Rank ratio (Articles/GDP per capita in economic group) USA 1. 9312 17.420 534.6 HIG 14 54,800 169.9 HIG 1 China 2. 1813 10.360 175.0 MIG 4 12,900 140.5 MIG 1 UK 3. 1900 2.848 667.1 HIG 12 37,700 50.4 HIG 2 Italy 4. 1672 2.129 785.3 HIG 9 34,500 48.5 HIG 3 Japan 5. 1673 4.770 350.7 HIG 22 37,800 44.3 HIG 4 Germany 6. 1717 3.820 449.5 HIG 18 44,700 38.4 HIG 5 Canada 7. 1286 1.794 716.8 HIG 11 44,500 28.9 HIG 6 India 8. 150 2.048 73.2 MIG 6 5800 25.9 MIG 2 France 9. 878 2.902 302.5 HIG 24 40,400 21.7 HIG 7 Poland 10. 458 0.552 829.4 HIG 8 24,400 18.8 HIG 8 Australia 11. 742 1.483 500.3 HIG 16 46,000 16.1 HIG 9 Netherlands 12. 762 0.880 865.5 HIG 7 47,400 16.1 HIG 10 South Korea 13. 561 1.410 397.9 HIG 20 35,400 15.8 HIG 11 Turkey 14. 290 0.813 356.6 MIG 1 19,600 14.8 MIG 3 Sweden 15. 522 0.559 933.6 HIG 6 44,700 11.7 HIG 12 Israel 16. 388 0.305 1272.1 HIG 2 33,400 11.6 HIG 13 Greece 17. 295 0.246 1197.2 HIG 4 25,800 11.4 HIG 14 Spain 18. 370 1.400 264.3 HIG 25 33,000 11.2 HIG 15 Denmark 19. 449 0.347 1293.2 HIG 1 44,300 10.1 HIG 16 Austria 20. 450 0.436 1031.9 HIG 5 45,400 9.9 HIG 17 Finland 21. 337 0.276 1219.7 HIG 3 40,500 8.3 HIG 18 Belgium 22. 332 0.528 629.0 HIG 13 41,700 8.0 HIG 19 Russia 23. 166 2.057 80.7 HIG 28 24,800 6.7 HIG 20 Taiwan 24. 265 0.530 500.5 HIG 15 43,600 6.1 HIG 21 Norway 25. 391 0.512 764.3 HIG 10 65,900 5.9 HIG 22 South Africa 26. 71 0.341 208.1 MIG 2 12,700 5.6 MIG 4 Switzerland 27. 306 0.679 450.7 HIG 17 55,200 5.5 HIG 23 Thailand 28. 74 0.374 198.0 MIG 3 14,400 5.1 MIG 5 Brazil 29. 70 2.244 31.2 MIG 7 15,200 4.6 MIG 6 Iran 30. 62 0.403 154.0 MIG 5 16,500 3.8 MIG 7 Czech Republic 31. 74 0.206 359.9 HIG 21 28,400 2.6 HIG 24 Hungary 32. 57 0.130 439.5 HIG 19 24,300 2.3 HIG 25 Portugal 33. 54 0.228 236.6 HIG 26 26,300 2.1 HIG 26 Ireland 34. 75 0.246 305.1 HIG 23 46,800 1.6 HIG 27 Singapore 35. 70 0.308 227.3 HIG 27 81,300 0.9 HIG 28 Socio-economic analysis of ovarian cancer research of the most active countries in ovarian cancer research. Source for GDP (Currency in 1000 Billion US Dollars) and GDP per capita (Currency in 1000 US Dollars) in 2014 was the World Economic Outlook Database of the International Monetary Fund of 2014. (Threshold: 50 ovarian cancer-specific publications) For the total economic power index GDP, Denmark was positioned at the first place with a total of 1293.2 ovarian cancer-specific articles per 1000 billion US-$ GDP (Bio US-$ GDP, Q2), followed by Israel (Q2: 1272). Amongst the high-income countries, the UK ranked at position 12 (Q2: 667.1), followed by Belgium (Q2: 629) and the USA (Q2: 534.6). China (Q2: 157) occupied the 4th rank of the middle-income countries and the 32nd position of all countries with more than 50 ovarian cancer-specific articles (Table  3 ). Denmark was positioned first when the ovarian cancer research output was related to population size. Here, 80.3 ovarian cancer-specific publications were authored per 1 million citizens. It was followed by Norway (77.9 publications/1 million citizens), Iceland (65.5 publications/1 million citizens), Finland (62.2 publications/1 million citizens) and Sweden at position 5 (54.8 publications/1 million citizens). Other productive countries were the USA (29.7 publications/1 million citizens), UK (29.8 publications/1 million citizens) and China (13.4 publications/1 million citizens). Furthermore, Israel took the lead having published one article per newly diagnosed ovarian cancer case based on the incidence data of 2012. It was followed by Norway (0.94 articles per new ovarian cancer case), Denmark (0.83 articles per new ovarian cancer case), Sweden (0.79 articles per new ovarian cancer case), the Netherlands (0.74 articles per new ovarian cancer case) and Finland (0.74 articles per new ovarian cancer case). The USA was ranked 11th; countries such as China, Russia and India were ranked last amongst the 25 investigated nations (Table  4 ). A DEMP shows the absolute ovarian cancer incidence numbers of the 25 counties that have published more than 100 