A total of 1799 patients were evaluated in this study. Positive surgical margins were found in 205 patients (11.4%) and positive PNI occurred in 350 cases (19.5%). Based on the status of surgical margin and PNI, the low-risk group contained 1335 patients (74.2%) and the high-risk group included 464 patients (25.8%). Significant differences were detected based on gender, CEA, surgery, pathologic T stage, pathologic N stage, and histologic grade between the two groups (all P < 0.05). To balance the baseline characteristics, 1:1 propensity score matching was performed (Table 1).
Table 1
Patient and tumor characteristics before and after matching
| Before matching | After matching | |
Characteristic - No. (%) | Low risk group (n = 1335) | High risk group (n = 464) | P-value | SMD | Low risk group (n = 464) | High risk group (n = 464) | P-value | SMD |
Age | | | 0.21 | 0.07 | | | 1.00 | 0.01 |
≤ 65 | 823 (61.6%) | 302 (65.1%) | | | 301 (64.9%) | 302 (65.1%) | | |
> 65 | 512 (38.4%) | 162 (34.9%) | | | 163 (35.1%) | 162 (34.9%) | | |
Gender | | | 0.02 | 0.13 | | | 0.94 | 0.01 |
Male | 894 (67.0%) | 339 (73.1%) | | | 337 (72.6%) | 339 (73.1%) | | |
Female | 441 (33.0%) | 125 (26.9%) | | | 127 (27.4%) | 125 (26.9%) | | |
CEA, ng/mL | | | < 0.01 | 0.31 | | | 0.26 | 0.08 |
≤ 5 | 926 (69.4%) | 252 (54.3%) | | | 270 (58.2%) | 252 (54.3%) | | |
> 5 | 409 (30.6%) | 212 (45.7%) | | | 194 (41.8%) | 212 (45.7%) | | |
Surgery | | | < 0.01 | 0.19 | | | 0.38 | 0.07 |
LAR | 1229 (92.1%) | 400 (86.2%) | | | 410 (88.4%) | 400 (86.2%) | | |
APR | 106 (7.9%) | 64 (13.8%) | | | 54 (11.6%) | 64 (13.8%) | | |
Pathologic T classification | | | < 0.01 | 0.96 | | | 1.00 | < 0.001 |
pT1-2 | 715 (53.6%) | 60 (12.9%) | | | 60 (12.9%) | 60 (12.9%) | | |
pT3-4 | 620 (46.4%) | 404 (87.1%) | | | 404 (87.1%) | 404 (87.1%) | | |
Pathologic N classification | | | < 0.01 | 0.56 | | | 0.56 | 0.04 |
cN0 | 1000 (74.9%) | 227 (48.9%) | | | 237 (51.1%) | 227 (48.9%) | | |
cN+ | 335 (25.1%) | 237 (51.1%) | | | 227 (48.9%) | 237 (51.1%) | | |
Histologic grade | | | 0.02 | 0.13 | | | 0.63 | 0.04 |
Low | 1263 (94.6%) | 424 (91.4%) | | | 429 (92.5%) | 424 (91.4%) | | |
High | 72 (5.4%) | 40 (8.6%) | | | 35 (7.5%) | 40 (8.6%) | | |
Surgical margin | | | < 0.01 | 1.26 | | | < 0.01 | 1.26 |
Negative | 1335 (100.0%) | 259 (55.8%) | | | 464 (100.0%) | 259 (55.8%) | | |
Positive | 0 (0.0%) | 205 (44.2%) | | | 0 ( 0.0%) | 205 (44.2%) | | |
Perineural invasion | | | < 0.01 | 2.48 | | | < 0.01 | 2.48 |
Negative | 1335 (100.0%) | 114 (24.6%) | | | 464 (100.0%) | 114 (24.6%) | | |
Positive | 0 (0.0%) | 350 (75.4%) | | | 0 (0.0%) | 350 (75.4%) | | |
APR Abdominoperineal resection, CEA Carcinoembryonic antigen, FU Fluorouracil, LAR Low anterior resection, SMD Standardized mean difference |
Next, 924 patients were allocated to each high-risk group (n = 462) or low-risk group (n = 462). The matching model was relatively well calibrated. Hosmer-Lemeshow goodness score for this model was 3.640 (P = 0.89). The c-index was 0.895 (P < 0.01). After matching, the standardized differences were reduced to less than 0.1 in all demographic factors except surgical margin and PNI. No significant difference was observed in age (P = 1.00), gender (P = 0.94), serum CEA level (P = 0.26), surgery type (P = 0.38), pathologic T stage (P = 1.00), pathologic N stage (P = 0.56), or histologic grade (P = 0.63). In matched cohort, adjuvant chemotherapy was administered to 423 (91.6%) patients in low-risk group and 416 (90.1%) in high-risk group. The regimen of adjuvant chemotherapy did not significantly differ between the low-risk and high-risk groups; FOLFOX, 8.2% versus 12.1%; LF, 74.1% versus 68.8%; Xeloda, 9.3% versus 9.3%; P = 0.17).
