Prognostic Implications of Occult Nodal Metastases in Patients with Clinically N0 Primary Parotid Gland Cancer

<i>Background and Objectives:</i> The role of occult nodal metastases in patients with parotid gland cancers remains unclear; such metastases are histologically diverse and exhibit unpredictable clinical courses. Here, we evaluated the prognostic utilities of such metastases, including metastases in the intraparenchymal lymph nodes (PARs). <i>Materials and Methods:</i> We retrospectively reviewed the medical charts of patients who underwent surgery to treat clinically N0 primary parotid gland cancers from 2000 to 2022. The primary outcome variables were 5-year overall survival (OS) and 5-year disease-free survival (DFS). We explored the effects of occult nodal metastases, including metastases in the PARs, especially in terms of the pathological T (pT) classification. <i>Results:</i> Among 74 patients, 48 (64.8%) were pT1/2 cases, and 26 (35.2%) were pT3/4 cases. Both perineural and lymphatic invasion were negatively associated with the 5-year DFS (hazard ratio [HR] = 3.533, 95% confidence interval [CI] = 1.325-9.421, <i>p</i> = 0.012; HR = 4.028, 95% CI = 1.497-10.839, <i>p</i> = 0.006, respectively). During pathological review, PAR metastases were observed in 12 patients (16.2%), and other occult metastases were present in 8 patients (10.8%). PAR metastases reduced the 5-year DFS in pT1/2 cases (87.2% vs. 22.2%, <i>p</i> = 0.001) but not in pT3/4 cases. <i>Conclusions:</i> PAR metastases significantly reduced the 5-year DFS in patients with clinically N0 primary parotid gland cancer. On subgroup analysis according to pT classification, this effect was significant among patients with early pT1/2 status but not patients with advanced pT3/4 status.