articles during the investigated time span (Additional file 2 : Figure S1). Table 4 The table depicts the absolute incidence numbers and the crude rate (defined as the new cancer cases diagnosed in a specific year per 100,000 persons at risk) of ovarian cancer of the 25 countries having published more than 100 related items and the ratio of country-specific articles per each new ovarian cancer case Rank Country Article count Incidence in 2012 Crude rate in 2012 Article/new case in 2012 1 Israel 388 380 9.8 1.02 2 Norway 391 418 16.9 0.94 3 Denmark 449 544 19.3 0.83 4 Sweden 522 659 13.8 0.79 5 Netherlands 762 1025 12.2 0.74 6 Finland 337 457 16.6 0.74 7 Austria 450 636 14.8 0.71 8 Australia 742 1424 12.4 0.52 9 Switzerland 306 621 15.8 0.49 10 Canada 1286 2648 15.2 0.49 11 United States 9312 20,874 13.1 0.45 12 Belgium 332 840 15.3 0.40 13 Greece 295 915 15.9 0.32 14 United Kingdom 1900 6692 21 0.28 15 Italy 1672 5911 19 0.28 16 Germany 1717 6673 16.1 0.26 17 South Korea 561 2349 9.8 0.24 18 France 878 4592 14.1 0.19 19 Japan 1673 8921 13.7 0.19 20 Turkey 290 2400 6.4 0.12 21 Spain 370 3236 13.7 0.11 22 Poland 458 4456 22.5 0.10 23 China 1813 34,575 5.3 0.05 24 Russia 166 13,373 17.4 0.01 25 India 150 26,834 4.4 0.01 The data reflect the incidence of ovarian cancer in 2012 The table depicts the absolute incidence numbers and the crude rate (defined as the new cancer cases diagnosed in a specific year per 100,000 persons at risk) of ovarian cancer of the 25 countries having published more than 100 related items and the ratio of country-specific articles per each new ovarian cancer case The data reflect the incidence of ovarian cancer in 2012 1685 journals published ovarian cancer-related articles since 1900. The most prolific journal was “Gynecologic Oncology” with 2710 articles and a related citation rate (CR) of 23.91 followed by “International Journal of Gynecological Cancer” (968 articles/CR = 11.06) and “Cancer Research” (637 articles/CR = 80.81). We displayed the top 15 journals including number of articles, citations and CR (Additional file 3 : Table S1, Additional file 4 : Figure S2) and identified the ten most cited articles in the area of ovarian cancer research (Additional file 5 : Table S2). The leading subject categories of ovarian cancer research were “Oncology” with 13,649 publications cited 363,896 times, “Obstetrics & Gynecology” (6878 publications, 128,161 citations), and—following with a considerable gap—“Pathology” (1238 publications and 31,921 citations) (Additional file 6 : Figure S3A). The areas “General & Internal Medicine” (35,221 citations) and “Genetics & Hereditary” (33,842 citations) showed a high CR relative to the total number of publications indicating a high impact of published work in the field. We performed a subject area analysis for the ten most active countries in ovarian cancer research to identify their particular scientific focus: Up to 80% of all publications in nine of the ten countries were attributed to “Oncology” and “Obstetrics & Gynecology”. China published a high percentage of articles in “Research and Experimental Medicine”, “Biochemistry and Molecular Biology” as well as “Cell Biology”. Researchers from the UK, Australia, France and Canada focused on the area of “Genetics”. Japanese scientists dedicated a high percentage of their work to the subject category of “Pathology”. “General and Internal Medicine” was popular among researchers from France and the UK (Additional file 6 : Figure S3B). We identified 3697 international collaborations publishing on ovarian cancer, 74% were bilateral (2733 items) and 15.4% trilateral co-operations (568 items). Joint research efforts were clearly dominated by scientists and institutions situated in the USA. US-American authors published 24% of all publications in co-operation with other countries, and collaborated with 13 different countries in total. The most active collaborations were established between the USA and Canada (433 collaborative papers), followed by