Clinical reports of pulmonary metastasectomy for colorectal cancer: a citation network analysis.

We examined the reference lists of the 51 papers incorporated into a systematic review and quantitative synthesis (SRQS) ( Fiorentino et al , 2010 ). We first excluded standard references to statistical methods and to books and book chapters. All the remaining citations were individually characterised by two of the authors (TT and FF). We defined 4 categories of particular interest: clinical follow-up studies of PMCRC, reports regarding hepatic metastasectomy, historical references, papers questioning the practice, and 10 other categories, including epidemiology, adjuvant therapies, pathological studies, and surgical techniques. We then constructed a citation network representing all papers as nodes and citations as links from one node to another ( Greenberg, 2009 ). A custom-designed relational database (MS Access, Microsoft, Redmond, WA, USA) was used to store and query the citation network data. We used Pajek, a freely available special-purpose social network analysis software tool (available from http://pajek.imfm.si/doku.php?id=pajek/ ), to perform visualisations of the citation networks ( de Nooy et al , 2005 ).

The 51 follow-up reports included in the SRQS were published as follows: 1 in 1971, 4 in the 1980s, 19 in the 1990s, and 27 between 2000 and 2006. These papers each cited between 10 and 36 references to a total of 1132. After planned exclusions, there were 972 citations to 293 other unique publications. The frequency distribution of citations was markedly skewed with 203 of 344 cited only once. The 5 most referenced papers were cited 31, 30, 27, and 2 × 22 times ( McCormack and Martini, 1979 ; Mansel et al , 1986 ; Goya et al , 1989 ; McAfee et al , 1992 ; Okumura et al , 1996 ). The network of the citations of 51 SRQS papers to 344 references in the network is illustrated in Figure 1 following the method of Greenberg. A cascade such as the one he described can be discerned. Of the 51 papers in the systematic review, 32 were classified for citation analysis as exclusively concerning and supporting PMCRC (both citing and cited), 19 were otherwise classified (e.g., mixed series of hepatic and pulmonary metastasectomy), and 21 further PMCRC papers were cited by SRQS papers ( Figure 2 ). The citations among these 72 papers are shown in Figure 3 and in addition, the 4 papers questioning practice. Of these, for example, Aberg et al (1980) and Aberg (1997) argued that apparent longer-than-expected survival is a matter of case selection. Two other authors expressed caution attributing survival to surgery ( Casciato et al , 1983 ; Todd, 1997 ). These are rarely cited compared with the dense network of citations between the index papers and other supportive PMCRC publications.

Our search of the literature for evidence regarding PMCRC was prompted in part by a NICE Cancer Services Manual update stating, ‘Surgery for metastases confined to the liver or lung can be curative…for a minority of patients, it can increase five-year survival rates from close to zero to over 30%’ ( National Institute for Health and Clinical Excellence (NICE), 2004 ). We speculated as to the evidence for this statement, but the single reference cited contained no information regarding pulmonary metastasectomy ( Stangl et al , 1994 ). This is a clear instance of not only selective but also inappropriate citation. The manner by which ‘history’ is cited is of interest ( Pastorino and Treasure, 2010 ). In our citation analysis, we categorised four papers as historical. As an example, the report by Blalock (1944) in the 1944 New England Journal of Medicine is cited by 14 of 51 of the index papers and is the thirteenth most frequent of 334 cited papers. Typical citations to it are: ‘The role of pulmonary metastasectomy for colorectal carcinoma was first introduced in 1944.’ ( Patel et al , 2003 ) and ‘Since Blalock reported the first pulmonary resection for colorectal metastases in 1944, lung metastases…have been considered to be cured by resection in selected cases.’( Ike et al , 2002 ). In summarising a guest lecture to The Massachusetts Medical Society, Blalock wrote ‘It is only eleven years since the first one stage removal of an entire lung...’. He was informing his colleagues that pneumonectomy is achievable; the fact that it was for a metastasis was coincidental. He tells us nothing about control of colorectal cancer or the eventual outcome beyond recovery from the operation. Furthermore, in current practice, pneumonectomy is considered an inappropriate operation for metastasectomy ( Migliore et al , 2010 ). Blalock's account of a pneumonectomy has no relevance to the current practice of pulmonary metastasectomy, and is not even claimed to be a first; hence, why is this such a popular citation? A possible answer is that among surgeons, his name adds authority: he went on to become extremely famous for the Blalock-Taussig shunt to palliate cyanotic heart disease ( Blalock and Taussig, 1945 ). His paper, which would not have been found by a formal search for metastasectomy but is passed on as folklore, gains a significance that Blalock did not intend. In marked contrast, Aberg's publications are barely mentioned. His paper in 1980 and editorial in 1997 which challenge the effect of pulmonary metastasectomy are cited by only two of the index papers, and yet, unlike Blalock's paper, Aberg's publications have metastasectomy in their title and could not be missed in a literature search. The failure to cite suggests bias against his paper rather than that it was not retrievable. In addition, US citations may have a higher perceived status ( Link, 1998 ). Blalock was from a prestigious American institution, Johns Hopkins, whereas Aberg worked in a less well-known Swedish hospital. To quote from a remarkable essay on the whole question of citation ( MacRoberts and MacRoberts, 1996 ), ‘The cumulative effect of citing more and more people who similarly agree with the author is to concretize the universality of the knowledge claim.’ Greenberg describes information cascades: authors write their clinical experience, citing similar practices and thus gain affirmation. However, it is like rolling a snowball: it gets bigger and bigger – but it is just more snow.