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life at all, and all three used different measurement instruments. In particular, the amount of available and
used quality of life instruments (which are often not specific to renal cancer) makes it extremely difficult to
[9]
draw conclusions about these data across studies .

Since the problem is clear - how can we overcome outcome reporting heterogeneity? A methodologically
sound and sustainable solution is a core outcome set (COS). A COS is an agreed minimum set of outcomes
[1]
to be reported in all trials in a clinical area . There exists extensive methodological guidance on how to
develop COS and how to report the protocols and results papers . A methodological hub called the
[1]
[13]
[12]
Core Outcome Measures in Effectiveness Trials (COMET) Initiative maintains a register of a completed and
ongoing COS. The stages of creating a COS are outlined in [Figure 1].
COS development in the rheumatoid arthritis field is an example that gives reason for optimism and hopes
that this strategy can improve the situation in the field of renal cancer as well. Their COS development
programme started in the mid-1990s [14,15] , and between 2002 and 2016, 81% of completed registered trials
reported the full rheumatoid arthritis COS with a decreasing rate of selective reporting [16,17] .

The good news for the field of renal cancer effectiveness research is the current development of a COS each
for localised, locally advanced, and metastatic renal cancer, broadly following the COMET COS
development guidance (registration available here: https://www.comet-initiative.org/studies/details/1406,
[18]
step 2; all steps are referring to Figure 1). After the publication of the study protocol (step 3) and the
literature review for localised renal cancer , qualitative methods will ensure that the views of the patients
[11]
are adequately captured (step 4ii) before a Delphi study (step 4iii) and consensus meeting (step 4iv) are
conducted. In this way, the outcomes that are considered core by stakeholders, including urology healthcare
professionals, researchers, and patients, will be determined, including their definitions and
recommendations for their formal assessment (step 5). Regarding the latter, a current project of the
European Organisation for Research and Treatment of Cancer (EORTC) Quality of Life Group, in
accordance with its methodological requirements, is developing a patient-reported outcome measure
specific for renal cell cancer patients [19,20] . The preliminary module is about to be tested and will likely result
in an acceptable and feasible tool.

Awareness-raising efforts are already ongoing to promote COS implementation in general. For instance, the
National Institute for Health and Care Research (NIHR), a major UK research funder, requests applicants to
utilise applicable COSs in their funding applications. In another example, one of the Core Outcomes in
Women’s and Newborn Health (CROWN) initiative COS implementation strategies was to create a
consortium of over 80 gynaecology-obstetrics journals to endorse the use of COSs in studies submitted to
their journals . Similarly, the group responsible for co-ordinating Cochrane reviews in the dermatology
[21]
setting has embedded a COS initiative within their review group, which aims to facilitate
implementation . These three examples are useful to bear in mind to address barriers to uptake.
[22]

In conclusion, outcome reporting heterogeneity is problematic for evidence synthesis in renal cancer
treatment effectiveness research. It creates difficulties in succinctly summarising the evidence base, which,
in turn, makes it difficult for guideline panels to recommend best practices and for clinicians to advise their
patients. A solution to this problem exists, and a renal cancer COS is in development. Examples of COS
uptake in other disciplines offer optimism. We can learn from other successful instances of COS
implementation, such as awareness raising, encouraging researchers to use COS through editorial
endorsement and policy, and via research funders requiring a specific statement on the use of or
justification to not use the existing COS.