Cancer Patients' Survival: Standard Calculation Methods And Some Considerations Regarding Their Interpretation: POPULACIJSKO PREŽIVETJE BOLNIKOV Z RAKOM: UPORABA RAZLIČNIH PRISTOPOV IN PROBLEMI INTERPRETACIJE REZULTATOV.
BACKGROUND
Cancer patients' survival is an extremely important but complex indicator for assessing regional or global inequalities in diagnosis practices and clinical management of cancer patients. The population-based cancer survival comparisons are available through international projects (i.e. CONCORD, EUROCARE, OECD Health Reports) and online systems (SEER, NORDCAN, SLORA). In our research we aimed to show that noticeable differences in cancer patients' survival may not always reflect the real inequalities in cancer care, but can also appear due to variations in the applied methodology for relative survival calculation.


METHODS
Four different approaches for relative survival calculation (cohort, complete, period and hybrid) have been implemented on the data set of Slovenian breast cancer patients diagnosed between 2000 and 2009, and the differences in survival estimates have been quantified. The major cancer survival comparison studies have been reviewed according to the selected relative survival calculation approach.


RESULTS
The gap between four survival curves widens with time; after ten years of follow up the difference increases to more than 10 percent points between the highest (hybrid) and the lowest (cohort) estimates. In population-based comparison studies, the choice of the calculation approach is not uniformed; we noticed a tendency of simply using the approach which yields numerically better survival estimates.


CONCLUSIONS
The population-based cancer relative survival, which is continually reported by recognised research groups, could not be compared directly as the methodology is different, and, consequently, final country scores differ. A uniform survival measure would be of great benefit in the cancer care surveillance.
Institute of Oncology Ljubljana, Epidemiology and Cancer Registry, Zaloska 2, 1000 Ljubljana, Slovenia.
Vesna
V
Zadnik
Institute of Oncology Ljubljana, Epidemiology and Cancer Registry, Zaloska 2, 1000 Ljubljana, Slovenia.
Tina
T
Žagar
Institute of Oncology Ljubljana, Epidemiology and Cancer Registry, Zaloska 2, 1000 Ljubljana, Slovenia.
Maja Primic
MP
Žakelj
bias; cancer registries; cancer survival; populacijska analiza preživetja; population-based survival analysis; preživetje bolnikov z rakom; pristranskost; register raka; relative survival; relativno preživetje