A total of 105 patients with pN1 stage III OSCC were enrolled in this study. The clinical characteristics of the patients are summarized in Table 1. The median age of the patients was 54 years (range 32–77). The study sample included 91 (86.7%) male patients and 14 (13.3%) female patients. In tumor differentiation, most of the patients had moderately differentiated carcinoma (n = 62, 59%), followed by well-differentiated carcinoma (n = 38, 36.2%) and poorly differentiated carcinoma (n = 5, 4.8%). The median of the greatest tumor size was 22 mm (range 6–55), and the median of the depth of invasion (DOI) was 8 mm (range 1–19). Perineural invasion and lymphovascular invasion were reported in 32.4% and 21.9% of the patients, respectively. In total, 97 patients (92.4%) received ipsilateral neck dissection, while only 8 patients (7.6%) underwent bilateral neck dissection because of the cross midline of the tumor. Thirty-seven patients had clinical N0 disease and received neck dissections. Furthermore, 35 patients received adjuvant radiotherapy after surgery, 17 patients received adjuvant concurrent chemoradiotherapy, and 53 patients preferred not to receive any postoperative adjuvant therapy. Our study cohort included 28 patients (26.7%) with pT1N1 disease, 49 patients (46.6%) with pT2N1 disease, and 28 patients (26.7%) with pT3N1 disease. The patients were followed up for a median of 69.5 months in this cohort. Tumor recurrence was observed in 35 (33.3%) patients, including local, regional, locoregional recurrence and distant metastasis occurred in 11, 16, 4, and 4 patients, respectively.

For pT classification, we found that patients in pT1N1 group had favorable outcomes than pT2N1 and pT3N1 groups (pT1N1 vs. pT2N1, OS: p = 0.009; DSS: p = 0.035; DFS: p = 0.039; pT1N1 vs pT3N1, OS: p = 0.001; DSS: p = 0.013; DFS: p = 0.023). However, pT2N1 group and pT3N1 group had very closed trend of Kaplan Meier survival curves (pT2N1 vs. pT3N1, OS, p = 0.252; DSS: p = 0.555; DFS: p = 0.717) (See Additional file 1: Table S1). Under this circumstance, we categorized our cohort into pT1N1 and pT2-3N1 as pT classification risk factor for further analysis.

The five-year survival rates of OS, DSS, and DFS were estimated for all clinico-pathological variables listed in Table 2. These results showed that pT1N1 disease was significantly associated with higher rates of five-year OS, DSS, and DFS compared with pT2-3N1 disease in univariate analysis (OS: p = 0.004, DSS: p = 0.021, and DFS: p = 0.023). The Kaplan–Meier curves of each strata (pT1N1 vs. pT2-3N1) was shown in Fig. 1. We also examined the relationships between the pT classification and other clinicopathological factors. The pT classification did not have significant association with age, histologic grade, PNI, LVI, surgical margin, positive nodal size and received adjuvant therapy or not (all p > 0.05) (See Additional file 2: Table S2).

Kaplan–Meier survival curves according to pT classification. A Overall survival curves, B disease-specific survival curves, and C disease-free survival curves

In this cohort, no significant difference in survival was observed (all p > 0.05) between patients who received adjuvant therapy and those who did not in both pT1N1 and pT2-3N1 strata. In pT2-3N1 subgroup, the survival outcomes for the surgery-with-adjuvant-therapy patients and the surgery-alone patients had no significant difference (OS: 80.1% vs. 63.2%, p = 0.671, DSS: 82.9% vs. 70.6%, p = 0.182, and DFS: 68.2% vs. 55.6%, p = 0.175). Similar results were observed for pT1N1 stratum, the survival rates for surgery-with-adjuvant-therapy group vs. the surgery-alone group were, OS: 100% vs. 94.1%, p = 0.421; DSS: 100% vs. 94.1%, p = 0.421, and DFS: 90.9% vs. 80.2%, p = 0.56 (Table 3).

Given the significantly better survival of pT1N1 disease patients among those with stage III OSCC, another cohort of patients with pathologically confirmed T1N1 disease (n = 28) and T2N0 disease (stage II, n = 270) from our cancer database between January 2007 and March 2016 were enrolled for comparison in terms of survival rates. The outcomes showed no significant differences in terms of five-year OS, DSS, and DFS between pT1N1 (n = 28) and pT2N0 (n = 270) patients (OS: 96.4% vs. 84.8%, p = 0.128; DSS: 96.4% vs 93.7%, p = 0.578; DFS: 85.7% vs. 88.5%, p = 0.63). The Kaplan–Meier curves of OS, DSS, and DFS in patients with pT1N1 and pT2N0 disease (according to the AJCC eighth edition TNM staging system) were represented in Fig. 2.

Kaplan–Meier survival curves of OSCC patients with pT1N1 disease and pT2N0 disease. A Overall survival curves, B disease-specific survival curves, and C disease-free survival curves