Survival Analyses Of High-risk Group
The 5-year OS and RFS rates in high-risk group at a median follow-up of 47.1 months were 71.4% and 51.4%, respectively. The 5-year OS rates were 73.4% in adjuvant chemotherapy arm and 53.9% in observation arm (Fig. 1A). The statistical difference was significant (P < 0.01). The 5-year RFS was significantly higher in adjuvant chemotherapy arm than in observation arm (52.7% vs. 40.9%, P = 0.01) (Fig. 1B). Patients in adjuvant chemotherapy arm also showed significantly improved LRFS (69.2% vs. 49.7%, P = 0.01, Fig. 1C) and DMFS (56.5% vs. 42.8%, P = 0.01, Fig. 1D) at five years compared with observation arm. Multivariate analysis revealed that the adjuvant chemotherapy was significantly associated with OS [hazard ratio (HR), 0.39 and 95% of confidence interval (CI), 0.23–0.66; P < 0.01] after adjusting for gender, serum CEA, surgery, pathologic T stage, and histologic grade (Table 2). Other significant prognostic factors were age (P < 0.01) and pathologic N stage (P < 0.01). Adjuvant chemotherapy (HR, 0.61 and 95% of CI, 0.39–0.94; P = 0.03) was also significantly associated with RFS in the multivariate analysis (Table 3).
Table 2
Prognostic factors associated with overall survival in high risk group
Variable | Univariate (P) Hazard ratio (95% CI) | Multivariate (P) Hazard ratio (95% CI) |
Age, year | 0.02 | < 0.01 |
≤ 65 | 1 | |
> 65 | 1.74 (1.22–2.49) | 1.87 (1.30–2.69) |
Gender | 0.96 | |
Male | 1 | |
Female | 1.01 (0.68–1.49) | |
CEA, ng/mL | 0.82 | |
≤ 5 | 1 | |
> 5 | 1.04 (0.73–1.48) | |
Surgery | 0.01 | |
LAR | 1 | |
APR | 1.77 (1.16–2.72) | |
Pathologic T | 0.03 | |
ypT0-2 | 1 | |
ypT3-4 | 2.12 (1.08–4.18) | |
Pathologic N | < 0.01 | < 0.01 |
ypN0 | 1 | 1 |
ypN+ | 2.57 (1.76–3.75) | 2.33 (1.57–3.46) |
Histologic grade | 0.04 | |
Low | 1 | |
High | 2.05 (1.24–3.38) | |
Adjuvant Chemotherapy | < 0.01 | < 0.01 |
No | 1 | 1 |
Yes | 0.41 (0.24–0.68) | 0.39 (0.23–0.66) |
APR Abdominoperineal resection, CEA Carcinoembryonic antigen, CI Confidence interval, LAR Low anterior resection |
Table 3
Prognostic factors associated with recurrence-free survival in high risk group
Variable | Univariate (P) Hazard ratio (95% CI) | Multivariate (P) Hazard ratio (95% CI) |
Age, year | 0.07 | 0.04 |
≤ 65 | 1 | 1 |
> 65 | 1.30 (0.98–1.72) | 1.34 (1.01–1.77) |
Gender | 0.14 | |
Male | 1 | |
Female | 1.25 (0.93–1.68) | |
CEA, ng/mL | 0.55 | |
≤ 5 | 1 | |
> 5 | 1.09 (0.83–1.43) | |
Surgery | < 0.01 | < 0.01 |
LAR | 1 | 1 |
APR | 1.94 (1.38–2.73) | 1.74 (1.21–2.51) |
Pathologic T | < 0.01 | |
ypT0-2 | 1 | |
ypT3-4 | 2.11 (1.27–3.52) | |
Pathologic N | < 0.01 | < 0.01 |
ypN0 | 1 | 1 |
ypN1-2 | 2.04 (1.53–2.70) | 1.81 (1.35–2.44) |
Histologic grade | 0.04 | |
Low | 1 | |
High | 1.55 (1.01–2.37) | |
Adjuvant Chemotherapy | 0.01 | 0.03 |
No | 1 | 1 |
Yes | 0.59 (0.38–0.90) | 0.61 (0.39–0.94) |
APR Abdominoperineal resection, CEA Carcinoembryonic antigen, CI Confidence interval, LAR Low anterior resection |
Survival Analyses Of Low-risk Group
The 5-year OS rates of the low-risk group were 86.6% in adjuvant chemotherapy arm and 75.3% in observation arm (Fig. 2A). The difference was not statistically significant (P = 0.61). The 5-year RFS rates were not statistically different between the two arms (66.9% vs. 53.1%, P = 0.75, Fig. 2B). The 5-year LRFS (82.5% vs. 64.4%, P = 0.36, Fig. 2C) and DMFS (70.4% vs. 63.4%, P = 0.93, Fig. 2D) showed no statistically significant difference.
Age (P = 0.04) was a significant prognostic factor for OS in the multivariate analysis. Adjuvant chemotherapy was not significantly associated with OS and RFS in the multivariate analysis (Supplementary Table).