US-American co-operations with the UK (385 papers), China (300 papers), Italy (291 papers) and Germany (284 papers) (Fig.  3 ). Fig. 3 International ovarian cancer research collaborations. Greyscale and bar thickness indicate intensity of collaborations. First ciphers in brackets indicate total publication numbers. Second ciphers indicate number of collaborative publications. Threshold: 5 collaborations International ovarian cancer research collaborations. Greyscale and bar thickness indicate intensity of collaborations. First ciphers in brackets indicate total publication numbers. Second ciphers indicate number of collaborative publications. Threshold: 5 collaborations

Ovarian cancer is the most lethal gynecological tumor in high income-countries; it represents the seventh-most common female cancer worldwide [ 1 , 2 ]. In the United States, approximately 22,000 new ovarian cancer cases are diagnosed annually, 14,200 related deaths occur each year [ 3 ]. The majority of invasive ovarian malignancies originate from epithelial cells. Each histotype—high-grade serous, low-grade serous, mucinous, clear cell and endometrioid—exhibits distinct clinical and pathological characteristics [ 4 ]. During the last three decades, multiple breakthrough discoveries have been reported in the field: for the last 10 years it has been accepted that two types of epithelial ovarian cancers exist [ 1 , 5 ]. Type I tumors include low-grade serous, endometrioid and clear cell histologies [ 6 , 7 ]. The association of Type I cancers with endometriosis was found in 2012. This benign condition increases the risk of low-grade serous and endometrioid cancers by approximately twofold, for clear cell subtypes by threefold [ 1 , 8 ]. Also, ARID1A gene mutations were described for endometriosis-associated endometrioid and clear cell cancers [ 9 ]. Type II high-grade serous carcinomas are the most common ovarian malignancies. In 2006, Medeiros et al. presumed their origin from the fimbriae of the fallopian tube [ 10 ]. In the last years, the identification of relevant somatic and germline mutations gained relevance as a first step towards screening strategies and novel targeted therapies: KRAS , BRAF , ERBB2 , CTNNB1 , PTEN , PIK3CA , ARID1A , PPP2R1A , and BCL2 mutations were found in Type I carcinomas. 96% of high-grade serous Type II tumors had TP53 mutations [ 1 , 5 , 11 ]. In 1994 and 1995, BRCA 1 / 2 mutations were described in hereditary Type II cancers; since then they have gained importance for clinical risk prediction and patient counseling [ 12 , 13 ]. The volume of scientific literature in oncology increased rapidly during the last 50 years [ 14 ]. Systematic evaluation of research output is necessary to guide individual reading, to plan research activities according to shortcomings and to quantify individual and collaborative productivity on national and international level. These assessments play an integral role in career decisions, allocation of grant funding and prioritizing research resources [ 14 ]. Scientometric methods provide the standardized analysis of journal articles in reference to their content and citations describing developments in origin and dissemination of published data. Specific to ovarian cancer, no systematic evaluation of the global scientific output is available to date, and no efforts have been made to understand trends in the related literature. Therefore, the topic of ovarian cancer was elected by the New Quality and Quantity Indices in Science (NewQIS) project [ 15 ] for a scientometric in-depth analysis. The study objectives included (1) the assessment of the worldwide publication output regarding quantitative aspects, parameters describing the recognition within the scientific community (e.g. citation rates) and research networks as well as (2) the evaluation of the country-specific productivity related to socio-economic variables. Also, we identified the leading journals publishing in the field and the most recognized articles since 1900.

