Introduction : Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal malignances. Its larger mass size is widely acknowledged with higher potential of lymph node (LN) metastasis. However, the quantitative relationships between tumor size and LN metastasis or survival remains unknown.

Methods : 9,958 resected PDAC patients with no distal metastasis in this study were retrieved from the Surveillance, Epidemiology, and End Results database registry of National Cancer Institute. The prevalence of LN metastases, lymph node ratio (LNR) and N2/N1 ratio were assessed in different tumor sizes.

Results : The prevalence of LN metastases conforms to Y=0.01396588X3-0.15149701X2+0.57556259X-0.00371747 within 1-40mm in size, and then fluctuated between 70.0% and 80.0% when beyond 40mm. The average LNR increased stepwise as tumor size increased from size 1-5mm (0.024) to size 41-45mm (0.177); then, beyond 45mm, it fluctuated near 0.170. N2/N1 ratio gradually increased along with tumor size from 1-5mm (0.286) to 41-45mm (1.016), and when tumor size came up to 41-45mm or even more, the ratio stably fluctuated around 1.000. Survival benefits from chemoradiotherapy showed that radiotherapy and chemotherapy could significantly prolong OS and CSS for patients with tumors within 21-50mm and 11-70mm in size, respectively.

Conclusions : The probability of metastasis in each regional LN for resectable PDAC patients with tumors more than 40mm in size was near 17.0% and its prevalence of LN metastasis was 70%-80%, and among which, 50% of patients may have a N2 stage. Furthermore, tumors within 11-70mm in size should be highly recommended for chemotherapy, and meanwhile within 21-50mm, radiotherapy can be additionally combined.