Frequency of a selection of variables (p<0.05) that remained in 1000 bootstrap samples following stepwise Cox procedure
| Covariate | Bootstrap inclusion frequency | |
| With PD-L1 | Without PD-L1 | |
| ECOG PS | 100 | 100 |
| NLR | 100 | 100 |
| Liver metastases | 99.5 | 100 |
| Alkaline phosphatase | 99.5 | 99.8 |
| Hemoglobin | 86.9 | 95.5 |
| Time from last chemotherapy | 91.1 | 91.2 |
| PD-L1 expression | 80.4 | – |
| Bone metastases | 48.4 | 78.3 |
| TNM at diagnosis | 49 | 44.8 |
| Perioperative chemotherapy | 24.6 | 28.8 |
| Number of previous lines for advanced disease | 25.1 | 20.9 |
| Pelvic irradiation | 14.8 | 20.3 |
| Number of disease sites | 21.7 | 16 |
| Previous chemotherapy | 8.1 | 10.3 |
| Non-visceral metastases | 5.8 | 6.4 |
ECOG PS, Eastern Cooperative Oncology Group performance status; NLR, neutrophil-to-lymphocyte ratio; TNM, tumor–node–metastases.