Statistical analysis

For comparisons patients were grouped in two groups based on the PTS: left-sided and right-sided CRC. OS was defined as the interval from first diagnosis of CRC, diagnosis of mCRC and diagnosis of BM, respectively, until death or last date of follow-up and estimated with the Kaplan–Meier product limit method. To test for differences between two parameters, the chi-square test was used for binary variables and the Mann–Whitney U test for differences in mean ranks between two variables. To test for differences between OS curves, the log-rank test was used. BM-free survival (BMFS) was defined as the interval from diagnosis of CRC until diagnosis of BM. Two-tailed P values <0.05 were considered to indicate statistical significance. The association of PTS with OS from diagnosis of CRC BM was the main point of interest of the present study.

The graded prognostic assessment (GPA) including Karnofsky performance status (KPS) (<70, 70-89, 90-100), age (<50, 50-59, ≥60 years), extracranial metastases (present, absent) and number of BM (1, 2-3, >3) and the recently updated GPA for gastrointestinal cancers (GI-GPA), respectively, including KPS (<80, 80, 90-100), age (<60, ≥60 years), extracranial metastases (present, absent) and number of BM (1, 2-3, >3) are the best established prognosticators of outcome in CRC BM patients.19 Therefore, we predefined a priori the inclusion of the PTS together with either the GPA or the GI-GPA into the multivariate model, depending on their significance in the univariate analysis. A multivariate analysis was carried out using the Cox regression model. Due to the exploratory and hypothesis-generating design of the present study, no adjustment for multiple testing was applied and no formal sample-size calculation was conducted.21 All statistics were calculated using statistical package for the social sciences (SPSS®) 26.0 software (SPSS Inc., Chicago, IL).