Background: Medical decisions made by oncology clinicians have serious implications, even when made collaboratively with the patient. Clinicians often use the Eastern Clinical Oncology Group (ECOG) performance status (PS) scores to help them make treatment-related decisions.

Methods: The current study explores the variability of the ECOG score when applied to 12 predetermined specially designed clinical case vignettes presented to a group of oncology clinicians (n = 72). The quantitative analysis included evaluation of variability of ECOG PS scores and exploration of rater and patient-related factors which may influence the final ECOG rating. In-depth interviews were conducted with oncology clinicians to ascertain factors that they felt were important while making treatment-related decisions. Basic and global themes were generated following qualitative data analysis.

Results: Quantitative results showed that there was poor agreement in ECOG rating between raters. Overall concordance with the gold standard rating ranged between 19.4% and 56.9% for the vignettes. Moreover, patients deemed to have socially desirable qualities (p < 0.004) were rated to have better PS and women patients (p < 0.004) to have worse PS. Clinicians having international work experience had increased concordance with ECOG PS rating. Qualitative results showed that ‘perceived socio-economic background of the patient’, ‘age of the patient’, ‘patient’s and family’s preferences’ and ‘past treatment response’ were the major themes highlighted by respondents that influenced the treatment-related decisions made by clinicians.

Conclusion: There is considerable variability in ECOG PS determined by clinicians. Decision-making in oncology is complex, multifactorial and is influenced by rater and patient-related factors.