During 115 years, a total of 23,378 original research articles were published in the WoS. The number of publications rose slowly until the seventies, when a steep and steady increase of research productivity started. This pattern is detected for most biomedical research as exemplified by studies on medical curare use or bacterial meningitis [ 39 , 40 ]. From 1900 to 1950, only 139 articles related to ovarian cancer were part of the WoS database. This is attributed to the following: Overall research activities were lower since the recognition and funding of scientists were not predominantly determined by their productivity. In 1915, Japanese researchers could provoke cancer in an animal model for the first time. Since then pathogenetic mechanisms of cancer shifted into the scientific focus paving the way to today’s understanding of the disease [ 41 ]. English was not the common scientific language at this time. Hence, a considerable amount of non-English publications issued before 1950 is not represented in our analysis. The late 1970s was an era when ovarian cancer-associated research gained increasing popularity (Fig.  1 ). Then, major scientific progress happened in the field as indicated by the first highly cited publication linking ovarian cancer and incessant ovulation [ 42 – 46 ]. The output grew dramatically in the nineties, which coincided with more landmark findings such as the discovery of the BRCA genes [ 47 ]. After 2008, annual research productivity increased to more than 1000 papers when new hypotheses regarding the origin of high-grade serous subtypes [ 10 ] and the association of ovarian cancers with ARID1A mutations and endometriosis were proposed [ 8 , 9 ]. Also, publications assessing novel treatment strategies such as pathway inhibitor (e.g. PARP inhibitors) or antibody-based therapies were mainly released in the last 7 years. Resembling the growing volume of published papers, the absolute citation count of ovarian cancer-related publications showed a steady increase until 2005 (Fig.  2 a). Landmark papers (included in Additional file 5 : Table S2) contributed to peaks in the graph: In 1979, Casagrande et al. [ 42 ] proposed the link between ovarian cancer and incessant ovulation. In 1989, two highly cited papers were published, which explored the pathogenetic relevance of HER2neu receptors and the efficacy of taxol as ovarian cancer treatment [ 48 , 49 ]. In 1994 and 1995, the ovarian cancer susceptibility genes—BRCA 1 and 2—were identified. Related articles lead to citation peaks in 1994 and 1996 [ 12 , 13 ]. A meta-analysis investigating the ovarian cancer risk of 8139 patients with BRCA1 and BRCA2 mutations was published in 2003 and associated with the citation peak in 2004 [ 50 ]. The decrease in citations after 2005 is linked to a delay of up to 8 years between publication and appropriate scientific recognition of an article represented by a maximum number of citations [ 51 ]. When country-specific ovarian cancer research productivity was analyzed, the leading position of the USA became evident. This finding aligns with a benchmarking study assessing the scientific output from 1961 to 2007 related to 22 organ systems. With 1,893,800 of 5,527,558 publications, the USA identified as the most productive nation [ 52 ]. The success of the USA points to its commitment to allocate major resources towards biomedical research, e.g. the NCI awarded $100.6 million ovarian cancer funding in 2003 ( http://www.cancer.gov/research/progress/snapshots/ovarian ). In our study, the USA was followed by the UK, China, Germany and Japan regarding research productivity. Glynn et al. [ 14 ] described a similar pattern for breast cancer, where the USA, the UK, Germany and Japan were also among the top five countries. China constitutes an exception: It ranked second for ovarian cancer research productivity but dropped to position 12 for breast cancer. Citations indicate the relevance of a published item [ 14 ]. In our study, the USA, the UK and Canada dominated the ranking in term of citation counts, HI and CR. These results correspond to other studies in obstetrics and gynecology, e.g. on smoking and pregnancy. Here, the USA, the UK and Canada also achieved the highest modified HI of 128, 79 and 62 and the highest citation rates of 41.4, 8.6 and 5.3%, respectively [ 38 ]. In order to define the commitment of single countries in ovarian cancer research, we investigated scientific productivity in terms of socio-economic