Is guideline adherence the key to increased survival in patients with breast cancer?

Recently, Varga et al. reported in an observational study that guideline adherence could prevent 56.3-78.9% of all deaths in patients with early onset breast cancer [Oncology 2010;78:189-195]. This would mean that nearly all deaths due to breast cancer can be avoided by guideline-adherent treatment. We argue that some methodological issues like immortal time bias or healthy adherer bias may have contributed to these implausible findings. However, the non-transparent reporting of the methods, especially regarding the operationalization of guideline adherence, hampers critical assessment of this study.