abilities and demonstrated two important features: First, the USA lost its leading position and other—mostly European—nations gained importance. Denmark, Norway, Iceland, Finland and Sweden ranked in the top five when article count was analyzed in relation to number of citizens. When the total number of articles was related to the total economic power, Israel, Iceland, Denmark and Finland were leading the field. Second, when we focused on countries with a large population and high total GDPs such as China, their relative contribution to the global research output remained small compared to the USA and Europe. Taken together, the worldwide research architecture on ovarian cancer revealed that the USA and Western European nations, China and Japan constitute the scientific power players. They publish the majority of highly cited ovarian cancer-related articles and dominate international collaborative efforts. A strong dedication of single countries to ovarian cancer research is also indicated by the prominent position of European nations—e.g. the Scandinavian countries in particular—when research productivity was related to socio-economic benchmarks. These findings coincide with other benchmarking studies [ 52 ] and with the fact that the highest incidence rates of ovarian cancer (e.g. Northern and Western Europe with incidences of 13.3 and 11.3 per 100,000 person-years as well as Northern America with an incidence of 10.7 per 100,000 person-years) are found in areas with the greatest research productivity. This association underlines that the nations, which should prioritize ovarian cancer research to alleviate the burden among its female inhabitants, actually do so. Taking the ratio of articles per every new ovarian cancer case (based on the absolute incidence data of 2012) into account, we can demonstrate a similar picture. The Scandinavian countries and the Netherlands were among the leading nations. By contrast, Canada and the USA were ranked only in the middle field with a publication output of around one article per two new ovarian cancer cases. Although China had a low incidence rate of 3.2 per 100,000 person-years reported for 2002, secular epidemiological trends project increasing numbers for the future [ 53 ]. This might explain why China supports ovarian cancer research as indicated by the strong research productivity [ 54 ]. The public health burden of ovarian cancer is significant. No considerable improvements of survival rates or decrease in morbidity and mortality have been seen over the past decades. Hence, research activity needs to be fostered, and collaborative research efforts are crucial to tackle the challenges in the field. National and international networks are equipped to do this by sharing resources, facilities and ideas leading to landmark publications [ 55 ]. In our study, the USA was the most preferred nation for collaborations based on its outstanding financial support and scientific infrastructure. We identified the most productive co-operations between the USA, Canada as well as the UK. This finding is linked to the areal proximity and cultural/language similarities, which contribute to the high productivity and quality of research produced by each of these countries. Additionally, our observation of increasing author numbers per ovarian cancer publication reflects the development of strong research networks around the globe. The Ovarian Cancer Association Consortium (OCAC) serves as an example for highly prolific global networks. Founded in 2005, this multidisciplinary, international group published more than 60 high impact papers in the areas of genetics and epidemiology. According to the International Agency for Research on Cancer in 2012, 58% of ovarian cancer cases occurred in less developed nations [ 56 ]. Countries with the highest incidence rates of ovarian cancer were Fiji (age-standardized rate per 100,000: 14.9), Latvia (age-standardized rate per 100,000: 14.2) and Bulgaria (age-standardized rate per 100,000: 14.0) [ 56 ]. Although these countries experience a significant burden due to the disease, they were underrepresented on our map of ovarian cancer research. We did not identify one article published by Fiji. Latvia published 10 articles cited 112 times and Bulgaria issued 10 articles cited 90 times. Fiji was not part of any collaborative network. Latvia participated in only five multinational collaborations and Bulgaria in eight bilateral collaborations. We want to point out the necessity—and almost the ethical responsibility—to include lower-resource countries with high incidence rates in scientific collaborations. Here epidemiological data, ideas and gained knowledge can be exchanged and benefit all participants. To date, most ovarian cancer related articles were published in the subject categories of Oncology, Obstetrics/Gynecology as well as Pathology. This is not surprising. However, we found a shift in publication activity to areas such as Genetics and Internal Medicine. This development is linked to highly cited articles published by recently founded genetic-epidemiological consortia, i.e. OCAC or Ovarian Tumor Tissue Array Consortium. The increase in the subject category of Internal Medicine is explained by the growing number of high quality publications in leading journals such as the “New England Journal of Medicine”, “Nature” or “Lancet”, which are attributed to this category. Also, it reflects the growing interest of internal medicine physicians in the care of ovarian cancer patients. In this study, we analyzed ovarian cancer research by assessing the country-specific publication output associated with this topic. DEMP analysis provided the visualization of computed geospatial information regarding our findings, which is a unique strength of this study. Researchers from different countries and continents can benefit from our data since they provide objective insights about the status of ovarian cancer research in their homeland or a specific country of interest. In particular, they are able to plan future research initiatives and collaborations tailored to meet the identified needs. Further, representatives of funding institution can use the presented results for the strategic allocation of resources according to obvious shortcomings. A limitation of this study is linked to the preference of the WoS to index mostly English publications. This translates into an underrepresentation of non-English items and an underestimation of the total article number, which seems to skew our findings. Since high quality research is mostly published in English journals and the WoS catalogs 90% of cited and 80% of published items related to a specific topic [ 57 ], our search identified the majority of relevant published items linked to ovarian cancer. Hence, the bias can be considered as limited. Overall, we assessed three types of bibliometric indicators gauging the publication activity on ovarian cancer: Quantitative aspects to measure the productivity of the research community, performance indicators to reflect the quality of scientific output and structural indicators to visualize the interconnectedness of research [ 58 ]. Limitations are linked to the evaluation of “qualitative” citation parameters. It is generally accepted that high citation numbers reflect outstanding scientific recognition. This relationship might be skewed due to the Matthew effect: Scientists of acknowledged standing will be cited more than little-known authors, and the citation count of their papers will increase disproportionally after their publications gained some initial popularity [ 59 ]. Also, the use of performance indicators (e.g. the citation rate) only helps gauge the quality of published research. Because citation habits and dynamics are highly variable in the investigated fields of research, all variables based on citation frequency are problematic and rather mirror the recognition of fellow scientists than truly reflect quality [ 59 ].

This density-equalizing mapping study represents the first concise analysis of the global ovarian cancer research architecture and illustrates the benefits of scientometrics to assess research output in a standardized way. Our study identifies historically interesting aspects in the research dynamics and relates these to landmark publications in the field. The identification of key manuscripts, subject areas as well as journals with high publication and citation rates guides individual reading and the future direction of scientific endeavors. Also, our observations highlight the outstanding importance of collaborative networks—such as OCAC—that are able to produce high quality research and apply for grant funding successfully in a joint effort. Further, lower-resource countries with a high disease burden in their population should be included in collaborative networks leading to mutual benefits due to the exchange of samples, epidemiologic data, ideas and gained knowledge.